Mocking with unittest.mock — Pytest for Python Testing | Sabaoon Academy

Mocking replaces a real dependency (a database call, an HTTP request, a file read) with a controlled fake. Your unit test then runs in isolation — fast, deterministic, no network required.

The Problem Without Mocking

def get_user_name(user_id: int) -> str:
    row = database.query(f"SELECT name FROM users WHERE id={user_id}")
    return row["name"]

def test_get_user_name():
    # ❌ requires a running database with test data
    assert get_user_name(1) == "Alice"

unittest.mock.patch

from unittest.mock import patch, MagicMock

def test_get_user_name():
    with patch("myapp.database.query") as mock_query:
        mock_query.return_value = {"name": "Alice"}
        result = get_user_name(1)

    assert result == "Alice"
    mock_query.assert_called_once_with("SELECT name FROM users WHERE id=1")

patch replaces the target for the duration of the with block and restores the original afterwards.

pytest-mock (Cleaner Syntax)

pip install pytest-mock

def test_get_user_name(mocker):
    mock_query = mocker.patch("myapp.database.query")
    mock_query.return_value = {"name": "Alice"}

    assert get_user_name(1) == "Alice"
    mock_query.assert_called_once()

The mocker fixture handles cleanup automatically — no with block needed.

Common Mock Assertions

mock.assert_called()                         # called at least once
mock.assert_called_once()                    # called exactly once
mock.assert_called_with(arg1, key=val)       # called with specific args
mock.assert_not_called()                     # never called

# Inspect call history
print(mock.call_count)                       # number of calls
print(mock.call_args)                        # args of last call
print(mock.call_args_list)                   # args of all calls

Mocking HTTP Requests

import requests

def fetch_quote():
    resp = requests.get("https://api.quotable.io/random")
    return resp.json()["content"]

def test_fetch_quote(mocker):
    mock_get = mocker.patch("requests.get")
    mock_get.return_value.json.return_value = {"content": "Test quote"}

    result = fetch_quote()
    assert result == "Test quote"
    mock_get.assert_called_once_with("https://api.quotable.io/random")

Mock vs Real Dependency

Mock isolation visualiser

Ctrl+Enter

HTML

CSS

* { box-sizing: border-box; }
body { font-family: sans-serif; padding: 1.25rem; background: #f9fafb; }
h2 { font-size: 1rem; margin-bottom: 0.875rem; }
.modes { display: flex; gap: 0.375rem; margin-bottom: 0.875rem; }
.modeBtn { padding: 0.35rem 0.875rem; border: 1px solid #d1d5db; background: white; cursor: pointer; font-size: 0.8rem; }
.modeBtn.active { background: #1e293b; color: white; border-color: #1e293b; }
.diagram { display: flex; align-items: center; gap: 0.5rem; margin-bottom: 0.875rem; flex-wrap: wrap; }
.box {
padding: 0.5rem 0.875rem; border: 1px solid #e5e7eb; background: white;
font-size: 0.78rem; text-align: center; min-width: 90px;
}
.box.highlight { border-color: #2563eb; background: #eff6ff; }
.box.mock-box  { border-color: #16a34a; background: #f0fdf4; }
.arrow { color: #9ca3af; font-size: 1.2rem; }
button#runBtn { padding: 0.4rem 1rem; background: #1e293b; color: white; border: none; cursor: pointer; font-size: 0.85rem; }
.result { padding: 0.5rem 0.875rem; font-family: monospace; font-size: 0.78rem; margin-top: 0.625rem; }
.result.pass { background: #f0fdf4; border: 1px solid #bbf7d0; color: #166534; }
.result.fail { background: #fef2f2; border: 1px solid #fca5a5; color: #dc2626; }

var mode = 'real';

document.getElementById('realMode').onclick = function() {
mode = 'real';
this.classList.add('active');
document.getElementById('mockMode').classList.remove('active');
document.getElementById('result').style.display = 'none';
renderDiagram();
};

document.getElementById('mockMode').onclick = function() {
mode = 'mock';
this.classList.add('active');
document.getElementById('realMode').classList.remove('active');
document.getElementById('result').style.display = 'none';
renderDiagram();
};

renderDiagram();

Preview

MagicMock for Complex Objects

mock_response = MagicMock()
mock_response.status_code = 200
mock_response.json.return_value = {"token": "abc123"}

# Supports context manager protocol
with patch("requests.post", return_value=mock_response) as mock_post:
    token = login("alice", "pass")
    assert token == "abc123"

MagicMock supports magic methods (__enter__, __exit__, __len__, etc.) automatically.