Testing Suite for Password Generator API

This document explains how to run and extend an automated test suite for the Password Generator API built with FastAPI.

The tutorial for the Password Generator API built with FastAPI can be found here.

Prerequisites

This article assumes you have already created the Password Generator API application found here.

Make sure you have the following installed in your development environment:

• Python 3.7+
• pytest library
• httpx library (used internally by FastAPI TestClient)
• Your FastAPI project dependencies installed

Install pytest and httpx (if not already):

pip install pytest httpx

Test Suite Overview

• The test suite is located in test_password_api.py
  NOTE: If this is your first time viewing this article, the file doess not yet exist. The file should be created based on the information outlined in this document.
• It uses FastAPI’s TestClient for simulating HTTP requests without starting the server.
• Tests cover both:
• GET /generate-password endpoint with query parameters via test_get_generate_password
• POST /generate-password endpoint with JSON payload via test_post_generate_password

How to Run Tests

Run all tests with:

pytest test_password_api.py

Recommended:

Use additional flags for better output:

pytest -s -v test_password_api.py

• -s: Shows print() output during tests (useful for debugging).
• -v: Verbose mode; lists each test and its result.

What the Tests Do

test_get_generate_password function

Sends requests with different parameters to the GET /generate-password endpoint and validates the results.

import pytest
from fastapi.testclient import TestClient
from main import app  # Your FastAPI application

client = TestClient(app)

@pytest.mark.parametrize("length, use_uppercase, use_numbers, use_special_chars", [
    (12, True, True, True),
    (4, False, False, False),  # minimal length, only lowercase
    (128, True, False, False),  # max length, uppercase + lowercase
    (20, False, True, True),
    (15, True, False, True),
])
def test_get_generate_password(length, use_uppercase, use_numbers, use_special_chars):
    params = {
        "length": length,
        "use_uppercase": str(use_uppercase).lower(),
        "use_numbers": str(use_numbers).lower(),
        "use_special_chars": str(use_special_chars).lower(),
    }
    response = client.get("/generate-password", params=params)
    print(f"GET Request URL: {response.request.url}")
    print(f"GET Response JSON: {response.json()}")

    assert response.status_code == 200
    data = response.json()
    assert "password" in data
    password = data["password"]
    assert isinstance(password, str)
    assert len(password) == length

test_post_generate_password function

Sends JSON payloads with different parameters to the POST /generate-password endpoint and validates the results.

@pytest.mark.parametrize("payload", [
    {"length": 12, "use_uppercase": True, "use_numbers": True, "use_special_chars": True},
    {"length": 4, "use_uppercase": False, "use_numbers": False, "use_special_chars": False},
    {"length": 128, "use_uppercase": True, "use_numbers": False, "use_special_chars": False},
    {"length": 20, "use_uppercase": False, "use_numbers": True, "use_special_chars": True},
    {"length": 15, "use_uppercase": True, "use_numbers": False, "use_special_chars": True},
])
def test_post_generate_password(payload):
    response = client.post("/generate-password", json=payload)
    print(f"POST Request URL: {response.request.url}")
    print(f"POST Request Body: {payload}")
    print(f"POST Response JSON: {response.json()}")

    assert response.status_code == 200
    data = response.json()
    assert "password" in data
    password = data["password"]
    assert isinstance(password, str)
    assert len(password) == payload["length"]

How to Add More Tests

• Use @pytest.mark.parametrize to easily test more combinations.
• Define new test_ prefixed functions for different cases.
• Use client.get() and client.post() to simulate API requests.
• Verify response status, data shape, content, and any business logic.

Example:

def test_invalid_length():
    response = client.get("/generate-password", params={"length": 3})  # below minimum allowed
    assert response.status_code == 422  # Expect validation error

Debugging Tests

• Use print() statements inside tests to output details.
• Run pytest with -s flag to see prints:

pytest -s -v test_password_api.py

• Use verbose flag (-v) to see detailed test results.

Summary

• The test suite ensures both GET and POST /generate-password endpoints behave correctly for various inputs.
• It runs fast and isolated from your running server via FastAPI’s TestClient.
• Use pytest CLI with -s and -v flags for best experience.
• Add more tests following the existing patterns to cover edge cases and invalid data.