Pretend: A Python Stubbing Library

Common errors

• AttributeError: 'Stub' object has no attribute 'some_method'

  cause You attempted to access a method or attribute on a `pretend.stub` object that was not explicitly defined when the stub was created.
  fix When initializing the stub, ensure all expected methods and attributes are provided. Example: `my_stub = stub(method_name=lambda: 'value', attribute_name='data')`.

• TypeError: <lambda>() missing 1 required positional argument: 'self'

  cause A function defined as a method on a `pretend.stub` was written to expect a `self` argument, but `pretend` does not bind stub functions to the instance in this way.
  fix Remove the `self` argument from functions assigned to stub methods. Example: `my_stub = stub(do_something=lambda: 'result')` instead of `my_stub = stub(do_something=lambda self: 'result')`.

• ValueError: API error (or similar exception message)

  cause You used `pretend.raiser` to make a stub method raise an exception, but the calling code did not include a `try...except` block to catch this expected exception.
  fix Wrap the call to the stub method (that uses `raiser`) in a `try...except` block to handle the expected exception in your test.

• ModuleNotFoundError: No module named 'pretend'

  cause The 'pretend' library is not installed in your current Python environment.
  fix Install the library using pip: `pip install pretend`.

Warnings

• gotcha Functions defined for `pretend.stub` objects do not automatically receive a `self` argument. They should be defined as regular functions or lambdas that do not expect a first positional argument for the instance.
  Fix: Define stubbed functions as `lambda *args, **kwargs: ...` or regular functions that explicitly don't take `self`. For example, `stub(method=lambda: 'result')` instead of `stub(method=lambda self: 'result')`.
• gotcha `pretend` is a stubbing library and explicitly does not include a `patch` method for monkey-patching. It promotes passing stubs as arguments or relying on your testing framework's patching utilities (e.g., `pytest` fixtures, `unittest.mock.patch`).
  Fix: Avoid searching for a `pretend.patch` function. Instead, refactor your code to accept dependencies as arguments, or integrate `pretend` stubs with your test runner's mocking/patching capabilities.
• gotcha Stubs created with `pretend.stub` are strict; they only respond to attributes and methods explicitly defined during their creation. Attempting to access an undefined attribute will result in an `AttributeError`, unlike some mocking libraries that might dynamically create attributes.
  Fix: Ensure all required attributes and methods are explicitly defined when creating your `pretend.stub`. If you need to record calls or assert interactions, use `pretend.call_recorder` for specific methods.

Install

• pip install pretend Install with pip

Imports

• stub

  from pretend import stub

  Used to create a basic stub object with defined attributes and methods.
• raiser

  from pretend import raiser

  A helper function to create a callable that raises a specified exception.
• call_recorder, call

  from pretend import call_recorder, call

  Used for recording calls made to a stubbed function, if behavior verification is needed.

Quickstart

Demonstrates how to create basic stubs with attributes, methods, and methods that raise exceptions using `pretend.stub` and `pretend.raiser`. Note that functions provided to stubs should not expect a `self` argument.

from pretend import stub, raiser

# Create a stub with a simple attribute
user_stub = stub(country_code="US")
print(f"User country code: {user_stub.country_code}")

# Create a stub with a method (note: functions on stubs don't take 'self')
data_service = stub(fetch_data=lambda: {'id': 1, 'value': 'test'})
print(f"Fetched data: {data_service.fetch_data()}")

# Create a stub method that raises an exception
error_prone_api = stub(fail_action=raiser(ValueError('API error')))
try:
    error_prone_api.fail_action()
except ValueError as e:
    print(f"Caught expected error: {e}")