Can't write to file using Flask and Docker volumes

Question

I'm trying to write to a file using Flask and docker. If the file has anything in it before running, the file gets emptied and overwritten with a blank page. But my flask app isn't actually able to write to it.

I believe my volumes aren't setting up correctly. Here's what I have:

In my path I have:

matrices/sampleMatrix.txt static/ templates/ app.py Dockerfile docker-compose.yml

Dockerfile

FROM python:3.6.1-alpine
ADD . .
RUN pip install -r requirements.txt
RUN pip install requests
#VOLUME [ "./matrices" ]     <- Do I need this? Doesn't seem to do anything more than the compose file
WORKDIR /
CMD ["python","app.py"]

docker-compose.yml

version: '2'

services:
  <container name>:
    build: . 
    ports: 
      - 82:80
    container_name: <container name>
    volumes:
      #- ./matrices:/var/lib/docker/matrices
      - ./matrices:/matrices

^^ This last line I think is the mistake. The left side is set up correctly to write to the file locally, but the path on the right isn't writing to the correct file.

Flask app:

    # This first block gets files in the matrices folder and confirms that my path matches a file which is there
    for item in glob.glob("/matrices/*"):
        print(item, file=sys.stderr)
        fileName= "/matrices/sampleMatrix.txt"
        print(item==fileName, file=sys.stderr)

    fileName= "/matrices/sampleMatrix.txt"
    print("Final path chosen: ",fileName, file=sys.stderr)

    try:
        #READ  <- reads nothing even when I've written something in the file ahead of time
        matrixFile= open(fileName, "w+")
        print("About to read: ",fileName, file=sys.stderr)
        print(matrixFile.read(), file=sys.stderr)
        print("Read from: ",fileName, file=sys.stderr)
        matrixFile.close() 

        #WRITE   
        matrixFile= open(fileName, "w+")
        matrixFile.write("text: 
")
        #matrixFile.write(str(distanceMatrix))
        print("wrote to: ",fileName, file=sys.stderr)
        matrixFile.close()

        #READ   <- Reads nothing right after writing to the fil
        matrixFile= open(fileName, "w+")
        print("About to read: ",fileName, file=sys.stderr)
        print(matrixFile.read(), file=sys.stderr)
        print("Read from: ",fileName, file=sys.stderr)
        matrixFile.close()   
    except: #<-   this except never triggers, which tells me the file is found, it's just to doing anything with it
        print("cant open file", file=sys.stderr)

My question here is, the Flask app clearly sees a file in matrices/ but for some reason, the docker path for the volume doesn't seem to be connecting to the right one.

question from:https://stackoverflow.com/questions/65926350/cant-write-to-file-using-flask-and-docker-volumes

Answer 1

In order to read the file, open it using r.
Opening file using w+ will erase its content.

You can read more about open file modes in this question