[TIP] [coverage.py] concept question about branch coverage, statement coverage, and percent coverage

[Enji Cooper]

Hi,
	I have a general question about a few of the tests that come packaged with coverage.py, related to “line coverage”, “statement coverage”, and “branch coverage”.
	The code snippet from BasicCoverageTest.test_simple is as follows:

“””\
            a = 1
            b = 2
            
            c = 4
            # Nothing here
            d = 6
“””
	Per the above example, lines 1, 2, 4, and 6 are executed, but nothing else. The line coverage is 4/4 => 100% and the number of statements executed is also 4/4, resulting in 100% line/statement coverage. There aren’t any branches in the above code, so I would put down something like “N/A”.
	The asserted report (abbreviated) is as follows:

	# The fields in `report` translate to "Statements, Miss, Branch, Partial Branches, Cover"
	lines=[1,2,4,6], report="4 0 0 0 100%

	The test asserts that lines {1, 2, 4, 6} are executed, which is a total of 4/4 lines/statements executed, no statements missed, no branches taken, no partial branches, resulting in 100% line/statement coverage. Seems legit (the expected results match my understanding).
	If my above understanding is correct, let’s continue on to the next example.
	The code snippet from CompoundStatementTest.test_elif is as follows:

"""\
            a = 1; b = 2; c = 3;
            if a == 1:
                x = 3
            elif b == 2:
                y = 5
            else:
                z = 7
            assert x == 3
“””

	In this case, there are 8 total lines, 11 statements (line 1 contains 3 statements) and 4 possible branches (`if a == 1`, `elif b == 2`, `else:` and `assert x == 3`). Since a=1, lines 1-3 and 8 are executed, but none of the other lines are executed. 6 of the 11 statements are executed, and only 2 of the branch statements (the first being `if a == 1`, the second being `assert x == 3`) are executed. If my understand was correct, this should result in 50% line coverage, 54.55% statement coverage, and 50% branch coverage. This however, isn’t what the test expects:

	# The fields in `report` translate to "Statements, Miss, Branch, Partial Branches, Cover, Missing"
	lines=[1,2,3,4,5,7,8], missing="4-7", report="7 3 4 1 45% 2->4, 4-7”

	I agree on the `lines` and `missing` value, but the rest of the values seem askew:

	* There are 10 statements, not 7 statements.
	* There are 4 missed statements/lines, not 3 statements/lines.
	* There are 4 total branches (I agree on that part at least).
	* There is 1 partial branch taken (err… this seems odd — which branch is that?).
	* The overall statement/line coverage is assumed to be 45% (how is that calculated?).
	* The missing lines are 2->4 (a jump statement), and 4-7.

	Could someone please explain why these expected values are as noted above?
Thank you,
-Enji

1. https://www.cs.odu.edu/~cs252/Book/stmtcov.html
2. https://www.cs.odu.edu/~cs252/Book/branchcov.html
3. https://www.froglogic.com/coco/statement-coverage/