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Impersonator is a Ruby library to record and replay object interactions.

When testing, you often find services that are expensive to invoke, and you need to use a double instead. Creating stubs and mocks for simple scenarios is easy, but, for complex interactions, things get messy fast. Stubbing elaborated canned response and orchestrating multiple expectations quickly degenerates in brittle tests that are hard to write and maintain.

Impersonator comes to the rescue. Given an object and a list of methods to impersonate:

  • The first time each method is invoked, it will record its invocations, including passed arguments, return values, and yielded values. This is known as record mode.
  • The next times, it will reproduce the recorded values and will validate that the method was invoked with the same arguments, in a specific order and the exact number of times. This is known as replay mode.

Impersonator only focuses on validating invocation signature and reproducing output values, which is perfect for many services. It won't work for services that trigger additional logic that is relevant to the test (e.g., if the method sends an email, the impersonated method won't send it).

Familiar with VCR? Impersonator is like VCR but for ruby objects instead of HTTP.


Add this line to your application's Gemfile:

gem 'impersonator', group: :test

And then execute:

$ bundle


Use Impersonator.impersonate passing in a list of methods to impersonate and a block that will instantiate the object at record time:

calculator = Impersonator.impersonate(:add, :divide) { }
  • At record time, Calculator will be instantiated and their methods normally invoked, recording the returned values (and yielded values if any).
  • At replay time, Calculator won't be instantiated. Instead, a double object will be generated on the fly that will replay the recorded values.
class Calculator
  def add(number_1, number_2)
    number_1 + number_2

# The first time it records...
Impersonator.recording('calculator add') do
  calculator = Impersonator.impersonate(:add) { }
  puts calculator.add(2, 3) # 5

# The next time it replays
Object.send :remove_const, :Calculator # Calculator does not even have to exist now
Impersonator.recording('calculator add') do
  calculator = Impersonator.impersonate(:add) { }
  puts calculator.add(2, 3) # 5

Typically you will use impersonate for testing, so this is how your test will look:

# The second time the test runs, impersonator will replay the
# recorded results
test 'sums the numbers' do
  Impersonator.recording('calculator add') do
    calculator = Impersonator.impersonate(:add){ }
    assert_equal 5, calculator.add(2, 3)

Impersonated methods will record and replay:

  • Arguments
  • Return values
  • Yielded values

Impersonate certain methods only

Use Impersonator#impersonate_methods to impersonate certain methods only. At replay time, the impersonated object will delegate to the actual object all the methods except the impersonated ones.

actual_calculator =
impersonator = Impersonator.impersonate(actual_calculator, :add)

In this case, in replay mode, Calculator gets instantiated normally and any method other than #add will be delegated to actual_calculator.


Recordings path

Impersonator works by recording method invocations in YAML format. By default, recordings are saved in:

  • spec/recordings if a spec folder is present in the project
  • test/recordings otherwise

You can configure this path with:

Impersonator.configure do |config|
  config.recordings_path = 'my/own/recording/path'

Ignore arguments when matching methods

By default, to determine if a method invocation was right, the list of arguments will be matched with ==. You can configure how this work by providing a list of argument indexes to ignore.

impersonator = Impersonator.impersonate(:add){ }
impersonator.configure_method_matching_for(:add) do |config|
  config.ignore_arguments_at 0

# Now the first parameter of #add will be ignored.
# In record mode:
impersonator.add(1, 2) # 3

# In replay mode
impersonator.add(9999, 2) # will still return 3 and won't fail because the first argument is ignored

Disabling record mode

You can disable impersonator by passing disable: true to Impersonator.recording:

Impersonator.recording('test recording', disabled: true) do
  # ...

This will effectively force record mode at all times. This is handy while you are figuring out how interactions with the mocked service go. It will save the recordings, but it will never use them.

Configuring attributes to serialize

Impersonator relies on Ruby standard YAML library for serializing/deserializing data. It works with simple attributes, arrays, hashes and objects which attributes are serializable in a recurring way. This means that you don't have to care when interchanging value objects, which is a common scenario when impersonating RPC-like clients.

However, there are some types, like Proc, anonymous classes, or IO classes like File, that will make the serialization process fail. You can customize which attributes are serialized by overriding init_with and encode_with in the class you want to serialize. You will typically exclude the problematic attributes by including only the compatible ones.

class MyClass
  # ...
  def init_with(coder) = coder['name']

  def encode_with(coder)
    coder['name'] = name

RSpec configuration

Impersonator is test-framework agnostic. If you are using RSpec, you can configure an around hook that will start a recording session automatically for each example that has an impersonator tag:

RSpec.configure do |config|
  config.around(:example, :impersonator) do |example|
    Impersonator.recording(example.full_description) do

Now you can just tag your tests with impersonator and an implicit recording named after the example will be available automatically, so you don't have to invoke Impersonator.recording anymore.

describe Calculator, :impersonator do
  it 'sums numbers' do
    # there is an implicit recording stored in 'calculator-sums-numbers.yaml'
    impersonator = Impersonator.impersonate(:add){ }
    expect(impersonator.add(1, 2)).to eq(3)


  • This library was heavily inspired by VCR. A gem that blew my mind years ago and that has been in my toolbox since then.



Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.`


Ruby library to record and replay object interactions





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