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Example: requests Client and FastAPI Provider with Pact Contract Testing

This example demonstrates contract testing between a synchronous requests-based client (consumer) and a FastAPI web server (provider). It is designed to be pedagogical, showing modern Python patterns, type hints, and best practices for contract-driven development.

Overview

Use the above links to view additional documentation within.

What This Example Demonstrates

Consumer Side

  • Synchronous HTTP client implementation with requests
  • Consumer contract testing with Pact mock servers
  • Handling different HTTP response scenarios (success, not found, etc.)
  • Modern Python patterns and type hints

Provider Side

  • FastAPI web server with RESTful endpoints
  • Provider verification against consumer contracts
  • Provider state setup for different test scenarios
  • Mock data management for testing

Testing Patterns

  • Independent consumer and provider testing
  • Contract-driven development workflow
  • Error handling and edge case testing
  • Type safety with Python type hints

Pedagogical Context

This example is intended for software engineers and engineering managers who want to understand:

  • How contract testing works in practice
  • Why consumer-driven contracts are valuable
  • How to structure Python code for clarity and testability
  • The benefits of using requests and FastAPI for simple, modern HTTP services

Prerequisites

  • Python 3.9 or higher
  • A dependency manager (uv recommended, pip also works)

Running the Example

We recommend using uv to manage the virtual env and manage dependencies. The following command will automatically set up the virtual environment, install dependencies, and then execute the command within the virtual environment:

uv run --group test pytest

Using pip

If using the venv module, the steps require are:

  1. Create and activate a virtual environment:

    python -m venv .venv
source .venv/bin/activate  # On macOS/Linux
.venv\Scripts\activate     # On Windows

  2. Install dependencies:

    pip install -U pip  # Pip 25.1 is required
pip install --group test -e .

  3. Run tests:

    pytest