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Testing & modern typing

pytest basics, mocks, type hints, TypedDict, Protocol, and static-checking patterns.

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How to Compare Files and Show a Diff in Python

Compare two text files and print a unified diff using Python's difflib module to highlight differences.

import difflib
from pathlib import Path

def compare_files(expected_path: str, actual_path: str) -> str:
    """Compare two text files and return a unified diff."""
    expected = Path(expected_path).read_text()
    actual = Path(actual_path).read_text()

    diff = difflib.unified_diff(
        expected.splitlines(ke…

11 0 Open

Testing & modern typing medium

How to Flag Unexpected Diff Changes in Python

Compares two snapshot lists, detects unexpected differences, and returns a flag indicating whether the snapshot should be updated.

diffing snapshot-testing difflib

Python

import difflib

def snapshot_diff(before, after, intentional_changes=None):
    """Compare snapshots and flag only unexpected differences."""
    intentional_changes = intentional_changes or set()
    diff = list(difflib.unified_diff(before, after, lineterm=""))
    has_unexpected = False

    for line in diff:
      …

16 0 Open

Testing & modern typing easy

How to use unittest mock side_effect with a sequence in Python

Demonstrates using Mock.side_effect with a list to return different values per call and raise an exception at a specific call in unittest.

unittest mock side_effect

Python

import unittest
from unittest.mock import Mock

class TestMockSideEffectSequence(unittest.TestCase):
    def test_side_effect_sequence(self):
        mock = Mock()
        mock.side_effect = [1, 2, 3, Exception("boom")]
        
        self.assertEqual(mock(), 1)
        self.assertEqual(mock(), 2)
        self.asser…

13 0 Open

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