Installing FastAPI

FastAPI is a Python framework for building APIs. This guide sets up an isolated Python environment, installs FastAPI and its standard extras, and verifies the installation by starting a small application.

1. Check Python

Install a Python version supported by the FastAPI and Pydantic versions you plan to use. Then confirm that Python is available in your terminal:

python --version

On some systems, use python3 --version or, in Windows PowerShell, py --version if python is not recognized.

2. Create a Virtual Environment

A virtual environment keeps this project’s packages separate from other Python projects and the system installation.

On macOS or Linux:

python3 -m venv .venv
source .venv/bin/activate

On Windows PowerShell:

py -m venv .venv
.\.venv\Scripts\Activate.ps1

After activation, the environment name usually appears in the terminal prompt. If PowerShell blocks the activation script, use Command Prompt and run .venv\Scripts\activate.bat, or invoke the environment’s Python directly as shown below.

3. Install FastAPI

For a typical development setup, install the standard extra. It includes FastAPI’s command-line development tools and a server, along with commonly used optional dependencies:

python -m pip install --upgrade pip
python -m pip install "fastapi[standard]"

Using python -m pip helps ensure packages are installed into the environment belonging to the selected Python interpreter. If the environment is not activated, invoke its interpreter explicitly:

.venv/bin/python -m pip install "fastapi[standard]"

On Windows, use .venv\Scripts\python.exe -m pip install "fastapi[standard]" instead.

The base fastapi package can also be installed without the standard extras, but you will need to install and configure a server and any optional dependencies separately. The standard extra is the simplest starting point for local development.

4. Verify the Installation

Check that FastAPI is installed in the active environment:

python -m pip show fastapi

Create a file named main.py with a minimal application:

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def root():
    return {"message": "FastAPI is ready"}

Start the development server from the directory containing main.py:

fastapi dev main.py

Open http://127.0.0.1:8000/ to see the response. The interactive API documentation is available at http://127.0.0.1:8000/docs. Stop the server with Ctrl+C.

5. Common Installation Issues

• The fastapi command is not found: activate the virtual environment, or run the command using the environment’s scripts directory. Confirm that fastapi[standard] was installed in the same environment.
• pip installs into the wrong Python: use python -m pip and check the interpreter with python --version and python -m pip --version.
• The environment is not activated: run its Python directly. Use .venv/bin/python on macOS or Linux, or .venv\Scripts\python.exe on Windows.
• A package or Python version conflict appears: check the package’s supported Python versions and recreate the environment with a compatible interpreter rather than changing the system Python installation.

Conclusion

Use a virtual environment for each FastAPI project, install fastapi[standard] for a convenient development setup, and verify it by starting a small app. Once the server and /docs page work, you are ready to build routes and request models.