Misleading branch coverage of empty methods

[nedbat]

Originally reported by Christian Heimes (Bitbucket: tiran, GitHub: tiran)

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Branch coverage of an abc.abstractmethod with just a doc string in the method, shows the decorator line as uncovered branch that never reaches "exit".

#!python
import abc

class AExample(object):
    @abc.abstractmethod  # exit
    def method(self):
        """doc
        """

class Example(AExample):
    def method(self):
        return True

Example().method()

When I replace the doc string with "pass", the decorator line is a covered branch. However the pass statement is not covered at all.

#!python

class AExample(object):
    @abc.abstractmethod 
    def method(self): 
        pass  # uncovered

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/129

[nedbat]

Turns out this isn't about abstract methods, it's about empty uncalled methods with only a docstring:

class Example(object):
    def method1(self):      # no branch to exit
        """doc"""

    def method2(self):
        pass                # not executed

    def method3(self):
        """doc"""
        pass                # not executed

[nedbat]

Original comment by Alex Gaynor (Bitbucket: alex_gaynor, GitHub: Unknown)

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It's unclear to me what the correct behavior is, it seems to me it's either a) no executed, or b) covered, but not a branch coverage thing. (b) is somewhat more convenient, but I'm not sure it's more correct.

[nedbat]

Original comment by Mike Nerone (Bitbucket: Manganeez, GitHub: Unknown)

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(a) would seem more appropriate for a bare function definition with only a docstring, while (b) seems right for an abstractmethod, so perhaps that should be special-cased as the OP suggested.

Incidentally, it's easy enough to do per-project because the "blamed" lines are the ones with the decorators. In your project .coveragerc:

#!
[report]
exclude_lines =
    @abstract
    <whatever_else>

[nedbat]

This will be fixed when the ast-branch code is merged.

[nedbat]

This is fixed in 44719bd (bb), which will be 4.1