List comprehension branch coverage relevance

[nedbat]

Originally reported by Steve Peak (Bitbucket: stevepeak, GitHub: stevepeak)

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Issue

Coverage for list comprehension is returned as a branch with seemingly irrelevant information.

Proof

Below is source/test/result to show the issue.

def never():
    return [_ for _ in []]

def full():
    a = [1]
    return [_ for _ in a]

def empty():
    a = []
    return [_ for _ in a]

empty()
full()

Python 3.6.1
Coverage.py, version 4.3.4 with C extension

$ coverage run --branch test.py && coverage xml && cat coverage.xml

<?xml version="1.0" ?>
<coverage branch-rate="0.6667" line-rate="0.9" timestamp="1493301584473" version="4.3.4">
	<!-- Generated by coverage.py: https://coverage.readthedocs.io -->
	<!-- Based on https://raw.githubusercontent.com/cobertura/web/f0366e5e2cf18f111cbd61fc34ef720a6584ba02/htdocs/xml/coverage-03.dtd -->
	<sources>
		<source>/home/ryan/Code/aspiredu/codecov-test</source>
	</sources>
	<packages>
		<package branch-rate="0.6667" complexity="0" line-rate="0.9" name=".">
			<classes>
				<class branch-rate="0.6667" complexity="0" filename="test.py" line-rate="0.9" name="test.py">
					<methods/>
					<lines>
						<line hits="1" number="1"/>
						<line branch="true" condition-coverage="0% (0/2)" hits="0" missing-branches="exit,exit" number="2"/>
						<line hits="1" number="4"/>
						<line hits="1" number="5"/>
						<line branch="true" condition-coverage="100% (2/2)" hits="1" number="6"/>
						<line hits="1" number="8"/>
						<line hits="1" number="9"/>
						<line branch="true" condition-coverage="100% (2/2)" hits="1" number="10"/>
						<line hits="1" number="12"/>
						<line hits="1" number="13"/>
					</lines>
				</class>
			</classes>
		</package>
	</packages>
</coverage>

Suggested fix options

(A) Remove branch coverage for lists

- <line branch="true" condition-coverage="100% (2/2)" hits="1" number="10"/>
+ <line branch="false" hits="1" number="10"/>

(B) Provide partial branch coverage when iterable was empty

- <line branch="true" condition-coverage="100% (2/2)" hits="1" number="10"/>
+ <line branch="true" condition-coverage="50% (1/2)" hits="1" number="10"/>

Credit @ryanhiebert with discovery

---

• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/574

[nedbat]

I thought the issue was the "missing-branches" attribute on the "never" case. But neither of your suggested fixes is about that.

[nedbat]

Original comment by Steve Peak (Bitbucket: stevepeak, GitHub: stevepeak)

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Oh, yes you are right. The condition-coverage and missing-branches need attention.

missing-branches="exit,exit" should be (for example) missing-branches="enter,exit"

Thanks

[nedbat]

Original comment by Jim G (Bitbucket: mrsynock, GitHub: mrsynock)

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I think I have the same bug to

#!python

    def liste(cls) -> Callable:
        def liste(patient_id: int) -> List:
            return [
                acte.dico 
                for acte in select(a for a in cls.model if a.patient.id == patient_id)
            ]
        return liste

is not covered but:

#!python

    def liste(cls) -> Callable:
        def liste(patient_id: int) -> List:
            return [
                acte.dico  for acte in select(a for a in cls.model if a.patient.id == patient_id)
            ]
        return liste

is covered