Skip to main content
`Ctrl`+`K`

# Source code for astropy.io.fits.hdu.hdulist

    # Licensed under a 3-clause BSD style license - see PYFITS.rst

    import gzip
    import itertools
    import os
    import re
    import shutil
    import sys
    import warnings
    from textwrap import indent

    import numpy as np

    from astropy.io.fits.file import FILE_MODES, _File
    from astropy.io.fits.header import _pad_length
    from astropy.io.fits.util import (
        _free_space_check,
        _get_array_mmap,
        _is_int,
        _tmp_name,
        fileobj_closed,
        fileobj_mode,
        ignore_sigint,
        isfile,
    )
    from astropy.io.fits.verify import VerifyError, VerifyWarning, _ErrList, _Verify
    from astropy.utils.compat.numpycompat import NUMPY_LT_2_0

    # NOTE: Python can be built without bz2.
    from astropy.utils.exceptions import AstropyUserWarning

    from .base import ExtensionHDU, _BaseHDU, _NonstandardHDU, _ValidHDU
    from .compressed.compressed import CompImageHDU
    from .groups import GroupsHDU
    from .image import ImageHDU, PrimaryHDU
    from .table import BinTableHDU

    __all__ = ["HDUList", "fitsopen"]

    # FITS file signature as per RFC 4047
    FITS_SIGNATURE = b"SIMPLE  =                    T"

    def fitsopen(
        name,
        mode="readonly",
        memmap=None,
        save_backup=False,
        cache=True,
        lazy_load_hdus=None,
        ignore_missing_simple=False,
        *,
        use_fsspec=None,
        fsspec_kwargs=None,
        decompress_in_memory=False,
        **kwargs,
    ):
        """Factory function to open a FITS file and return an `HDUList` object.

        Parameters
        ----------
        name : str, file-like or `pathlib.Path`
            File to be opened.

        mode : str, optional
            Open mode, 'readonly', 'update', 'append', 'denywrite', or
            'ostream'. Default is 'readonly'.

            If ``name`` is a file object that is already opened, ``mode`` must
            match the mode the file was opened with, readonly (rb), update (rb+),
            append (ab+), ostream (w), denywrite (rb)).

        memmap : bool, optional
            Is memory mapping to be used? This value is obtained from the
            configuration item ``astropy.io.fits.Conf.use_memmap``.
            Default is `True`.

        save_backup : bool, optional
            If the file was opened in update or append mode, this ensures that
            a backup of the original file is saved before any changes are flushed.
            The backup has the same name as the original file with ".bak" appended.
            If "file.bak" already exists then "file.bak.1" is used, and so on.
            Default is `False`.

        cache : bool, optional
            If the file name is a URL, `~astropy.utils.data.download_file` is used
            to open the file.  This specifies whether or not to save the file
            locally in Astropy's download cache. Default is `True`.

        lazy_load_hdus : bool, optional
            To avoid reading all the HDUs and headers in a FITS file immediately
            upon opening.  This is an optimization especially useful for large
            files, as FITS has no way of determining the number and offsets of all
            the HDUs in a file without scanning through the file and reading all
            the headers. Default is `True`.

            To disable lazy loading and read all HDUs immediately (the old
            behavior) use ``lazy_load_hdus=False``.  This can lead to fewer
            surprises--for example with lazy loading enabled, ``len(hdul)``
            can be slow, as it means the entire FITS file needs to be read in
            order to determine the number of HDUs.  ``lazy_load_hdus=False``
            ensures that all HDUs have already been loaded after the file has
            been opened.

            .. versionadded:: 1.3

        uint : bool, optional
            Interpret signed integer data where ``BZERO`` is the central value and
            ``BSCALE == 1`` as unsigned integer data.  For example, ``int16`` data
            with ``BZERO = 32768`` and ``BSCALE = 1`` would be treated as
            ``uint16`` data. Default is `True` so that the pseudo-unsigned
            integer convention is assumed.

        ignore_missing_end : bool, optional
            Do not raise an exception when opening a file that is missing an
            ``END`` card in the last header. Default is `False`.

        ignore_missing_simple : bool, optional
            Do not raise an exception when the SIMPLE keyword is missing. Note
            that io.fits will raise a warning if a SIMPLE card is present but
            written in a way that does not follow the FITS Standard.
            Default is `False`.

            .. versionadded:: 4.2

        checksum : bool, str, optional
            If `True`, verifies that both ``DATASUM`` and ``CHECKSUM`` card values
            (when present in the HDU header) match the header and data of all HDU's
            in the file.  Updates to a file that already has a checksum will
            preserve and update the existing checksums unless this argument is
            given a value of 'remove', in which case the CHECKSUM and DATASUM
            values are not checked, and are removed when saving changes to the
            file. Default is `False`.

        disable_image_compression : bool, optional
            If `True`, treats compressed image HDU's like normal binary table
            HDU's.  Default is `False`.

        do_not_scale_image_data : bool, optional
            If `True`, image data is not scaled using BSCALE/BZERO values
            when read.  Default is `False`.

        character_as_bytes : bool, optional
            Whether to return bytes for string columns, otherwise unicode strings
            are returned, but this does not respect memory mapping and loads the
            whole column in memory when accessed. Default is `False`.

        ignore_blank : bool, optional
            If `True`, the BLANK keyword is ignored if present.
            Default is `False`.

        scale_back : bool, optional
            If `True`, when saving changes to a file that contained scaled image
            data, restore the data to the original type and reapply the original
            BSCALE/BZERO values. This could lead to loss of accuracy if scaling
            back to integer values after performing floating point operations on
            the data. Default is `False`.

        output_verify : str
            Output verification option.  Must be one of ``"fix"``,
            ``"silentfix"``, ``"ignore"``, ``"warn"``, or
            ``"exception"``.  May also be any combination of ``"fix"`` or
            ``"silentfix"`` with ``"+ignore"``, ``+warn``, or ``+exception"
            (e.g. ``"fix+warn"``).  See :ref:`astropy:verify` for more info.

        use_fsspec : bool, optional
            Use `fsspec.open` to open the file? Defaults to `False` unless
            ``name`` starts with the Amazon S3 storage prefix ``s3://`` or the
            Google Cloud Storage prefix ``gs://``.  Can also be used for paths
            with other prefixes (e.g., ``http://``) but in this case you must
            explicitly pass ``use_fsspec=True``.
            Use of this feature requires the optional ``fsspec`` package.
            A ``ModuleNotFoundError`` will be raised if the dependency is missing.

