How to pass command line arguments to a python Docker container?

Before getting into the docker container arguments we must know about python command line arguments and how they are accessed by the developer. Command line arguments are of great use when we want our python script to be controlled outside of the program.

Access the Python Script's Command Line Arguments

Step 1: Create a Python Script main.py

First, let's create a Python script that can accept and process command line arguments ?

# sys will allow us to access the passed arguments
import sys

# sys.argv[0] access the first argument passed that is the python script name
print("\nFile or Script Name is :", sys.argv[0])

# print arguments other than the file name
print("\nArguments passed:", end=" ")
for i in range(1, len(sys.argv)):
    print(sys.argv[i], end=" ")

print("\n\nLowercase arguments:")
# Lowercase operation on the passed arguments
for i in range(1, len(sys.argv)):
    print(sys.argv[i].lower(), end=" ")

Step 2: Run on the Terminal with Arguments

Execute the Python script with command line arguments ?

# python3 main.py HELLO THIS IS TUTORIALSPOINT

Output

File or Script Name is : main.py

Arguments passed: HELLO THIS IS TUTORIALSPOINT

Lowercase arguments:
hello this is tutorialspoint

Pass Arguments to Docker Container

We have different ways to pass command line arguments to the docker container. Some of these are mentioned below stepwise.

Using Docker Arguments with ENTRYPOINT

The trick is to use entrypoints and dockerfile and redirect that entrypoint to the python file execution. After this just pass the required python arguments during the process of running the docker container.

Step 1: Create a Dockerfile

Create a Dockerfile that sets up the Python environment and defines the entry point ?

FROM python
WORKDIR /app
COPY . /app/
ENTRYPOINT ["python3", "main.py"]

Step 2: Build the Image

Build the Docker image using the Dockerfile ?

# docker build -t arg_py .

Output

Sending build context to Docker daemon 8.192kB
Step 1/4 : FROM python
---> fa9122485d1d
Step 2/4 : WORKDIR /app
---> Using cache
---> 9e9708fe1d43
Step 3/4 : COPY . /app/
---> aea9ecf32f55
Step 4/4 : ENTRYPOINT ["python3", "main.py"]
---> Running in 864440fa7988
Removing intermediate container 864440fa7988
---> d6e31e5606b8
Successfully built d6e31e5606b8
Successfully tagged arg_py:latest

Step 3: Run the Container

Run the container and pass arguments that will be forwarded to the Python script ?

# docker run --name mycontainer arg_py HELLO FROM TUTORIALSPOINT

Output

File or Script Name is : main.py

Arguments passed: HELLO FROM TUTORIALSPOINT

Lowercase arguments:
hello from tutorialspoint

Using Docker Environment Variables with ENTRYPOINT

Docker run command provides us with some extraordinary functionalities, one of which is environment variables. Here we will use these environment variables to pass the data to the inside python script on the docker container.

Step 1: Create a Python Script for Accessing Environment Variables

This time creation of a python script will be very similar to the first example. Instead of importing the sys module, we will import the os module for fetching environment variables ?

import os

# declare some variables for environment variable
# os.getenv will fetch the environment variables from the container.
userName = os.getenv("User_Name")
passWord = os.getenv("Pass_Word")

# Now print the variables that has been fetched.
print("Running with user: %s" % userName)
print("Your password: %s" % passWord)

# Apply some operation
print(userName.upper())
print(passWord.upper())

Save the above file as main.py

Step 2: Build the Dockerfile

Build the dockerfile to create a new image with this python code. This dockerfile is the same that we created before only the python code in main.py is changed ?

# docker build -t env_img .

Output

Sending build context to Docker daemon 3.072kB
Step 1/4 : FROM python
---> fa9122485d1d
Step 2/4 : WORKDIR /app
---> Using cache
---> 9e9708fe1d43
Step 3/4 : COPY . /app/
---> 31f98d53c161
Step 4/4 : ENTRYPOINT ["python3", "main.py"]
---> Running in bec1681a6842
Removing intermediate container bec1681a6842
---> 5dd89b0c7985
Successfully built 5dd89b0c7985
Successfully tagged env_img:latest

Step 3: Run the Container

Use the environment variables mentioned in the python script during running the container. Docker run has a -e flag to mention any environment variable, we can mention multiple environment variables at a time ?

# docker run -e User_Name="TutorialsPoint" -e Pass_Word="secret" --name env_cont env_img

Output

Running with user: TutorialsPoint
Your password: secret
TUTORIALSPOINT
SECRET

Conclusion

You can pass arguments to Python Docker containers using ENTRYPOINT for direct command line arguments or environment variables with the -e flag. Both methods provide flexible ways to control your Python application behavior from outside the container.