How to test code without a real database

During unit testing, connecting to a real DB is unnecessary:
• tests run slowly
• become unstable
• require a working server

It is much better to mock the call to pandas.read_sql and return dummy data

Example function:

def query_user_data(user_id):
    query = f"SELECT id, name FROM users WHERE id = {user_id}"
    return pd.read_sql(query, "postgresql://localhost/mydb")

Test with mock:

from unittest.mock import patch
import pandas as pd

@patch("pandas.read_sql")
def test_database_query_mocked(mock_read_sql):
    mock_read_sql.return_value = pd.DataFrame(
        {"id": [123], "name": ["Alice"]}
    )

    result = query_user_data(user_id=123)
    assert result["name"].iloc[0] == "Alice"

This way you test only the business logic — quickly, reliably, and without unnecessary dependencies

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