How Do I Print Debugging Information in Go Tests?

Logging in Go Tests Using the "testing" Package

When writing Go tests, there may be instances where you want to print information for debugging purposes. However, simply calling fmt.Println within a test function does not produce visible output.

By default, output from fmt.Println statements in tests is suppressed. To display such output, you must use the .Log or .Logf methods provided by the testing.T structure.

Using .Log and .Logf

Similar to fmt.Print and fmt.Printf, .Log and .Logf allow you to print formatted output. For instance, instead of using:

func TestPrintSomething(t *testing.T) {
    fmt.Println("Say hi")
}

You should use:

func TestPrintSomething(t *testing.T) {
    t.Log("Say hi")
}

Viewing Output with Verbosity

Even when using .Log, output may not be visible unless you provide the -v (verbosity) flag when running go test. This flag can be specified using the following syntax:

go test -v your_test_file.go

By adding the -v flag, the output from .Log statements will be displayed on the console.

Why Print Statements Are Hidden

fmt.X print statements are indeed functional within tests. However, their output is typically not displayed in the expected location because test results are processed by the Go testing infrastructure before being printed. The .Log methods, on the other hand, are specifically designed to display formatted output within the testing environment and can be controlled using the verbosity flag.

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