Guard against an exception in the wrong place (Python)

2017-06-25T17:17:43-07:00

Python recipe 580808 by Steven D'Aprano (context, exception, guard, manager). Revision 2.

Sometimes exception handling can obscure bugs unless you guard against a particular exception occurring in a certain place. One example is that accidentally raising StopIteration inside a generator will halt the generator instead of displaying a traceback. That was solved by PEP 479, which automatically has such StopIteration exceptions change to RuntimeError. See the discussion below for further examples.

Here is a class which can be used as either a decorator or context manager for guarding against the given exceptions. It takes an exception (or a tuple of exceptions) as argument, and if the wrapped code raises that exception, it is re-raised as another exception type (by default RuntimeError).

For example:

try:
    with exception_guard(ZeroDivisionError):
        1/0  # raises ZeroDivisionError
except RuntimeError:
    print ('ZeroDivisionError replaced by RuntimeError')


@exception_guard(KeyError)
def demo():
    return {}['key']  # raises KeyError

try:
    demo()
except RuntimeError:
    print ('KeyError replaced by RuntimeError')

Function guards for Python 3 (Python)

2016-05-01T13:11:05-07:00

Python recipe 580658 by Dmitry Dvoinikov (call, decorator, dispatch, function, guard).

This module implements a function guard - facility to redirect the the call to one of several function implementations at run time based on the actual call arguments.

Wrap each of the identically named functions in a @guard decorator and provide a _when parameter with a default value set to guarding expression.

See samples at the top of the module.

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Guard against an exception in the wrong place (Python)

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