I am using coverage.py tool to get coverage of python code. If I use the command without --branch flag like below,
coverage run test_cmd
I get the coverage report like this,
Name Stmts Miss Cover
--------------------------------------------------------------------------------------------------------------------------
/path/file.py 9 2 78%
From this I understand that the cover percentage value is derived as this
cover = (Stmts Covered/total stmts)*100 = (9-2/9)*100 = 77.77%
But when I run coverage with --branch flag enabled like this
coverage run --branch test_cmd
I get the coverage report like this,
Name Stmts Miss Branch BrPart Cover
----------------------------------------------------------------------------------------------------------------------------------------
/path/file.py 9 2 2 1 73%
From this report I am not able to understand the formula used to get Cover=73%
How is this number coming and is this correct value for code coverage?
While you probably shouldn't worry too much about the exact numbers, here is how they are calculated:
Reading from the output you gave, we have:
n_statements = 9
n_missing = 2
n_branches = 2
n_missing_branches = 1
Then it is calculated (I simplified the syntax a bit, in the real code everything is hidden behind properties.)
n_executed = n_statements - n_missing
n_executed_branches = n_branches - n_missing_branches
numerator = n_executed + n_executed_branches
denominator = n_statements + n_branches
https://github.com/nedbat/coveragepy/blob/master/coverage/results.py#L215-L222
pc_covered = (100.0 * numerator) / denominator
Now put all of this in a script, add print(pc_covered) and run it:
72.72727272727273
It is then rounded of course, so there you have your 73%.