Start docker container through an entry point function with arguments

Question

Suppose I have a file called handler.py inside a docker container (which is not yet running or up from image). Let the image name be testimage .

Inside handler.py , we have a function greet such that

def greet(username):
    print("Hello %s!"%(username))

Now I want to start my docker container from this image such that I invoke this function greet inside the file handler.py along with an argument. I want to call this while creating a running container itself.

Answer 1

Actually you're asking two things. One how to call a function in a python file from commandline. Two how to do this via Docker.

For the first, in the handler.py you'd need a main function to be able to do this. Something like this for example.

import sys


def greet(username):
    print("Hello %s!"%(username))

if __name__ == '__main__':
    greet(sys.argv[1])

Running it gives:

$ python handler.py harshvardhan
Hello harshvardhan!

Alternative and more complex is using OptionParser and switches based on that. Depending on your usecase, either works.

For the docker, I think you don't want to change the entrypoint, but the CMD. Dockerfile:

FROM python:2.7-alpine

WORKDIR /app

COPY handler.py .

ENTRYPOINT ["/usr/local/bin/python2.7"]
CMD ["/app/handler.py"]

Build an image:

$ docker build . -t local:dev

Run it, overriding the CMD :

$ docker run local:dev /app/handler.py itismemario
Hello itismemario!

Answer 2

You can provide the python script execution command during docker run as below,

docker run <image> python handler.py --user username_value .

And make sure you have handled that args using argparse in handler.py and call your greet function.

import argparse
parser = argparse.ArgumentParser(description='')
parser.add_argument('--user', help='username', required=True, type=str)