            .. versionadded:: 5.2

        fsspec_kwargs : dict, optional
            Keyword arguments passed on to `fsspec.open`. This can be used to
            configure cloud storage credentials and caching behavior.
            For example, pass ``fsspec_kwargs={"anon": True}`` to enable
            anonymous access to Amazon S3 open data buckets.
            See ``fsspec``'s documentation for available parameters.

            .. versionadded:: 5.2

        decompress_in_memory : bool, optional
            By default files are decompressed progressively depending on what data
            is needed.  This is good for memory usage, avoiding decompression of
            the whole file, but it can be slow. With decompress_in_memory=True it
            is possible to decompress instead the whole file in memory.

            .. versionadded:: 6.0

        Returns
        -------
        hdulist : `HDUList`
            `HDUList` containing all of the header data units in the file.

        """
        from astropy.io.fits import conf

        if memmap is None:
            # distinguish between True (kwarg explicitly set)
            # and None (preference for memmap in config, might be ignored)
            memmap = None if conf.use_memmap else False
        else:
            memmap = bool(memmap)

        if lazy_load_hdus is None:
            lazy_load_hdus = conf.lazy_load_hdus
        else:
            lazy_load_hdus = bool(lazy_load_hdus)

        if "uint" not in kwargs:
            kwargs["uint"] = conf.enable_uint

        if not name:
            raise ValueError(f"Empty filename: {name!r}")

        return HDUList.fromfile(
            name,
            mode,
            memmap,
            save_backup,
            cache,
            lazy_load_hdus,
            ignore_missing_simple,
            use_fsspec=use_fsspec,
            fsspec_kwargs=fsspec_kwargs,
            decompress_in_memory=decompress_in_memory,
            **kwargs,
        )

    class HDUList(list, _Verify):
        """
        HDU list class.  This is the top-level FITS object.  When a FITS
        file is opened, a `HDUList` object is returned.
        """

        def __init__(self, hdus=[], file=None):
            """
            Construct a `HDUList` object.

            Parameters
            ----------
            hdus : BaseHDU or sequence thereof, optional
                The HDU object(s) to comprise the `HDUList`.  Should be
                instances of HDU classes like `ImageHDU` or `BinTableHDU`.

            file : file-like, bytes, optional
                The opened physical file associated with the `HDUList`
                or a bytes object containing the contents of the FITS
                file.
            """
            if isinstance(file, bytes):
                self._data = file
                self._file = None
            else:
                self._file = file
                self._data = None

            # For internal use only--the keyword args passed to fitsopen /
            # HDUList.fromfile/string when opening the file
            self._open_kwargs = {}
            self._in_read_next_hdu = False

            # If we have read all the HDUs from the file or not
            # The assumes that all HDUs have been written when we first opened the
            # file; we do not currently support loading additional HDUs from a file
            # while it is being streamed to.  In the future that might be supported
            # but for now this is only used for the purpose of lazy-loading of
            # existing HDUs.
            if file is None:
                self._read_all = True
            elif self._file is not None:
                # Should never attempt to read HDUs in ostream mode
                self._read_all = self._file.mode == "ostream"
            else:
                self._read_all = False

            if hdus is None:
                hdus = []

            # can take one HDU, as well as a list of HDU's as input
            if isinstance(hdus, _ValidHDU):
                hdus = [hdus]
            elif not isinstance(hdus, (HDUList, list)):
                raise TypeError("Invalid input for HDUList.")

            for idx, hdu in enumerate(hdus):
                if not isinstance(hdu, _BaseHDU):
                    raise TypeError(f"Element {idx} in the HDUList input is not an HDU.")

            super().__init__(hdus)

            if file is None:
                # Only do this when initializing from an existing list of HDUs
                # When initializing from a file, this will be handled by the
                # append method after the first HDU is read
                self.update_extend()

        def __len__(self):
            if not self._in_read_next_hdu:
                self.readall()

            return super().__len__()

        def __repr__(self):
            # Special case: if the FITS file is located on a remote file system
            # and has not been fully read yet, we return a simplified repr to
            # avoid downloading the entire file.  We can tell that a file is remote
            # from the fact that the ``fsspec`` package was used to open it.
            is_fsspec_file = self._file and "fsspec" in str(
                self._file._file.__class__.__bases__
            )
            if not self._read_all and is_fsspec_file:
                return f"{type(self)} (partially read)"

            # In order to correctly repr an HDUList we need to load all the
            # HDUs as well
            self.readall()

            return super().__repr__()

        def __iter__(self):
            # While effectively this does the same as:
            # for idx in range(len(self)):
            #     yield self[idx]
            # the more complicated structure is here to prevent the use of len(),
            # which would break the lazy loading
            for idx in itertools.count():
                try:
                    yield self[idx]
                except IndexError:
                    break

        def __getitem__(self, key):
            """
            Get an HDU from the `HDUList`, indexed by number or name.
            """
            # If the key is a slice we need to make sure the necessary HDUs
            # have been loaded before passing the slice on to super.
            if isinstance(key, slice):
                max_idx = key.stop
                # Check for and handle the case when no maximum was
                # specified (e.g. [1:]).
                if max_idx is None:
                    # We need all of the HDUs, so load them
                    # and reset the maximum to the actual length.
                    max_idx = len(self)

                # Just in case the max_idx is negative...
                max_idx = self._positive_index_of(max_idx)

                number_loaded = super().__len__()

                if max_idx >= number_loaded:
                    # We need more than we have, try loading up to and including
                    # max_idx. Note we do not try to be clever about skipping HDUs
                    # even though key.step might conceivably allow it.
                    for i in range(number_loaded, max_idx):
                        # Read until max_idx or to the end of the file, whichever
                        # comes first.
                        if not self._read_next_hdu():
                            break

                try:
                    hdus = super().__getitem__(key)
                except IndexError as e:
                    # Raise a more helpful IndexError if the file was not fully read.
                    if self._read_all:
                        raise e
                    else:
                        raise IndexError(
                            "HDU not found, possibly because the index "
                            "is out of range, or because the file was "
                            "closed before all HDUs were read"
                        )
                else:
                    return HDUList(hdus)

            # Originally this used recursion, but hypothetically an HDU with
            # a very large number of HDUs could blow the stack, so use a loop
            # instead
            try:
                return self._try_while_unread_hdus(
                    super().__getitem__, self._positive_index_of(key)
                )
            except IndexError as e:
                # Raise a more helpful IndexError if the file was not fully read.
                if self._read_all:
                    raise e
                else:
                    raise IndexError(
                        "HDU not found, possibly because the index "
                        "is out of range, or because the file was "
                        "closed before all HDUs were read"
                    )

        def __contains__(self, item):
            """
            Returns `True` if ``item`` is an ``HDU`` _in_ ``self`` or a valid
            extension specification (e.g., integer extension number, extension
            name, or a tuple of extension name and an extension version)
            of a ``HDU`` in ``self``.

