CaptureLogger (capture_logger v1.0.0)

Functionality to capture logs for testing.

Examples

defmodule AssertionTest do
  use ExUnit.Case

  import CaptureLogger
  require Logger

  test "example" do
    {result, log} =
      with_log([formatter: LoggerJSON.Basic.new())], fn ->
        Logger.error("log msg")
        2 + 2
      end)

    assert result == 4
    assert Jason.decode!(log)["message"] == "log msg"
  end

  test "check multiple captures concurrently" do
    fun = fn ->
      for msg <- ["hello", "hi"] do
        log = assert capture_log(
          [formatter: LoggerJSON.Basic.new()],
          fn -> Logger.error(msg) end
        )
        assert Jason.decode!(log) message == msg
      end

      Logger.debug("testing")
    end

    assert capture_log(fun) =~ "hello"
    assert capture_log(fun) =~ "\"message\":\"testing\""
  end
end

Summary

Functions

capture_log(opts \\ [], fun)

Captures Logger messages generated when evaluating fun.

start()

Starts the CaptureLogger server under a supervisor. This should be called with ExUnit.start/1.

with_log(opts \\ [], fun)

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

Types

capture_log_opts()

@type capture_log_opts() :: [
  {:level, Logger.level() | nil}
  | {:formatter, {module(), term()} | module() | nil}
  | {atom(), term() | nil}
]

Functions

capture_log(opts \\ [], fun)

@spec capture_log(capture_log_opts(), (-> any())) :: String.t()

Captures Logger messages generated when evaluating fun.

Returns the binary which is the captured output.

This function mutes the default logger handler 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 on use ExUnit.Case, messages from other tests might be captured. This is OK as long you consider such cases in your assertions, typically by using the =~/2 operator to perform partial matches.

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

Options

:level: 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, then any message with a lower level will be ignored. The default level is nil, which will capture all messages.
Note this setting does not override the overall Logger.level/0 value. Therefore, if Logger.level/0 is set to a higher level than the one configured in this function, no message will be captured. The behaviour is undetermined if async tests change Logger level.
:formatter: The formatter to use for formatting log messages. May be provided either as module/0 or as {module, options}. If provided as a module, it must respond to new/1 and the remaining options will be provided as configuration to the formatter module.
If not provided, defaults to Logger.default_formatter/1 with any other options forwarded as a configuration to the default formatter.
The default formatter may also be set with the :formatter configuration key for :capture_logger. When set in a compile-time configuration file (config/test.exs), it should be provided as a {module, options} tuple. When set in config/runtime.exs or test/test_helper.exs, it may be possible to call module.new(options).

start()

Starts the CaptureLogger server under a supervisor. This should be called with ExUnit.start/1.

with_log(opts \\ [], fun)

@spec with_log(capture_log_opts(), (-> result)) :: {result, log :: String.t()}
when result: any()

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"