Branch coverage measurement

In addition to the usual statement coverage, coverage.py also supports branch coverage measurement. Where a line in your program could jump to more than one next line, coverage.py tracks which of those destinations are actually visited, and flags lines that haven’t visited all of their possible destinations.

For example:

def my_partial_fn(x):
    if x:
        y = 10
    return y

my_partial_fn(1)

In this code, line 2 is an if statement which can go next to either line 3 or line 4. Statement coverage would show all lines of the function as executed. But the if was never evaluated as false, so line 2 never jumps to line 4.

Branch coverage will flag this code as not fully covered because of the missing jump from line 2 to line 4. This is known as a partial branch.

How to measure branch coverage

To measure branch coverage, run coverage.py with the --branch flag:

coverage run --branch myprog.py

When you report on the results with coverage report or coverage html, the percentage of branch possibilities taken will be included in the percentage covered total for each file. The coverage percentage for a file is the actual executions divided by the execution opportunities. Each line in the file is an execution opportunity, as is each branch destination.

The HTML report gives information about which lines had missing branches. Lines that were missing some branches are shown in yellow, with an annotation at the far right showing branch destination line numbers that were not exercised.

The XML and JSON reports produced by coverage xml and coverage json also include branch information, including separate statement and branch coverage percentages.

How it works

When measuring branches, coverage.py collects pairs of line numbers, a source and destination for each transition from one line to another. Static analysis of the source provides a list of possible transitions. Comparing the measured to the possible indicates missing branches.

The idea of tracking how lines follow each other was from Titus Brown. Thanks, Titus!

Excluding code

If you have excluded code, a conditional will not be counted as a branch if one of its choices is excluded:

def only_one_choice(x):
    if x:
        blah1()
        blah2()
    else:  # pragma: no cover
        # x is always true.
        blah3()

Because the else clause is excluded, the if only has one possible next line, so it isn’t considered a branch at all.

Structurally partial branches

Sometimes branching constructs are used in unusual ways that don’t actually branch. For example:

while True:
    if cond:
        break
    do_something()

Here the while loop will never exit normally, so it doesn’t take both of its “possible” branches. For some of these constructs, such as “while True:” and “if 0:”, coverage.py understands what is going on. In these cases, the line will not be marked as a partial branch.

But there are many ways in your own code to write intentionally partial branches, and you don’t want coverage.py pestering you about them. You can tell coverage.py that you don’t want them flagged by marking them with a pragma:

i = 0
while i < 999999999:  # pragma: no branch
    if eventually():
        break

Here the while loop will never complete because the break will always be taken at some point. Coverage.py can’t work that out on its own, but the “no branch” pragma indicates that the branch is known to be partial, and the line is not flagged.