flexmock

Common errors

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

  cause You are trying to call a method or access an attribute on a mocked object that was not explicitly defined or allowed by the mock. This means the mock did not correctly intercept the call or the method wasn't mocked.
  fix Ensure the `should_receive()` chain correctly targets the method you intend to mock, including its name and any arguments if necessary. For attributes, use `and_return()` directly or `set()`.

• AssertionError: expected 'method_name' to be called once but was called 0 times

  cause This error occurs during mock verification (`should_have_received`) when the mocked method was never actually called by the code under test, or was called on a different object than the one mocked.
  fix Double-check that your code under test indeed calls the method you mocked, and that it operates on the exact object you set up the mock for. Pay attention to class vs. instance mocks.

• My tests are flaky or failing intermittently when run in a suite, but pass individually.

  cause This is a classic sign of mocks not being properly reset between test runs, leading to state leakage and interference between tests.
  fix Ensure `flexmock_teardown()` is called after every test. If using `unittest`, call it in `tearDown()`. If using `pytest`, use the `flexmock` fixture in your tests (`def test_my_feature(flexmock): ...`) or create an `autouse` fixture to ensure `flexmock_teardown()` runs.

• TypeError: 'Mock' object is not subscriptable

  cause You are attempting to access a mocked object using dictionary-like subscription (e.g., `mock_obj['key']`) without explicitly mocking the `__getitem__` method.
  fix To mock dictionary-like access, explicitly mock the `__getitem__` method: `flexmock(obj).should_receive('__getitem__').with_args('key').and_return('value')`.

Warnings

• breaking flexmock v0.13.0 dropped support for Python 3.8. Earlier versions dropped 3.6 and 3.7. Ensure your Python environment meets the `flexmock` version requirements.
  Fix: Upgrade your Python interpreter to 3.9 or newer, or pin your flexmock version to `flexmock < 0.13.0` for Python 3.8, or `flexmock < 0.12.0` for Python 3.6/3.7.
• gotcha Mocks created with `flexmock` do not automatically clean up in all testing contexts. Failure to explicitly call `flexmock_teardown()` or use appropriate test runner integration can lead to mocks leaking between tests, causing flaky or incorrect test results.
  Fix: For `unittest` tests, call `flexmock_teardown()` in `tearDown()`. For `pytest`, either use the `flexmock` fixture which handles cleanup, or implement an `autouse` fixture calling `flexmock_teardown()`.
• gotcha The `flexmock.new_instances()` method is specifically for mocking *class instantiation* (i.e., when `__init__` is called), not for mocking methods on an already existing class or a new instance after creation. Misunderstanding this can lead to mocks not being applied as expected.
  Fix: Use `flexmock(ClassName).new_instances(instance1, instance2)` to mock objects returned by `ClassName()`. To mock methods on an existing object or a class directly, use `flexmock(obj).should_receive(...)` or `flexmock(Class).should_receive(...)`.
• deprecated As of v0.12.0, flexmock's pytest integration switched to use native pytest hooks and a direct `flexmock` fixture. While older `pytest-flexmock` plugins might still function, the recommended approach is to rely on the library's built-in support.
  Fix: Remove explicit `pytest-flexmock` dependencies if you were using an older integration approach. Ensure `flexmock` is installed, and pytest will discover its plugin. Use `flexmock` as a fixture in your tests: `def test_something(flexmock): ...`.

Install

• pip install flexmock Install flexmock

Imports

• flexmock

  from flexmock import flexmock

• flexmock_teardown
  wrong:

  from flexmock.mock import flexmock_teardown

  correct:

  from flexmock import flexmock_teardown

  flexmock_teardown is exposed directly at the top level of the flexmock package, not within a 'mock' submodule.

Quickstart

This quickstart demonstrates how to mock a class method using `flexmock` to control the return value of `datetime.date.today()`. It highlights the importance of `flexmock_teardown()` for proper cleanup after tests, preventing mock leakage.

import datetime
from flexmock import flexmock, flexmock_teardown

def get_today_str():
    return datetime.date.today().isoformat()

# --- Example Test (usually in a test file) ---
# The flexmock_teardown() is crucial for cleaning up mocks.
# In pytest, use the flexmock fixture. In unittest, call it in tearDown.
# For a standalone script, call it manually.

try:
    # Mock the 'today' method of the datetime.date class
    flexmock(datetime.date).should_receive('today').and_return(datetime.date(2023, 10, 26))

    # Call the function under test
    result = get_today_str()

    # Assert the result
    assert result == '2023-10-26'
    print(f"Test passed: Today's date is mocked to {result}")

finally:
    # Ensure mocks are cleaned up
    flexmock_teardown()