Accumulators Global Counter Mock in Python

Shows an accumulator-style global counter with a mock patch to control its value in tests.

Medium Python 3.9+ Aug 9, 2026 Big data & Spark 14 views 0 copies

accumulator global state mock unittest

Python code

32 lines

Python 3.9+

import unittest
from unittest.mock import patch

# Module-level global counter accumulator
counter = 0

def increment(by=1):
    """Increment the global counter in place (accumulator pattern)."""
    global counter
    counter += by
    return counter

def reset():
    """Reset the counter to zero."""
    global counter
    counter = 0

# Example usage and test with mock
class TestCounter(unittest.TestCase):
    def test_increment(self):
        reset()
        self.assertEqual(increment(), 1)
        self.assertEqual(increment(5), 6)

    def test_mock_counter(self):
        reset()
        with patch("__main__.counter", new=100):
            self.assertEqual(increment(), 101)
        self.assertEqual(counter, 0)  # unchanged outside mock

if __name__ == "__main__":
    unittest.main(verbosity=2)

Output

stdout

test_increment (__main__.TestCounter) ... ok
test_mock_counter (__main__.TestCounter) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK

How it works

The global counter follows the accumulator pattern, updating in place with each increment call. unittest.mock.patch temporarily replaces the global variable with a controlled value, letting you test behavior under specific counter states. The with block ensures the patch is applied only inside it, leaving the real counter untouched afterward. This isolates tests from shared global state, which is crucial when multiple tests mutate the same global.

Common mistakes

Forgetting `global counter` inside the function, causing UnboundLocalError.
Not resetting the counter before tests, leading to order-dependent failures.
Patching the wrong name when the function is imported from another module.

Variations

Use a class-level accumulator with `@classmethod` instead of a global variable.
Use `unittest.mock.patch.object` to patch a counter attribute on a class.

Real-world use cases

Testing a Spark accumulator's value in unit tests by patching the global reference.
Simulating a processed-record count that increments across distributed tasks for validation.
Mocking a global error counter to verify logging behavior under injected failures.

Accumulators Global Counter Mock in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

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How it works

Common mistakes

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Real-world use cases

More from Big data & Spark

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