173 lines
4.1 KiB
Plaintext
173 lines
4.1 KiB
Plaintext
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PEP: 490
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Title: Chain exceptions at C level
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Version: $Revision$
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Last-Modified: $Date$
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Author: Victor Stinner <victor.stinner@gmail.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 25-March-2015
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Python-Version: 3.5
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Abstract
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========
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Python 3 chained exceptions: PEP 3134. This PEP makes the same change at C
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level.
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Rationale
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=========
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Python 3 chained exceptions: PEP 3134. This PEP makes the same change at C
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level.
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Proposal
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========
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Modify PyErr_*() funnctions to chain exceptions
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-----------------------------------------------
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Modify C functions raising exceptions of the Python C API to automatically
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chain exceptions.
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Modify functions to not chain exceptions
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----------------------------------------
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Keep the original exception is not always interesting. Example in Python
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(``os._Environ.__getitem__``)::
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try:
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value = self._data[self.encodekey(key)]
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except KeyError:
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# raise KeyError with the original key value
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raise KeyError(key) from None
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To remove the previous exception, PyErr_Clear() can be called before
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PyErr_SetString() (or other functions raising exceptions).
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Python C API
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------------
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Include/pyerror.h declares functions related to exceptions.
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Functions raising exceptions:
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* PyErr_SetNone(exc_type)
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* PyErr_SetObject(exc_type, exc_value)
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* PyErr_SetString(exc_type, message)
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* PyErr_Format(exc, format, ...)
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Helpers to raise specific exceptions:
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* PyErr_BadArgument()
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* PyErr_BadInternalCall()
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* PyErr_NoMemory()
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* PyErr_SetFromErrno(exc)
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* PyErr_SetFromWindowsErr(err)
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* PyErr_SetImportError(message, name, path)
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* _PyErr_SetKeyError(key)
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* _PyErr_TrySetFromCause(prefix_format, ...)
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Manage the current exception:
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* PyErr_Clear(): clear the current exception, like "except: pass"
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* PyErr_Fetch(exc_type, exc_value, exc_tb)
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* PyErr_Restore(exc_type, exc_value, exc_tb)
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* PyErr_GetExcInfo(exc_type, exc_value, exc_tb)
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* PyErr_SetExcInfo(exc_type, exc_value, exc_tb)
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Others function to handle exceptions:
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* PyErr_ExceptionMatches(exc): check to implement "except exc: ..."
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* PyErr_GivenExceptionMatches(exc1, exc2)
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* PyErr_NormalizeException(exc_type, exc_value, exc_tb)
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* _PyErr_ChainExceptions(exc_type, exc_value, exc_tb)
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Backward compatibility
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======================
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XXX
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Alternatives
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============
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No change
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---------
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Python 3.5 introduces a new private ``_PyErr_ChainExceptions()`` function which
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is enough to chain manually exceptions.
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New helpers to chain exceptions
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-------------------------------
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Functions like ``PyErr_SetString()`` don't chain automatically exceptions. To
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make usage of ``_PyErr_ChainExceptions()`` easier, new functions are added:
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* PyErr_SetStringChain(exc_type, message)
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* PyErr_FormatChaine(exc_type, format, ...)
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* PyErr_SetNoneChain(exc_type)
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* PyErr_SetObjectChain(exc_type, exc_value)
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Helper functions like _PyErr_SetKeyError(key) or PyErr_SetImportError(message,
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name, path) don't chain exceptions. The generic
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_PyErr_ChainExceptions(exc_type, exc_value, exc_tb) should be used.
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Appendix
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========
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Python Issues
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-------------
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Chain exceptions:
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* `Chain exceptions in C
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<http://bugs.python.org/issue23763>`_
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* `Chaining exceptions at C level
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<http://bugs.python.org/issue21715>`_: added _PyErr_ChainExceptions()
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* `zipimport: chain ImportError to OSError
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<http://bugs.python.org/issue23696>`_
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Prevent loosing exceptions:
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* `Raise SystemError if a function returns a result with an exception set
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<http://bugs.python.org/issue23571>`_
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PEPs
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----
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* `PEP 3134 -- Exception Chaining and Embedded Tracebacks
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<https://www.python.org/dev/peps/pep-3134/>`_ (Python 3.0):
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new ``__context__`` and ``__cause__`` attributes for exceptions
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* `PEP 415 - Implement context suppression with exception attributes
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<https://www.python.org/dev/peps/pep-0415/>`_ (Python 3.3):
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``raise exc from None``
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* `PEP 409 - Suppressing exception context
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<https://www.python.org/dev/peps/pep-0409/>`_ (superseded by the PEP 415)
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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