https://github.com/arxeiss/go-false-positive-coverage

How Code coverage in Go can give false positive results
https://github.com/arxeiss/go-false-positive-coverage

coverage golang measure

Last synced: 28 days ago
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How Code coverage in Go can give false positive results

• Host: GitHub
• URL: https://github.com/arxeiss/go-false-positive-coverage
• Owner: arxeiss
• Created: 2021-02-28T14:08:47.000Z (over 3 years ago)
• Default Branch: master
• Last Pushed: 2021-03-09T10:12:05.000Z (over 3 years ago)
• Last Synced: 2024-01-30T02:04:35.408Z (10 months ago)
• Topics: coverage, golang, measure
• Language: Go
• Homepage: https://dev.to/arxeiss/false-positive-go-code-coverage-3k7j
• Size: 3.91 KB
• Stars: 0
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Go code coverage and False-Positive percentage result

The basic `go test` with cooperation with `go tool cover` can measure and show overall Code coverage of your code.

However the result can be false positive and show **much higher percentage** of covered code that actually is.

## ✍️ Readme more

Read more why and how the code coverage is measured. I published the article in English on [Dev.to](https://dev.to/arxeiss/false-positive-go-code-coverage-3k7j) or in Czech on [kutac.cz](https://www.kutac.cz/pocitace-a-internety/falesne-pozitivni-code-coverage-v-go).

## Try it yourself

Clone this repo and try all 3 cases:

### Step 1 - tests only in `binary` package give 100%

```bash

go test -covermode=count -coverprofile=coverage.out ./...

# ?   	github.com/arxeiss/go-false-positive-coverage	[no test files]

# ok  	github.com/arxeiss/go-false-positive-coverage/binary	0.002s	coverage: 100.0% of statements

# ?   	github.com/arxeiss/go-false-positive-coverage/unary	[no test files]

go tool cover

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:3:	Add		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:7:	Sub		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:11:	Mul		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:15:	Div		100.0%

# total:									(statements)	100.0%

```

### Step 2 - With more tests the overall coverage goes down

```bash

# First "activate" tests in unary package by renaming .bak file

mv unary/operations_test.go.bak unary/operations_test.go

# Now run the tests

go test -covermode=count -coverprofile=coverage.out ./...

# ?   	github.com/arxeiss/go-false-positive-coverage	[no test files]

# ok  	github.com/arxeiss/go-false-positive-coverage/binary	0.002s	coverage: 100.0% of statements

# ok  	github.com/arxeiss/go-false-positive-coverage/unary	0.002s	coverage: 33.3% of statements

go tool cover

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:3:	Add		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:7:	Sub		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:11:	Mul		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:15:	Div		100.0%

# github.com/arxeiss/go-false-positive-coverage/unary/operations.go:5:	Abs		100.0%

# github.com/arxeiss/go-false-positive-coverage/unary/operations.go:9:	Sin		0.0%

# github.com/arxeiss/go-false-positive-coverage/unary/operations.go:13:	Cos		0.0%

# total:									(statements)	71.4%

```

### Step 3 - Measure "real" coverage with -coverpkg flag

```bash

# Now run the tests

go test -covermode=count -coverprofile=coverage.out -coverpkg=github.com/arxeiss/go-false-positive-coverage/... ./...

# ?   	github.com/arxeiss/go-false-positive-coverage	[no test files]

# ok  	github.com/arxeiss/go-false-positive-coverage/binary	0.002s	coverage: 44.4% of statements in github.com/arxeiss/go-false-positive-coverage/...

# ok  	github.com/arxeiss/go-false-positive-coverage/unary	0.002s	coverage: 11.1% of statements in github.com/arxeiss/go-false-positive-coverage/...

go tool cover

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:3:	Add		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:7:	Sub		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:11:	Mul		100.0%

# github.com/arxeiss/go-false-positive-coverage/binary/operations.go:15:	Div		100.0%

# github.com/arxeiss/go-false-positive-coverage/main.go:10:		main		0.0%

# github.com/arxeiss/go-false-positive-coverage/unary/operations.go:5:	Abs		100.0%

# github.com/arxeiss/go-false-positive-coverage/unary/operations.go:9:	Sin		0.0%

# github.com/arxeiss/go-false-positive-coverage/unary/operations.go:13:	Cos		0.0%

# total:									(statements)	55.6%

```