ExUnit

v1.8.1

ExUnit.CaptureLog View Source

Functionality to capture logs for testing.

Examples 

defmodule AssertionTest do
  use ExUnit.Case

  import ExUnit.CaptureLog
  require Logger

  test "example" do
    assert capture_log(fn ->
      Logger.error "log msg"
    end) =~ "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

Link to this section Functions

Link to this function

capture_log(opts \\ [], fun) View Source 
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.

However, capture_log does not guarantee to capture log messages originated from processes spawned using a low level Kernel spawn function (e.g. Kernel.spawn/1) and such processes exit with an exception or a throw. Therefore, prefer using a Task, or other OTP process, will send explicit logs before its exit or monitor signals and will not cause VM generated log messages.

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.