coverage of multiline generator/set/dict comprehensions is wrong when run with pytest's assertion rewriting

Originally reported by Andy Freeland (Bitbucket: rouge8, GitHub: rouge8)

Code/config to reproduce available as a gist. Fails on Python 2.7 but not Python 3.5.

Essentially, given this test:

def test_foo():
    # covered!
    assert {i for i in range(10)} == {i for i in range(10)}

    # "didn't finish the set comprehension"
    assert {i for i in range(10)} == {
        i for i in range(10)
    }

    # covered!
    assert True

When run under pytest with assertion rewriting (the default), the multiline set comprehension is reported as partially covered, even though the comprehension on oneline is fully covered. I think this is a bug in coverage, not pytest's assertion rewriting, because this code passes:

import ast
from _pytest.assertion.rewrite import rewrite_asserts

oneline = """assert {i for i in range(10)} == {i for i in range(10)}"""
multiline = """assert {i for i in range(10)} == {
    i for i in range(10)
}"""

# Parse the expressions
oneline_tree = ast.parse(oneline)
multiline_tree = ast.parse(multiline)

# Dump the pre-assertion rewrite ASTs
multiline_dump_prerewrite = ast.dump(multiline_tree)
oneline_dump_prerewrite = ast.dump(oneline_tree)

# The ASTs should be the same
assert multiline_dump_prerewrite == oneline_dump_prerewrite

# Rewrite the asserts
rewrite_asserts(oneline_tree)
rewrite_asserts(multiline_tree)

# Dump the rewritten ASTs
oneline_dump_rewrite = ast.dump(oneline_tree)
multiline_dump_rewrite = ast.dump(multiline_tree)

# The ASTs should be the same
assert oneline_dump_rewrite == multiline_dump_rewrite

• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/515

Start with the linked reproduction gist and compare coverage results for the one-line and multiline comprehensions when pytest assertion rewriting is enabled. Trace how coverage handles the rewritten AST and confirm the fix by showing that the multiline comprehension is reported as fully covered.