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bpo-45464: [doc] Explain that subclassing multiple exceptions is fragile (GH-29094)
Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>
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@ -34,6 +34,10 @@ class or one of its subclasses, and not from :exc:`BaseException`. More
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information on defining exceptions is available in the Python Tutorial under
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:ref:`tut-userexceptions`.
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Exception context
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-----------------
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When raising (or re-raising) an exception in an :keyword:`except` or
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:keyword:`finally` clause
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:attr:`__context__` is automatically set to the last exception caught; if the
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@ -67,6 +71,25 @@ exceptions so that the final line of the traceback always shows the last
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exception that was raised.
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Inheriting from built-in exceptions
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-----------------------------------
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User code can create subclasses that inherit from an exception type.
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It's recommended to only subclass one exception type at a time to avoid
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any possible conflicts between how the bases handle the ``args``
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attribute, as well as due to possible memory layout incompatibilities.
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.. impl-detail::
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Most built-in exceptions are implemented in C for efficiency, see:
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:source:`Objects/exceptions.c`. Some have custom memory layouts
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which makes it impossible to create a subclass that inherits from
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multiple exception types. The memory layout of a type is an implementation
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detail and might change between Python versions, leading to new
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conflicts in the future. Therefore, it's recommended to avoid
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subclassing multiple exception types altogether.
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Base classes
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------------
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@ -0,0 +1,4 @@
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Mention in the documentation of :ref:`Built-in Exceptions
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<bltin-exceptions>` that inheriting from multiple exception types in a
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single subclass is not recommended due to possible memory layout
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incompatibility.
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