Code coverage

Measuring what the tests execute: statement, branch, condition and path coverage, reading a coverage rate, and why 100% does not prove the code is right.

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Code coverage: what is it?
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The share of the source code that running the tests actually went through, measured by a tool and given as a percentage.
It tells what the tests execute, never what they check: code that runs without any assertion still counts as covered.
The only test calls remise(120, True). What coverage does it give?

```python
def remise(montant, membre):
if montant > 100:
montant = montant - 10
if membre:
montant = montant * 0.9
return montant
`
- 100% of statements, but 2 branches out of 4
- 100% of statements and 100% of branches
- 2 statements out of 5 and 2 branches out of 4
- 100% of statements, but 1 branch out of 4
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100% of statements, but 2 branches out of 4
The call goes through both if on the true side, so every statement runs. Both false sides (montant <= 100 and membre false) are never taken: 2 branches out of 4.
An if without else has … branches: one where the condition is true, and one where it is false and execution goes straight past the block.
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An if without else has two branches: one where the condition is true, and one where it is false and execution goes straight past the block.
The false branch exists even with no code written for it: it is the one a test forgets most often.
Statement coverage: what is it?
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The proportion of the program's statements run at least once by the whole set of tests.
It is the weakest criterion: an if without else is fully executed when its condition is true, even if the false case was never tried.

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