            """
            try:
                self._try_while_unread_hdus(self.index_of, item)
            except (KeyError, ValueError):
                return False

            return True

        def __setitem__(self, key, hdu):
            """
            Set an HDU to the `HDUList`, indexed by number or name.
            """
            _key = self._positive_index_of(key)
            if isinstance(hdu, (slice, list)):
                if _is_int(_key):
                    raise ValueError("An element in the HDUList must be an HDU.")
                for item in hdu:
                    if not isinstance(item, _BaseHDU):
                        raise ValueError(f"{item} is not an HDU.")
            else:
                if not isinstance(hdu, _BaseHDU):
                    raise ValueError(f"{hdu} is not an HDU.")

            try:
                self._try_while_unread_hdus(super().__setitem__, _key, hdu)
            except IndexError:
                raise IndexError(f"Extension {key} is out of bound or not found.")

            self._resize = True
            self._truncate = False

        def __delitem__(self, key):
            """
            Delete an HDU from the `HDUList`, indexed by number or name.
            """
            if isinstance(key, slice):
                end_index = len(self)
            else:
                key = self._positive_index_of(key)
                end_index = len(self) - 1

            self._try_while_unread_hdus(super().__delitem__, key)

            if key == end_index or (key == -1 and not self._resize):
                self._truncate = True
            else:
                self._truncate = False
                self._resize = True

        # Support the 'with' statement
        def __enter__(self):
            return self

        def __exit__(self, type, value, traceback):
            output_verify = self._open_kwargs.get("output_verify", "exception")
            self.close(output_verify=output_verify)

        @classmethod
        def fromfile(
            cls,
            fileobj,
            mode=None,
            memmap=None,
            save_backup=False,
            cache=True,
            lazy_load_hdus=True,
            ignore_missing_simple=False,
            **kwargs,
        ):
            """
            Creates an `HDUList` instance from a file-like object.

            The actual implementation of ``fitsopen()``, and generally shouldn't
            be used directly.  Use :func:`open` instead (and see its
            documentation for details of the parameters accepted by this method).
            """
            return cls._readfrom(
                fileobj=fileobj,
                mode=mode,
                memmap=memmap,
                save_backup=save_backup,
                cache=cache,
                ignore_missing_simple=ignore_missing_simple,
                lazy_load_hdus=lazy_load_hdus,
                **kwargs,
            )

        @classmethod
        def fromstring(cls, data, **kwargs):
            """
            Creates an `HDUList` instance from a string or other in-memory data
            buffer containing an entire FITS file.  Similar to
            :meth:`HDUList.fromfile`, but does not accept the mode or memmap
            arguments, as they are only relevant to reading from a file on disk.

            This is useful for interfacing with other libraries such as CFITSIO,
            and may also be useful for streaming applications.

            Parameters
            ----------
            data : str, buffer-like, etc.
                A string or other memory buffer containing an entire FITS file.
                Buffer-like objects include :class:`~bytes`, :class:`~bytearray`,
                :class:`~memoryview`, and :class:`~numpy.ndarray`.
                It should be noted that if that memory is read-only (such as a
                Python string) the returned :class:`HDUList`'s data portions will
                also be read-only.
            **kwargs : dict
                Optional keyword arguments.  See
                :func:`astropy.io.fits.open` for details.

            Returns
            -------
            hdul : HDUList
                An :class:`HDUList` object representing the in-memory FITS file.
            """
            try:
                # Test that the given object supports the buffer interface by
                # ensuring an ndarray can be created from it
                np.ndarray((), dtype="ubyte", buffer=data)
            except TypeError:
                raise TypeError(
                    f"The provided object {data} does not contain an underlying "
                    "memory buffer.  fromstring() requires an object that "
                    "supports the buffer interface such as bytes, buffer, "
                    "memoryview, ndarray, etc.  This restriction is to ensure "
                    "that efficient access to the array/table data is possible."
                )

            return cls._readfrom(data=data, **kwargs)

        def fileinfo(self, index):
            """
            Returns a dictionary detailing information about the locations
            of the indexed HDU within any associated file.  The values are
            only valid after a read or write of the associated file with
            no intervening changes to the `HDUList`.

            Parameters
            ----------
            index : int
                Index of HDU for which info is to be returned.

            Returns
            -------
            fileinfo : dict or None

                The dictionary details information about the locations of
                the indexed HDU within an associated file.  Returns `None`
                when the HDU is not associated with a file.

                Dictionary contents:

                ========== ========================================================
                Key        Value
                ========== ========================================================
                file       File object associated with the HDU
                filename   Name of associated file object
                filemode   Mode in which the file was opened (readonly,
                           update, append, denywrite, ostream)
                resized    Flag that when `True` indicates that the data has been
                           resized since the last read/write so the returned values
                           may not be valid.
                hdrLoc     Starting byte location of header in file
                datLoc     Starting byte location of data block in file
                datSpan    Data size including padding
                ========== ========================================================

            """
            if self._file is not None:
                output = self[index].fileinfo()

                if not output:
                    # OK, the HDU associated with this index is not yet
                    # tied to the file associated with the HDUList.  The only way
                    # to get the file object is to check each of the HDU's in the
                    # list until we find the one associated with the file.
                    f = None

                    for hdu in self:
                        info = hdu.fileinfo()

                        if info:
                            f = info["file"]
                            fm = info["filemode"]
                            break

                    output = {
                        "file": f,
                        "filemode": fm,
                        "hdrLoc": None,
                        "datLoc": None,
                        "datSpan": None,
                    }

                output["filename"] = self._file.name
                output["resized"] = self._wasresized()
            else:
                output = None

            return output

        def __copy__(self):
            """
            Return a shallow copy of an HDUList.

