Why is python not able to find required modules after "pip install" inside a Docker-image?

I have an issue with Python that it can’t find any dependencies when running in a container. In my case, I have a fastAPI-based application that runs perfectly on my local machine. When I start the docker image, it complains about every single module as long as I don’t do a separate "pip install xyz" within the image.

I have the following Dockerfile:

# Use the official Python image from the Docker Hub
FROM python:3.12-slim as builder

# Set environment variables
ENV PYTHONDONTWRITEBYTECODE=1
ENV PYTHONUNBUFFERED=1

# Install build dependencies
RUN apt-get update \
    && apt-get install -y --no-install-recommends gcc libpq-dev

# Create a working directory
WORKDIR /app

# Install pipenv and python dependencies in a virtual environment
COPY ./requirements.txt /app/
RUN python3 -m venv venv && bash -c "source venv/bin/activate"
RUN venv/bin/pip3 install -r requirements.txt

# Use the official Python image again for the final stage
FROM python:3.12-slim

# Set environment variables
ENV PYTHONDONTWRITEBYTECODE=1
ENV PYTHONUNBUFFERED=1

# Create a working directory
WORKDIR /app

# Copy installed dependencies from the builder stage
COPY --from=builder /usr/local/lib/python3.12/site-packages /usr/local/lib/python3.12/site-packages
COPY --from=builder /usr/local/bin/pip /usr/local/bin/pip

# Copy the application code
COPY . /app

# Install runtime dependencies (if any)
RUN pip install uvicorn gunicorn

# Expose the port the app runs on
EXPOSE 8000

# Command to run the application
CMD ["gunicorn", "-k", "uvicorn.workers.UvicornWorker", "main:app", "--bind", "0.0.0.0:8000", "--workers", "4"]

My file requirements.txt contains all necessary modules, like

...
fastapi==0.111.0
fastapi-cli==0.0.4
filelock==3.14.0
...
pydub==0.25.1
Pygments==2.18.0
python-dotenv==1.0.1
python-multipart==0.0.9
PyYAML==6.0.1
requests==2.32.3
rich==13.7.1
...

I built the container with:

docker build -t my-fastapi-app .

I run the container with:

docker run -p 8000:8000 my-fastapi-app

it prints a long call stack ending with:

ModuleNotFoundError: No module named 'requests'

Then I am going to add the modules with a separate "pip install" inside the Dockerfile:

RUN pip install pydub requests

Now it complains about missing fastAPI:

ModuleNotFoundError: No module named 'fastapi'

So I am going to add another pip install with fastAPI and so on and so forth.

Then I have tried using pipenv:

COPY ./Pipfile /app
COPY ./Pipfile.lock /app

RUN pip install --upgrade pip \
    && pip install pipenv \
    && pipenv install --deploy --ignore-pipfile

The necessary Pipfile contains all required modules, but then they are still missing after installation.

I thought that pip install -r requirements.txt would do all this automatically. But that’s not the case here. Where is my mistake?

Do I have to blow up my Dockerfile with separate "pip install"-commands for all modules that are listed within my "requirements.txt"?

Or is the virtual env just messed up somehow that Python can’t find any modules although they are installed?

>Solution :

You’re installing things into a virtualenv, and then copying over the system packages directory (which doesn’t have those deps then) into the runtime container.

Copy the virtualenv instead (and then use it).

RUN python3 -m venv /venv
RUN /venv/bin/pip3 install -r requirements.txt

# Use the official Python image again for the final stage
FROM python:3.12-slim
# ...
COPY --from=builder /venv /venv
# ...
RUN /venv/bin/python -m pip install uvicorn gunicorn
# ...
CMD ["/venv/bin/gunicorn", "-k", "uvicorn.workers.UvicornWorker", "main:app", "--bind", "0.0.0.0:8000", "--workers", "4"]