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- Python warnings system

# Python warnings system

Astropy uses the Python `warnings` module to issue warning messages. The details of using the warnings module are general to Python, and apply to any Python software that uses this system. The user can suppress the warnings using the python command line argument `-W"ignore"` when starting an interactive python session. For example:

    $ python -W"ignore"

The user may also use the command line argument when running a python script as follows:

    $ python -W"ignore" myscript.py

It is also possible to suppress warnings from within a python script. For instance, the warnings issued from a single call to the `astropy.io.fits.writeto` function may be suppressed from within a Python script using the `warnings.filterwarnings` function as follows:

    >>> import warnings
    >>> from astropy.io import fits
    >>> warnings.filterwarnings('ignore', category=UserWarning, append=True)
    >>> fits.writeto(filename, data, overwrite=True)

An equivalent way to insert an entry into the list of warning filter specifications for simple call `warnings.simplefilter`:

    >>> warnings.simplefilter('ignore', UserWarning)

Astropy includes its own warning classes, `AstropyWarning` and `AstropyUserWarning`. All warnings from Astropy are based on these warning classes (see below for the distinction between them). One can thus ignore all warnings from Astropy (while still allowing through warnings from other libraries like Numpy) by using something like:

    >>> from astropy.utils.exceptions import AstropyWarning
    >>> warnings.simplefilter('ignore', category=AstropyWarning)

Warning filters may also be modified just within a certain context using the `warnings.catch_warnings` context manager:

    >>> with warnings.catch_warnings():
    ...     warnings.simplefilter('ignore', AstropyWarning)
    ...     fits.writeto(filename, data, overwrite=True)

As mentioned above, there are actually two base classes for Astropy warnings. The main distinction is that `AstropyUserWarning` is for warnings that are intended for typical users (e.g. “Warning: Ambiguous unit”, something that might be because of improper input). In contrast, `AstropyWarning` warnings that are not `AstropyUserWarning` may be for lower-level warnings more useful for developers writing code that uses Astropy (e.g., the deprecation warnings discussed below). So if you’re a user that just wants to silence everything, the code above will suffice, but if you are a developer and want to hide development-related warnings from your users, you may wish to still allow through `AstropyUserWarning`.

Astropy also issues warnings when deprecated API features are used. If you wish to squelch deprecation warnings, you can start Python with `-Wi::Deprecation`. This sets all deprecation warnings to ignored. There is also an Astropy-specific `AstropyDeprecationWarning` which can be used to disable deprecation warnings from Astropy only.

See the CPython documentation for more information on the -W argument.