132 lines
3.8 KiB
Plaintext
132 lines
3.8 KiB
Plaintext
This rule raises an issue when:
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* an ``++__exit__++`` method has a bare ``++raise++`` outside of an ``++except++`` block.
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* an ``++__exit__++`` method raises the exception provided as parameter.
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== Why is this an issue?
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:link-with-uscores1: https://docs.python.org/3/reference/datamodel.html?highlight=__exit__%20special#object.__exit__
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Methods ``++__enter__++`` and ``++__exit__++`` make it possible to implement objects which can be used as the expression of a ``++with++`` statement:
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[source,python]
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----
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with MyContextManager() as c :
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... # do something with c
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----
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This statement can be rewritten as a ``++try...finally++`` and an explicit call to the ``++__enter__++`` and ``++__exit__++`` methods:
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[source,python]
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----
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c = MyContextManager()
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c.__enter__()
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try:
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... # do something with c
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finally:
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c.__exit__()
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----
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The ``++__exit__++`` is the method of a statement context manager which is called when exiting the runtime context related to this object.
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If an exception is supplied as an argument, its propagation can be suppressed by having the method return a truthy value. Otherwise, the exception will be processed normally upon exit from the method.
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The special method {link-with-uscores1}[``++__exit__++``] should only raise an exception when it fails. It should never raise the provided exception, it is the caller's responsibility. The ``++__exit__++`` method can filter provided exceptions by simply returning True or False. Raising this exception will make the stack trace difficult to understand.
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== How to fix it
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To fix this issue, make sure to avoid raising the exception provided to an ``++__exit__++`` method.
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=== Code examples
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==== Noncompliant code example
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[source,python,diff-id=1,diff-type=noncompliant]
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----
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class MyContextManager:
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def __enter__(self):
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return self
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def __exit__(self, *args):
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raise # Noncompliant: __exit__ method has a bare raise outside of an except block.
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class MyContextManager:
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def __enter__(self):
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return self
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def __exit__(self, *args):
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raise args[2] # Noncompliant: __exit__() methods should not reraise the provided exception; this is the caller’s responsibility.
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class MyContextManager:
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def __enter__(self):
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return self
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def __exit__(self, exc_type, exc_value, traceback):
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raise exc_value # Noncompliant: __exit__() methods should not reraise the provided exception; this is the caller’s responsibility.
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----
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==== Compliant solution
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[source,python,diff-id=1,diff-type=compliant]
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----
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class MyContextManager:
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def __enter__(self, stop_exceptions):
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return self
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def __exit__(self, *args):
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try:
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...
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except:
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raise # No issue when raising another exception. The __exit__ method can fail and raise an exception
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class MyContextManager:
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def __enter__(self):
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return self
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def __exit__(self, exc_type, exc_value, traceback):
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pass # by default the function will return None, which is always False, and the exc_value will naturally raise.
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class MyContextManager:
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def __enter__(self, stop_exceptions):
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return self
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def __exit__(self, *args):
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raise MemoryError("No more memory") # This is ok too.
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----
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:link-with-uscores1: https://docs.python.org/3/reference/datamodel.html?highlight=__exit__%20special#object.__exit__
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== Resources
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=== Documentation
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* Python documentation – {link-with-uscores1}[The ``++__exit__++`` special method]
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* PEP 343 – https://www.python.org/dev/peps/pep-0343/[The "with" Statement]
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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=== Message
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remove this "raise" statement and return "False" instead.
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=== Highlighting
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The "raise" statement.
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'''
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== Comments And Links
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(visible only on this page)
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=== is related to: S5747
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endif::env-github,rspecator-view[]
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