            Returns
            -------
            copy : `HDUList`
                A shallow copy of this `HDUList` object.

            """
            return self[:]

        # Syntactic sugar for `__copy__()` magic method
        copy = __copy__

        def __deepcopy__(self, memo=None):
            return HDUList([hdu.copy() for hdu in self])

        def pop(self, index=-1):
            """Remove an item from the list and return it.

            Parameters
            ----------
            index : int, str, tuple of (string, int), optional
                An integer value of ``index`` indicates the position from which
                ``pop()`` removes and returns an HDU. A string value or a tuple
                of ``(string, int)`` functions as a key for identifying the
                HDU to be removed and returned. If ``key`` is a tuple, it is
                of the form ``(key, ver)`` where ``ver`` is an ``EXTVER``
                value that must match the HDU being searched for.

                If the key is ambiguous (e.g. there are multiple 'SCI' extensions)
                the first match is returned.  For a more precise match use the
                ``(name, ver)`` pair.

                If even the ``(name, ver)`` pair is ambiguous the numeric index
                must be used to index the duplicate HDU.

            Returns
            -------
            hdu : BaseHDU
                The HDU object at position indicated by ``index`` or having name
                and version specified by ``index``.
            """
            # Make sure that HDUs are loaded before attempting to pop
            self.readall()
            list_index = self.index_of(index)
            return super().pop(list_index)

        def insert(self, index, hdu):
            """
            Insert an HDU into the `HDUList` at the given ``index``.

            Parameters
            ----------
            index : int
                Index before which to insert the new HDU.

            hdu : BaseHDU
                The HDU object to insert
            """
            if not isinstance(hdu, _BaseHDU):
                raise ValueError(f"{hdu} is not an HDU.")

            num_hdus = len(self)

            if index == 0 or num_hdus == 0:
                if num_hdus != 0:
                    # We are inserting a new Primary HDU so we need to
                    # make the current Primary HDU into an extension HDU.
                    if isinstance(self[0], GroupsHDU):
                        raise ValueError(
                            "The current Primary HDU is a GroupsHDU.  "
                            "It can't be made into an extension HDU, "
                            "so another HDU cannot be inserted before it."
                        )

                    hdu1 = ImageHDU(self[0].data, self[0].header)

                    # Insert it into position 1, then delete HDU at position 0.
                    super().insert(1, hdu1)
                    super().__delitem__(0)

                if not isinstance(hdu, (PrimaryHDU, _NonstandardHDU)):
                    # You passed in an Extension HDU but we need a Primary HDU.
                    # If you provided an ImageHDU then we can convert it to
                    # a primary HDU and use that.
                    if isinstance(hdu, ImageHDU):
                        hdu = PrimaryHDU(hdu.data, hdu.header)
                    else:
                        # You didn't provide an ImageHDU so we create a
                        # simple Primary HDU and append that first before
                        # we append the new Extension HDU.
                        phdu = PrimaryHDU()

                        super().insert(0, phdu)
                        index = 1
            else:
                if isinstance(hdu, GroupsHDU):
                    raise ValueError("A GroupsHDU must be inserted as a Primary HDU.")

                if isinstance(hdu, PrimaryHDU):
                    # You passed a Primary HDU but we need an Extension HDU
                    # so create an Extension HDU from the input Primary HDU.
                    hdu = ImageHDU(hdu.data, hdu.header)

            super().insert(index, hdu)
            hdu._new = True
            self._resize = True
            self._truncate = False
            # make sure the EXTEND keyword is in primary HDU if there is extension
            self.update_extend()

        def append(self, hdu):
            """
            Append a new HDU to the `HDUList`.

            Parameters
            ----------
            hdu : BaseHDU
                HDU to add to the `HDUList`.
            """
            if not isinstance(hdu, _BaseHDU):
                raise ValueError("HDUList can only append an HDU.")

            # store BZERO and BSCALE if present
            bzero = hdu.header.get("BZERO")
            bscale = hdu.header.get("BSCALE")

            if len(self) > 0:
                if isinstance(hdu, GroupsHDU):
                    raise ValueError("Can't append a GroupsHDU to a non-empty HDUList")

                if isinstance(hdu, PrimaryHDU):
                    # You passed a Primary HDU but we need an Extension HDU
                    # so create an Extension HDU from the input Primary HDU.
                    # TODO: This isn't necessarily sufficient to copy the HDU;
                    # _header_offset and friends need to be copied too.
                    hdu = ImageHDU(
                        hdu.data,
                        hdu.header,
                        do_not_scale_image_data=hdu._do_not_scale_image_data,
                    )
            else:
                if not isinstance(hdu, (PrimaryHDU, _NonstandardHDU)):
                    # You passed in an Extension HDU but we need a Primary
                    # HDU.
                    # If you provided an ImageHDU then we can convert it to
                    # a primary HDU and use that.
                    if isinstance(hdu, ImageHDU):
                        hdu = PrimaryHDU(
                            hdu.data,
                            hdu.header,
                            do_not_scale_image_data=hdu._do_not_scale_image_data,
                        )
                    else:
                        # You didn't provide an ImageHDU so we create a
                        # simple Primary HDU and append that first before
                        # we append the new Extension HDU.
                        phdu = PrimaryHDU()
                        super().append(phdu)

            # Add back BZERO and BSCALE if relevant
            if getattr(hdu, "_do_not_scale_image_data", False):
                if bzero is not None:
                    hdu.header["BZERO"] = bzero
                if bscale is not None:
                    hdu.header["BSCALE"] = bscale

            super().append(hdu)
            hdu._new = True
            self._resize = True
            self._truncate = False

            # make sure the EXTEND keyword is in primary HDU if there is extension
            self.update_extend()

        def index_of(self, key):
            """
            Get the index of an HDU from the `HDUList`.

            Parameters
            ----------
            key : int, str, tuple of (string, int) or BaseHDU
                The key identifying the HDU.  If ``key`` is a tuple, it is of the
                form ``(name, ver)`` where ``ver`` is an ``EXTVER`` value that must
                match the HDU being searched for.

