FastAPI docker: No module named "app" error

The Problem

I have a Dockerfile for my FastAPI project that looks like this:

FROM python:3.8
WORKDIR /app

COPY requirements.txt /
RUN pip install --requirement /requirements.txt

COPY ./app /app

EXPOSE 8000
CMD ["uvicorn", "app.main:app", "--host=0.0.0.0" , "--reload" , "--port", "8000"]

When I start the container from an image build with this Dockerfile with a command like docker-compose up -d, it produces the following Python error:

ModuleNotFoundError: No module named 'app'

The entry point for my project is a script named main.py inside the app folder.

How do I fix this error?

The Solution

The docker container this Dockerfile creates will run the command specified by CMD in the directory specified by WORKDIR, i.e. app. The command gives the project entry point as app.main:app, but the first part is unnecessary as we’re already in the app directory. Therefore, we can fix the error by removing the app. part of the command

FROM python:3.8
WORKDIR /app

COPY requirements.txt /
RUN pip install --requirement /requirements.txt

COPY ./app /app

EXPOSE 8000
CMD ["uvicorn", "main:app", "--host=0.0.0.0" , "--reload" , "--port", "8000"]
#                ^removed preceding "app."

To avoid these kinds of errors, we must ensure that commands are executed in the correct working directory. We can test this before creating our Docker container by running any CMD commands in their intended working directory. Note that the working directory can be changed at any point in the Dockerfile by using an additional WORKDIR directive.