running docker container is not overwritting a txt file with python

Question

I have a python script which changes the first line of a file, and it works locally but not when running the docker container.

Directory structure

my_workdir
├─── Dockerfile
├─── docker-compose.yml
├─── script.py
└─── data
    └─── r2.txt

this is my python code

a_file = open("data/r2.txt", "r")
list_of_lines = a_file.readlines()
list_of_lines[0] = "1\n"
a_file = open("data/r2.txt", "a")
a_file.writelines(list_of_lines)
a_file.close()

it basically changes the first line inside "r2.txt" to "1" but when I check the txt file is not changing it. I was reading that you need to mount a volume so I added the following to my docker-compose.yml file

version: "3"
services:
  app:
    image: imagen-c1
    container_name: c1
    privileged: true
    deploy:
      replicas: 1
      update_config:
        parallelism: 1
        delay: 5s
      restart_policy:
        condition: on-failure
    build: .
    environment:
      - DISPLAY=${DISPLAY}
      - LEGACY_IPTABLES=true 
    volumes:
      - /tmp/.X11-unix:/tmp/.X11-unix
      - ./:/data #HERE IS THE VOLUME

this is my dockerfile

FROM python:3.9-alpine

RUN pip install selenium

RUN apk add chromium
RUN apk add chromium-chromedriver


ENV CHROME_BIN=/usr/bin/chromium-browser \
    CHROME_PATH=/usr/lib/chromium/
ENV HOME /home/user-machine
    
RUN pip install webdriver_manager
WORKDIR ./
COPY ["script.py", "data/", "./"]
CMD python3 script.py

maybe I'm not doing it correctly because the file remains the same.

Answer 1

I think there are 2 problems in your current code.

1. COPY ["script.py", "data/", "./"]

   inside_container_root
   ├─── script.py
   └─── r2.txt
   

   Above is what will be outcome, according to this SO answer

   Try do ls -l to see if this is really the case:

   docker build -t my_image
   docker run my_image ls -l
   

   This might be what you wanted:

   COPY script.py ./
   ADD data data
   

   ADD can copy directories

2. I think file data/r2.txt will be read-only because it is a file inside the image. Once you resolved (1), your script.py should hit error saying you are trying to write a read-only file. I think the whole data/ directory is not writable after image is created. The updated file needs to be a folder not existing in the image. Like data_out. If you do not need the contents of your new file to be used outside the container, you do not need to mount volume. Mounting volume is to enable retrieval of file from outside the container. To get a writable directory, just make sure that writeable directory does not exist in the image.

    a_file = open("data_out/r2.txt", "a")
   

   You might want something like this.