Coverage issue with python short-circuit for loop

[nedbat]

Originally reported by Glen Nelson (Bitbucket: glen_nelson, GitHub: Unknown)

---

Sample code below was run with:

• python 3.5.2
• coverage 4.4.1
• nose 1.3.7

Consider the following two files:

#!python

#foo.py
def foo(items):
    for item in items:
        v1 = 5 if item > 5 else None
        v2 = item
        if not v1 or not v2:
            continue
        print(v1)
        print(v2)

#!python

#test_foo.py

from unittest import TestCase

from reproduce import foo


class TestFoo(TestCase):
    def test_foo(self):
        foo.foo([1, 2, 3, 4, 5, 6, 7])

We would expect full coverage of 'foo' here from the test. As we can see, when item is 1-5, we should hit the continue, else we should reach the prints.

Running coverage through nosetests was done with the following command

nosetests -P --with-coverage --cover-erase --cover-inclusive --cover-package reproduce --cover-html-dir=test_coverage_unit --cover-html tests/unit/reproduce

...
Name                                            Stmts   Miss  Cover
-------------------------------------------------------------------
reproduce/foo.py                                    8      1    88%

The missing line is the 'continue', due to short-circuting.

---

• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/593

[nedbat]

Original comment by Glen Nelson (Bitbucket: glen_nelson, GitHub: Unknown)

---

The assembly:

  3           0 SETUP_LOOP              81 (to 84)
              3 LOAD_FAST                0 (items)
              6 GET_ITER
        >>    7 FOR_ITER                73 (to 83)
             10 STORE_FAST               1 (item)

  4          13 LOAD_FAST                1 (item)
             16 LOAD_CONST               1 (5)
             19 COMPARE_OP               4 (>)
             22 POP_JUMP_IF_FALSE       31
             25 LOAD_CONST               1 (5)
             28 JUMP_FORWARD             3 (to 34)
        >>   31 LOAD_CONST               0 (None)
        >>   34 STORE_FAST               2 (v1)

  5          37 LOAD_FAST                1 (item)
             40 STORE_FAST               3 (v2)

  6          43 LOAD_FAST                2 (v1)
             46 UNARY_NOT
             47 POP_JUMP_IF_TRUE         7
             50 LOAD_FAST                3 (v2)
             53 UNARY_NOT
             54 POP_JUMP_IF_FALSE       60

  7          57 JUMP_ABSOLUTE            7

  8     >>   60 LOAD_GLOBAL              0 (print)
             63 LOAD_FAST                2 (v1)
             66 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             69 POP_TOP

  9          70 LOAD_GLOBAL              0 (print)
             73 LOAD_FAST                3 (v2)
             76 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             79 POP_TOP
             80 JUMP_ABSOLUTE            7
        >>   83 POP_BLOCK
        >>   84 LOAD_CONST               0 (None)
             87 RETURN_VALUE

We see on 47, we jump directly back to 7 (the for loop iterator) if v1 is false. If instead v2 is false, we fall through the jump to 60, hitting the continue (line 57), which brings us back to line 7. It looks the coverage is only counting it if we hit line 57.

[nedbat]

Duplicate of #198.