Fix Python ImportError: No Module Named FastAPI

You see an ImportError in Python. It says no module named fastapi. This is a common issue. It stops your API development. Let's fix it step by step.

The error means Python cannot find FastAPI. The module is not in your path. This guide will help you solve it. We cover all important causes.

Understanding the ImportError

The error looks like this. It appears when you run your script.

# Example error traceback
import fastapi

Traceback (most recent call last):
  File "app.py", line 1, in 
    import fastapi
ModuleNotFoundError: No module named 'fastapi'

This is a ModuleNotFoundError. Python's import system failed. The fastapi package is missing. Your code cannot proceed.

Primary Cause: FastAPI Not Installed

The most common reason is simple. You did not install FastAPI. You must install it with pip. Use the package installer for Python.

First, check if FastAPI is installed. Open your terminal or command prompt. Run the following command.

pip show fastapi

If you get "Package(s) not found", it's not installed. You need to install it. Follow our guide on How to Install FastAPI in Python.

The basic installation command is below. Run it in your terminal.

pip install fastapi

This downloads and installs FastAPI from PyPI. It also installs required dependencies. After this, try importing again.

Using a Virtual Environment

You might be using a virtual environment. This is a best practice. It isolates project dependencies.

Ensure your virtual environment is activated. The error occurs if it's not active. Your pip install went to the global Python.

On macOS/Linux, activate with source.

source venv/bin/activate

On Windows, activate differently.

venv\Scripts\activate

After activation, install FastAPI inside it. Then the import should work. The module will be available.

Checking Python Path and Environment

Multiple Python versions can cause confusion. You might have Python 2 and Python 3. Or different installations.

Verify which Python and pip you are using. Use these commands in your terminal.

python --version
pip --version

Ensure they match. If you use python3, use pip3. Install FastAPI with the correct pip. For example, use pip3 install fastapi.

Also check your IDE or editor settings. It might be using a different interpreter. Point it to the correct Python with FastAPI installed.

Reinstalling FastAPI

A corrupted installation can cause issues. Uninstall and reinstall FastAPI. This often fixes strange errors.

First, uninstall the package. Use the pip uninstall command.

pip uninstall fastapi

Confirm the removal. Then install it again cleanly. For a complete cleanup, see How to Uninstall FastAPI in Python.

Reinstall with the standard command.

pip install fastapi

This ensures a fresh copy. It replaces any broken files.

IDE and Project-Specific Issues

Your IDE might not see the installed package. This happens in PyCharm, VS Code, or others. You need to configure the project interpreter.

In PyCharm, go to Settings. Find Project: YourProjectName. Click Python Interpreter. Select the correct interpreter from the list.

In VS Code, open the command palette. Select "Python: Select Interpreter". Choose the Python path where FastAPI is installed.

Restart your IDE after changes. This reloads the environment. The import error should disappear.

Example: Successful FastAPI Application

After fixing the error, your code should run. Here is a minimal FastAPI app to test.

# main.py - A simple FastAPI test
from fastapi import FastAPI  # This import should now work

app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="0.0.0.0", port=8000)

Run this application with uvicorn. First, install uvicorn if needed.

pip install uvicorn

Then run your script.

python main.py

INFO:     Started server process [1234]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)

Open your browser. Go to http://localhost:8000. You will see the JSON response. This confirms FastAPI is working.

Conclusion

The "No module named fastapi" error is fixable. Always check the installation first. Use the correct Python environment.

Activate your virtual environment. Verify the Python path. Reinstall if necessary. Configure your IDE properly.

Following these steps will resolve the issue. You can then build your FastAPI applications smoothly. Happy coding!