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==============
Error Handling
==============
GLib has its own method of handling errors using :obj:`GLib.Error`. These are
raised as Python exceptions, but with a few small differences.
It's common in Python for exception subclasses to be used (e.g.,
:obj:`ValueError` versus :obj:`IOError`) to distinguish different types of
errors. Libraries often define their own :obj:`Exception` subclasses, and library
users will handle these cases explicitly.
In GLib-using libraries, errors are all :obj:`GLib.Error` instances, with no
subclassing for different error types. Instead, every :obj:`GLib.Error`
instance has attributes that distinguish types of error:
* :attr:`domain` is the error domain, usually a string that you can covert to
a `GLib` quark with :func:`GLib.quark_to_string`
* :attr:`code` identifies a specific error within the domain
* :attr:`message` is a human-readable description of the error
Error domains are defined per-module, and you can get an error domain from
:func:`*_error_quark` functions on the relevant module. For example, IO errors
from `Gio` are in the domain returned by :func:`Gio.io_error_quark`, and the
possible values for :attr:`code` are enumerated in :obj:`Gio.IOErrorEnum`.
Once you've caught a :obj:`GLib.Error`, you can call
:meth:`GLib.Error.matches` to see whether it matches the specific error you
want to handle.
Examples
--------
Catching a specific error:
.. code:: pycon
>>> from gi.repository import GLib, Gio
>>> f = Gio.File.new_for_path('missing-path')
>>> try:
... f.read()
... except GLib.Error as err:
... if err.matches(Gio.io_error_quark(), Gio.IOErrorEnum.NOT_FOUND):
... print('File not found')
... else:
... raise
File not found
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