                If the key is ambiguous (e.g. there are multiple 'SCI' extensions)
                the first match is returned.  For a more precise match use the
                ``(name, ver)`` pair.

                If even the ``(name, ver)`` pair is ambiguous (it shouldn't be
                but it's not impossible) the numeric index must be used to index
                the duplicate HDU.

                When ``key`` is an HDU object, this function returns the
                index of that HDU object in the ``HDUList``.

            Returns
            -------
            index : int
                The index of the HDU in the `HDUList`.

            Raises
            ------
            ValueError
                If ``key`` is an HDU object and it is not found in the ``HDUList``.
            KeyError
                If an HDU specified by the ``key`` that is an extension number,
                extension name, or a tuple of extension name and version is not
                found in the ``HDUList``.

            """
            if _is_int(key):
                return key
            elif isinstance(key, tuple):
                _key, _ver = key
            elif isinstance(key, _BaseHDU):
                return self.index(key)
            else:
                _key = key
                _ver = None

            if not isinstance(_key, str):
                raise KeyError(
                    f"{type(self).__name__} indices must be integers, extension "
                    f"names as strings, or (extname, version) tuples; got {_key}"
                )

            _key = (_key.strip()).upper()

            found = None
            for idx, hdu in enumerate(self):
                name = hdu.name
                if isinstance(name, str):
                    name = name.strip().upper()
                # 'PRIMARY' should always work as a reference to the first HDU
                if (name == _key or (_key == "PRIMARY" and idx == 0)) and (
                    _ver is None or _ver == hdu.ver
                ):
                    found = idx
                    break

            if found is None:
                raise KeyError(f"Extension {key!r} not found.")
            else:
                return found

        def _positive_index_of(self, key):
            """
            Same as index_of, but ensures always returning a positive index
            or zero.

            (Really this should be called non_negative_index_of but it felt
            too long.)

            This means that if the key is a negative integer, we have to
            convert it to the corresponding positive index.  This means
            knowing the length of the HDUList, which in turn means loading
            all HDUs.  Therefore using negative indices on HDULists is inherently
            inefficient.
            """
            index = self.index_of(key)

            if index >= 0:
                return index

            if abs(index) > len(self):
                raise IndexError(f"Extension {index} is out of bound or not found.")

            return len(self) + index

        def readall(self):
            """
            Read data of all HDUs into memory.
            """
            while self._read_next_hdu():
                pass

        @ignore_sigint
        def flush(self, output_verify="fix", verbose=False):
            """
            Force a write of the `HDUList` back to the file (for append and
            update modes only).

            Parameters
            ----------
            output_verify : str
                Output verification option.  Must be one of ``"fix"``,
                ``"silentfix"``, ``"ignore"``, ``"warn"``, or
                ``"exception"``.  May also be any combination of ``"fix"`` or
                ``"silentfix"`` with ``"+ignore"``, ``+warn``, or ``+exception"
                (e.g. ``"fix+warn"``).  See :ref:`astropy:verify` for more info.

            verbose : bool
                When `True`, print verbose messages
            """
            if self._file.mode not in ("append", "update", "ostream"):
                warnings.warn(
                    f"Flush for '{self._file.mode}' mode is not supported.",
                    AstropyUserWarning,
                )
                return

            save_backup = self._open_kwargs.get("save_backup", False)
            if save_backup and self._file.mode in ("append", "update"):
                filename = self._file.name
                if os.path.exists(filename):
                    # The file doesn't actually exist anymore for some reason
                    # then there's no point in trying to make a backup
                    backup = filename + ".bak"
                    idx = 1
                    while os.path.exists(backup):
                        backup = filename + ".bak." + str(idx)
                        idx += 1
                    warnings.warn(
                        f"Saving a backup of {filename} to {backup}.", AstropyUserWarning
                    )
                    try:
                        shutil.copy(filename, backup)
                    except OSError as exc:
                        raise OSError(
                            f"Failed to save backup to destination {filename}"
                        ) from exc

            self.verify(option=output_verify)

            if self._file.mode in ("append", "ostream"):
                for hdu in self:
                    if verbose:
                        try:
                            extver = str(hdu._header["extver"])
                        except KeyError:
                            extver = ""

                    # only append HDU's which are "new"
                    if hdu._new:
                        hdu._prewriteto()
                        with _free_space_check(self):
                            hdu._writeto(self._file)
                            if verbose:
                                print("append HDU", hdu.name, extver)
                            hdu._new = False
                        hdu._postwriteto()

            elif self._file.mode == "update":
                self._flush_update()

        def update_extend(self):
            """
            Make sure that if the primary header needs the keyword ``EXTEND`` that
            it has it and it is correct.
            """
            if not len(self):
                return

            if not isinstance(self[0], PrimaryHDU):
                # A PrimaryHDU will be automatically inserted at some point, but it
                # might not have been added yet
                return

            hdr = self[0].header

            def get_first_ext():
                try:
                    return self[1]
                except IndexError:
                    return None

            if "EXTEND" in hdr:
                if not hdr["EXTEND"] and get_first_ext() is not None:
                    hdr["EXTEND"] = True
            elif get_first_ext() is not None:
                if hdr["NAXIS"] == 0:
                    hdr.set("EXTEND", True, after="NAXIS")
                else:
                    n = hdr["NAXIS"]
                    hdr.set("EXTEND", True, after="NAXIS" + str(n))

        def writeto(
            self, fileobj, output_verify="exception", overwrite=False, checksum=False
        ):
            """
            Write the `HDUList` to a new file.

            Parameters
            ----------
            fileobj : str, file-like or `pathlib.Path`
                File to write to.  If a file object, must be opened in a
                writeable mode.

            output_verify : str
                Output verification option.  Must be one of ``"fix"``,
                ``"silentfix"``, ``"ignore"``, ``"warn"``, or
                ``"exception"``.  May also be any combination of ``"fix"`` or
                ``"silentfix"`` with ``"+ignore"``, ``+warn``, or ``+exception"
                (e.g. ``"fix+warn"``).  See :ref:`astropy:verify` for more info.

            overwrite : bool, optional
                If ``True``, overwrite the output file if it exists. Raises an
                ``OSError`` if ``False`` and the output file exists. Default is
                ``False``.

            checksum : bool
                When `True` adds both ``DATASUM`` and ``CHECKSUM`` cards
                to the headers of all HDU's written to the file.

