ExUnit
ExUnit
v1.13.0

View Source ExUnit.CaptureLog (ExUnit v1.13.0)

Functionality to capture logs for testing.

Examples 

defmodule AssertionTest do
  use ExUnit.Case

  import ExUnit.CaptureLog
  require Logger

  test "example" do
    {result, log} =
      with_log(fn ->
        Logger.error("log msg")
        2 + 2
      end)

    assert result == 4
    assert log =~ "log msg"
  end

  test "check multiple captures concurrently" do
    fun = fn ->
      for msg <- ["hello", "hi"] do
        assert capture_log(fn -> Logger.error(msg) end) =~ msg
      end

      Logger.debug("testing")
    end

    assert capture_log(fun) =~ "hello"
    assert capture_log(fun) =~ "testing"
  end
end

Link to this section Summary

Functions

capture_log(opts \\ [], fun)

Captures Logger messages generated when evaluating fun.

with_log(opts \\ [], fun)

Invokes the given fun and returns the result and captured log.

Link to this section Functions

Link to this function

capture_log(opts \\ [], fun)

View Source 
@spec capture_log(
  keyword(),
  (() -> any())
) :: String.t()

Captures Logger messages generated when evaluating fun.

Returns the binary which is the captured output.

This function mutes the :console backend and captures any log messages sent to Logger from the calling processes. It is possible to ensure explicit log messages from other processes are captured by waiting for their exit or monitor signal.

Note that when the async is set to true, the messages from another test might be captured. This is OK as long you consider such cases in your assertions.

It is possible to configure the level to capture with :level, which will set the capturing level for the duration of the capture, for instance, if the log level is set to :error any message with the lower level will be ignored. The default level is nil, which will capture all messages. The behaviour is undetermined if async tests change Logger level.

The format, metadata and colors can be configured with :format, :metadata and :colors respectively. These three options defaults to the :console backend configuration parameters.

To get the result of the evaluation along with the captured log, use with_log/2.

Link to this function

with_log(opts \\ [], fun)

View Source (since 1.13.0) 
@spec with_log(
  keyword(),
  (() -> any())
) :: {any(), String.t()}

Invokes the given fun and returns the result and captured log.

It accepts the same arguments and options as capture_log/2.

Examples 

{result, log} =
  with_log(fn ->
    Logger.error("log msg")
    2 + 2
  end)

assert result == 4
assert log =~ "log msg"