            Notes
            -----
            gzip, zip, bzip2 and lzma compression algorithms are natively supported.
            Compression mode is determined from the filename extension
            ('.gz', '.zip', '.bz2' or '.xz' respectively).  It is also possible to
            pass a compressed file object, e.g. `gzip.GzipFile`.
            """
            if len(self) == 0:
                warnings.warn("There is nothing to write.", AstropyUserWarning)
                return

            self.verify(option=output_verify)

            # make sure the EXTEND keyword is there if there is extension
            self.update_extend()

            if fileobj is sys.stdout:
                # special case stdout for debugging convenience
                # see https://github.com/astropy/astropy/issues/3427
                fileobj = fileobj.buffer

            # make note of whether the input file object is already open, in which
            # case we should not close it after writing (that should be the job
            # of the caller)
            closed = isinstance(fileobj, str) or fileobj_closed(fileobj)

            mode = FILE_MODES[fileobj_mode(fileobj)] if isfile(fileobj) else "ostream"

            # This can accept an open file object that's open to write only, or in
            # append/update modes but only if the file doesn't exist.
            fileobj = _File(fileobj, mode=mode, overwrite=overwrite)
            hdulist = self.fromfile(fileobj)
            try:
                dirname = os.path.dirname(hdulist._file.name)
            except (AttributeError, TypeError):
                dirname = None

            try:
                with _free_space_check(self, dirname=dirname):
                    for hdu in self:
                        hdu._output_checksum = checksum
                        hdu._prewriteto()
                        hdu._writeto(hdulist._file)
                        hdu._postwriteto()
            finally:
                hdulist.close(output_verify=output_verify, closed=closed)

        def close(self, output_verify="exception", verbose=False, closed=True):
            """
            Close the associated FITS file and memmap object, if any.

            Parameters
            ----------
            output_verify : str
                Output verification option.  Must be one of ``"fix"``,
                ``"silentfix"``, ``"ignore"``, ``"warn"``, or
                ``"exception"``.  May also be any combination of ``"fix"`` or
                ``"silentfix"`` with ``"+ignore"``, ``+warn``, or ``+exception"
                (e.g. ``"fix+warn"``).  See :ref:`astropy:verify` for more info.

            verbose : bool
                When `True`, print out verbose messages.

            closed : bool
                When `True`, close the underlying file object.
            """
            try:
                if (
                    self._file
                    and self._file.mode in ("append", "update")
                    and not self._file.closed
                ):
                    self.flush(output_verify=output_verify, verbose=verbose)
            finally:
                if self._file and closed and hasattr(self._file, "close"):
                    self._file.close()

                # Give individual HDUs an opportunity to do on-close cleanup
                for hdu in self:
                    hdu._close(closed=closed)

        def info(self, output=None):
            """
            Summarize the info of the HDUs in this `HDUList`.

            Note that this function prints its results to the console---it
            does not return a value.

            Parameters
            ----------
            output : file-like or bool, optional
                A file-like object to write the output to.  If `False`, does not
                output to a file and instead returns a list of tuples representing
                the HDU info.  Writes to ``sys.stdout`` by default.
            """
            if output is None:
                output = sys.stdout

            if self._file is None:
                name = "(No file associated with this HDUList)"
            else:
                name = self._file.name

            results = [
                f"Filename: {name}",
                "No.    Name      Ver    Type      Cards   Dimensions   Format",
            ]

            format = "{:3d}  {:10}  {:3} {:11}  {:5d}   {}   {}   {}"
            default = ("", "", "", 0, (), "", "")
            for idx, hdu in enumerate(self):
                summary = hdu._summary()
                if len(summary) < len(default):
                    summary += default[len(summary) :]
                summary = (idx,) + summary
                if output:
                    results.append(format.format(*summary))
                else:
                    results.append(summary)

            if output:
                output.write("\n".join(results))
                output.write("\n")
                output.flush()
            else:
                return results[2:]

        def filename(self):
            """
            Return the file name associated with the HDUList object if one exists.
            Otherwise returns None.

            Returns
            -------
            filename : str
                A string containing the file name associated with the HDUList
                object if an association exists.  Otherwise returns None.

            """
            if self._file is not None:
                if hasattr(self._file, "name"):
                    return self._file.name
            return None

        @classmethod
        def _readfrom(
            cls,
            fileobj=None,
            data=None,
            mode=None,
            memmap=None,
            cache=True,
            lazy_load_hdus=True,
            ignore_missing_simple=False,
            *,
            use_fsspec=None,
            fsspec_kwargs=None,
            decompress_in_memory=False,
            **kwargs,
        ):
            """
            Provides the implementations from HDUList.fromfile and
            HDUList.fromstring, both of which wrap this method, as their
            implementations are largely the same.
            """
            if fileobj is not None:
                if not isinstance(fileobj, _File):
                    # instantiate a FITS file object (ffo)
                    fileobj = _File(
                        fileobj,
                        mode=mode,
                        memmap=memmap,
                        cache=cache,
                        use_fsspec=use_fsspec,
                        fsspec_kwargs=fsspec_kwargs,
                        decompress_in_memory=decompress_in_memory,
                    )
                # The Astropy mode is determined by the _File initializer if the
                # supplied mode was None
                mode = fileobj.mode
                hdulist = cls(file=fileobj)
            else:
                if mode is None:
                    # The default mode
                    mode = "readonly"

                hdulist = cls(file=data)
                # This method is currently only called from HDUList.fromstring and
                # HDUList.fromfile.  If fileobj is None then this must be the
                # fromstring case; the data type of ``data`` will be checked in the
                # _BaseHDU.fromstring call.

            if (
                not ignore_missing_simple
                and hdulist._file
                and hdulist._file.mode != "ostream"
                and hdulist._file.size > 0
            ):
                pos = hdulist._file.tell()
                # FITS signature is supposed to be in the first 30 bytes, but to
                # allow reading various invalid files we will check in the first
                # card (80 bytes).
                simple = hdulist._file.read(80)
                match_sig = simple[:29] == FITS_SIGNATURE[:-1] and simple[29:30] in (
                    b"T",
                    b"F",
                )

                if not match_sig:
                    # Check the SIMPLE card is there but not written correctly
                    match_sig_relaxed = re.match(rb"SIMPLE\s*=\s*[T|F]", simple)

                    if match_sig_relaxed:
                        warnings.warn(
                            "Found a SIMPLE card but its format doesn't"
                            " respect the FITS Standard",
                            VerifyWarning,
                        )
                    else:
                        if hdulist._file.close_on_error:
                            hdulist._file.close()
                        raise OSError(
                            "No SIMPLE card found, this file does not appear to "
                            "be a valid FITS file. If this is really a FITS file, "
                            "try with ignore_missing_simple=True"
                        )

                hdulist._file.seek(pos)

            # Store additional keyword args that were passed to fits.open
            hdulist._open_kwargs = kwargs

            if fileobj is not None and fileobj.writeonly:
                # Output stream--not interested in reading/parsing
                # the HDUs--just writing to the output file
                return hdulist

            # Make sure at least the PRIMARY HDU can be read
            read_one = hdulist._read_next_hdu()

            # If we're trying to read only and no header units were found,
            # raise an exception
            if not read_one and mode in ("readonly", "denywrite"):
                # Close the file if necessary (issue #6168)
                if hdulist._file.close_on_error:
                    hdulist._file.close()

                raise OSError("Empty or corrupt FITS file")

            if not lazy_load_hdus or kwargs.get("checksum") is True:
                # Go ahead and load all HDUs
                while hdulist._read_next_hdu():
                    pass

            # initialize/reset attributes to be used in "update/append" mode
            hdulist._resize = False
            hdulist._truncate = False

            return hdulist

        def _try_while_unread_hdus(self, func, *args, **kwargs):
            """
            Attempt an operation that accesses an HDU by index/name
            that can fail if not all HDUs have been read yet.  Keep
            reading HDUs until the operation succeeds or there are no
            more HDUs to read.
            """
            while True:
                try:
                    return func(*args, **kwargs)
                except Exception:
                    if self._read_next_hdu():
                        continue
                    else:
                        raise

        def _read_next_hdu(self):
            """
            Lazily load a single HDU from the fileobj or data string the `HDUList`
            was opened from, unless no further HDUs are found.

            Returns True if a new HDU was loaded, or False otherwise.
            """
            if self._read_all:
                return False

            fileobj, data, kwargs = self._file, self._data, self._open_kwargs

            if fileobj is not None and fileobj.closed:
                return False

            try:
                self._in_read_next_hdu = True

                # read all HDUs
                try:
                    if fileobj is not None:
                        try:
                            # Make sure we're back to the end of the last read
                            # HDU
                            if len(self) > 0:
                                last = self[len(self) - 1]
                                if last._data_offset is not None:
                                    offset = last._data_offset + last._data_size
                                    fileobj.seek(offset, os.SEEK_SET)

                            hdu = _BaseHDU.readfrom(fileobj, **kwargs)
                        except EOFError:
                            self._read_all = True
                            return False
                        except OSError:
                            # Close the file: see
                            # https://github.com/astropy/astropy/issues/6168
                            #
                            if self._file.close_on_error:
                                self._file.close()

                            if fileobj.writeonly:
                                self._read_all = True
                                return False
                            else:
                                raise
                    else:
                        if not data:
                            self._read_all = True
                            return False
                        hdu = _BaseHDU.fromstring(data, **kwargs)
                        self._data = data[hdu._data_offset + hdu._data_size :]

                    if not kwargs.get("disable_image_compression", False):
                        if isinstance(hdu, BinTableHDU) and CompImageHDU.match_header(
                            hdu.header
                        ):
                            kwargs_comp = {
                                key: val
                                for key, val in kwargs.items()
                                if key in ("scale_back", "uint", "do_not_scale_image_data")
                            }
                            hdu = CompImageHDU(bintable=hdu, **kwargs_comp)

                    super().append(hdu)
                    if len(self) == 1:
                        # Check for an extension HDU and update the EXTEND
                        # keyword of the primary HDU accordingly
                        self.update_extend()

                    hdu._new = False
                    if "checksum" in kwargs:
                        hdu._output_checksum = kwargs["checksum"]
                # check in the case there is extra space after the last HDU or
                # corrupted HDU
                except (VerifyError, ValueError) as exc:
                    warnings.warn(
                        f"Error validating header for HDU #{len(self)} (note: Astropy "
                        f"uses zero-based indexing).\n{indent(str(exc), 4 * ' ')}\n"
                        "There may be extra bytes after the last HDU or the "
                        "file is corrupted.",
                        VerifyWarning,
                    )
                    del exc
                    self._read_all = True
                    return False
            finally:
                self._in_read_next_hdu = False

            return True

        def _verify(self, option="warn"):
            errs = _ErrList([], unit="HDU")

            # the first (0th) element must be a primary HDU
            if (
                len(self) > 0
                and (not isinstance(self[0], PrimaryHDU))
                and (not isinstance(self[0], _NonstandardHDU))
            ):
                err_text = "HDUList's 0th element is not a primary HDU."
                fix_text = "Fixed by inserting one as 0th HDU."

                def fix(self=self):
                    self.insert(0, PrimaryHDU())

                err = self.run_option(option, err_text=err_text, fix_text=fix_text, fix=fix)
                errs.append(err)

            if len(self) > 1 and (
                "EXTEND" not in self[0].header or self[0].header["EXTEND"] is not True
            ):
                err_text = (
                    "Primary HDU does not contain an EXTEND keyword "
                    "equal to T even though there are extension HDUs."
                )
                fix_text = "Fixed by inserting or updating the EXTEND keyword."

                def fix(header=self[0].header):
                    naxis = header["NAXIS"]
                    if naxis == 0:
                        after = "NAXIS"
                    else:
                        after = "NAXIS" + str(naxis)
                    header.set("EXTEND", value=True, after=after)

                errs.append(
                    self.run_option(option, err_text=err_text, fix_text=fix_text, fix=fix)
                )

            # each element calls their own verify
            for idx, hdu in enumerate(self):
                if idx > 0 and (not isinstance(hdu, ExtensionHDU)):
                    err_text = f"HDUList's element {idx} is not an extension HDU."

                    err = self.run_option(option, err_text=err_text, fixable=False)
                    errs.append(err)

                else:
                    result = hdu._verify(option)
                    if result:
                        errs.append(result)
            return errs

        def _flush_update(self):
            """Implements flushing changes to a file in update mode."""
            for hdu in self:
                # Need to all _prewriteto() for each HDU first to determine if
                # resizing will be necessary
                hdu._prewriteto(inplace=True)

            try:
                self._wasresized()

                # if the HDUList is resized, need to write out the entire contents of
                # the hdulist to the file.
                if self._resize or self._file.compression:
                    self._flush_resize()
                else:
                    # if not resized, update in place
                    for hdu in self:
                        hdu._writeto(self._file, inplace=True)

                # reset the modification attributes after updating
                for hdu in self:
                    hdu._header._modified = False
            finally:
                for hdu in self:
                    hdu._postwriteto()

        def _flush_resize(self):
            """
            Implements flushing changes in update mode when parts of one or more HDU
            need to be resized.
            """
            old_name = self._file.name
            old_memmap = self._file.memmap
            name = _tmp_name(old_name)

            if not self._file.file_like:
                old_mode = os.stat(old_name).st_mode
                # The underlying file is an actual file object.  The HDUList is
                # resized, so we need to write it to a tmp file, delete the
                # original file, and rename the tmp file to the original file.
                if self._file.compression == "gzip":
                    new_file = gzip.GzipFile(name, mode="ab+")
                else:
                    new_file = name

                with self.fromfile(new_file, mode="append") as hdulist:
                    for hdu in self:
                        hdu._writeto(hdulist._file, inplace=True, copy=True)
                    if sys.platform.startswith("win"):
                        # Collect a list of open mmaps to the data; this well be
                        # used later.  See below.
                        mmaps = [
                            (idx, _get_array_mmap(hdu.data), hdu.data)
                            for idx, hdu in enumerate(self)
                            if hdu._has_data
                        ]

                    hdulist._file.close()
                    self._file.close()
                if sys.platform.startswith("win"):
                    # Close all open mmaps to the data.  This is only necessary on
                    # Windows, which will not allow a file to be renamed or deleted
                    # until all handles to that file have been closed.
                    for idx, mmap, arr in mmaps:
                        if mmap is not None:
                            mmap.close()

                os.remove(self._file.name)

                # reopen the renamed new file with "update" mode
                os.rename(name, old_name)
                os.chmod(old_name, old_mode)

                if isinstance(new_file, gzip.GzipFile):
                    old_file = gzip.GzipFile(old_name, mode="rb+")
                else:
                    old_file = old_name

                ffo = _File(old_file, mode="update", memmap=old_memmap)

                self._file = ffo

                for hdu in self:
                    # Need to update the _file attribute and close any open mmaps
                    # on each HDU
                    if hdu._has_data and _get_array_mmap(hdu.data) is not None:
                        del hdu.data
                    hdu._file = ffo

                if sys.platform.startswith("win"):
                    # On Windows, all the original data mmaps were closed above.
                    # However, it's possible that the user still has references to
                    # the old data which would no longer work (possibly even cause
                    # a segfault if they try to access it).  This replaces the
                    # buffers used by the original arrays with the buffers of mmap
                    # arrays created from the new file.  This seems to work, but
                    # it's a flaming hack and carries no guarantees that it won't
                    # lead to odd behavior in practice.  Better to just not keep
                    # references to data from files that had to be resized upon
                    # flushing (on Windows--again, this is no problem on Linux).
                    for idx, mmap, arr in mmaps:
                        if mmap is None:
                            continue
                        if NUMPY_LT_2_0:
                            # Note that this hack is only possible on numpy 1.x:
                            # in 2.x, we cannot write directly to the data attribute
                            # https://github.com/numpy/numpy/issues/8628
                            with warnings.catch_warnings():
                                warnings.simplefilter("ignore", category=DeprecationWarning)
                                arr.data = self[idx].data.data
                        elif sys.getrefcount(arr) > 2:
                            # 2 is the minimum number of references to this object
                            # counting `arr` and a reference as an argument to getrefcount(),
                            # see  https://docs.python.org/3/library/sys.html#sys.getrefcount
                            warnings.warn(
                                "Memory map object was closed but appears to still "
                                "be referenced. Further access will result in undefined "
                                "behavior (possibly including segmentation faults).",
                                category=UserWarning,
                                stacklevel=2,
                            )
                    del mmaps  # Just to be sure

            else:
                # The underlying file is not a file object, it is a file like
                # object.  We can't write out to a file, we must update the file
                # like object in place.  To do this, we write out to a temporary
                # file, then delete the contents in our file like object, then
                # write the contents of the temporary file to the now empty file
                # like object.
                self.writeto(name)
                hdulist = self.fromfile(name)
                ffo = self._file

                ffo.truncate(0)
                ffo.seek(0)

                for hdu in hdulist:
                    hdu._writeto(ffo, inplace=True, copy=True)

                # Close the temporary file and delete it.
                hdulist.close()
                os.remove(hdulist._file.name)

            # reset the resize attributes after updating
            self._resize = False
            self._truncate = False
            for hdu in self:
                hdu._header._modified = False
                hdu._new = False
                hdu._file = ffo

        def _wasresized(self, verbose=False):
            """
            Determine if any changes to the HDUList will require a file resize
            when flushing the file.

            Side effect of setting the objects _resize attribute.
            """
            if not self._resize:
                # determine if any of the HDU is resized
                for hdu in self:
                    # for CompImageHDU, we need to handle things a little differently
                    # because the HDU matching the header/data on disk is hdu._bintable
                    if isinstance(hdu, CompImageHDU):
                        hdu = hdu._bintable
                        if hdu is None:
                            continue

                    # Header:
                    nbytes = len(str(hdu._header))
                    if nbytes != (hdu._data_offset - hdu._header_offset):
                        self._resize = True
                        self._truncate = False
                        if verbose:
                            print("One or more header is resized.")
                        break

                    # Data:
                    if not hdu._has_data:
                        continue

                    nbytes = hdu.size
                    nbytes = nbytes + _pad_length(nbytes)
                    if nbytes != hdu._data_size:
                        self._resize = True
                        self._truncate = False
                        if verbose:
                            print("One or more data area is resized.")
                        break

                if self._truncate:
                    try:
                        self._file.truncate(hdu._data_offset + hdu._data_size)
                    except OSError:
                        self._resize = True
                    self._truncate = False

            return self._resize