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Create a file outside of Docker container while container is running in Python

I currently have a Python application running in a Docker container on Ubuntu 20.04.
In this Python application I want to create a text file every few minutes for use in other applications on the Ubuntu server. However, I am finding it challenging to create a file and save it on the server from inside a containerised Python application.
The application Dockerfile/start.sh/main.py files reside in /var/www/my_app_name/ and I would like to have the output.txt file that main.py creates in that same folder, the location of the Dockerfile/main.py source.
The text file is created in Python using a simple line:
text_file = open("my_text_file.txt", "wt")
I have seen that the best way to do this is to use a volume. My current docker run which is called by batch script start.sh includes the line:
docker run -d --name=${app} -v $PWD:/app ${app}
However I am not having much luck and the file is not created in the working directory where main.py resides.

A bind mount is the way to go and your -v $PWD:/app should work.
If you run a command like
docker run --rm -v $PWD:/app alpine /bin/sh -c 'echo Hello > /app/hello.txt'
you'll get a file in your current directory called hello.txt.
The command runs an Alpine image, maps the current directory to /app in the container and then echoes 'Hello' to /app/hello.txt in the container.

Ended up solving this myself, thanks to those who answered to get me on the right path.
For those finding this question in the future, creating a bind mound using the -v flag in docker run is indeed the correct way to go, just ensure that you have the correct path.
To get the file my_text_file.txt in the folder where my main.py and Dockerfile files are located, I changed my above to:
-v $PWD:/src/output ${app}
The /src/output is a folder structure within the container where my_text_file.txt is created, so in the Python, you should be saving the file using:
text_file = open("/src/output/my_text_file.txt", "wt")
The ${PWD} is where the my_text_file.txt is located on the local machine.
In short, I was not saving my file to the correct folder within the container in the Python code, it should have been saved to app in the container, and then my solution in the OP would have worked.

Docker Run: Mounted Volume not showing change in files

I am struggling with running the latest changes. Below are the details.
Dockerfile
FROM python:3.7.3
RUN mkdir -p /usr/apps
COPY test.py /usr/apps
RUN pip install mindsdb
CMD [ "python","test.py" ]
Build
docker build -t py37:custom .
Run
docker run -it -v /Development/PetProjects/mindsdb:/usr/apps/ py37:custom
But it shows only the changes at the time of build.

First of all while starting your container you are not using volumes but bind mounts. So you mount directory /Development/PetProjects/mindsdb on your host machine to /usr/apps/ directory. Every change made to files on your host machine in this directory, will be visible in the container, and the other way round.
If you wanted to use volumes, you could create one using docker volume create command and then running container with this volume : docker container run -v volume_name:path_in_container image_name. Then you would be able to stop container and run it again by passing this volume to run command and changes to path_in_container directory could be stored across container creations.
Another thing is that you are trying to mount /usr/apps/ in your container and you copied a python script there using Dockerfile. Note that in you current docker run command contents of /Development/PetProjects/mindsdb will replace content of /usr/apps/ in your container and if you do not have your script in /Development/PetProjects/mindsdb - script will not be visible in the container.
Moreover your CMD seems not to work because of path relativeness. You should change your CMD to CMD [ "python","/usr/apps/test.py" ] or use WORKDIR option - WORKDIR /usr/apps/ so your python command could be executed from this directory and script could be visible there.
More information about differences between volumes and bind mounts can be found in docker documentation.

How to run a simple Python script without writing a complete Dockerfile?

I have set up Docker Toolbox on a Win 10 machine. I have some simple single file Python scripts that I want to run in Docker, just for learning purpose.
Started learning Docker today, and Python 3 days ago.
I assume I have set up Docker correctly, I can run the example hello-world image. No error messages during setup.
I am following an instruction from here https://runnable.com/docker/python/dockerize-your-python-application,
which says:
If you only need to run a simple script (with a single file), you can avoid writing a complete Dockerfile. In the examples below, assume you store my_script.py in /usr/src/widget_app/, and you want to name the container my-first-python-script:
docker run -it --rm --name my-first-python-script -v "$PWD":/usr/src/widget_app python:3 python my_script.py
If I type pwd, it shows:
/c/Program Files/Docker Toolbox
And the script I want to run is located here:
C:\Docker\Python\my_script.py
This is what I think should work:
docker run -it --rm --name my-first-python-script -v "$PWD":/c/Docker/Python python:3 python my_script.py
No matter how I try to specify the file directory, I get an error:
python: can't open file 'my_script.py': [Errno 2] No such file or directory

When you run -v "$PWD":/c/Docker/Python, you are saying you want to link your current working directory to the path /c/Docker/Python in the container, which isn't what you want to do. What you are trying to do is link C:\Docker\Python\ on your host to the container folder /usr/src/widget_app.
This command will put your script inside the container path /usr/src/widget_app, then run it:
docker run -it --rm --name my-first-python-script -v /c/Docker/Python:/usr/src/widget_app python:3 python /usr/src/widget_app/my_script.py

Docker- Do we need to include RUN command in Dockerfile

I have a python code and to convert it to docker image, I can use below command:
sudo docker build -t customdocker .
This converts python code to docker image. To convert it I use a Dockerfile with below commands:
FROM python:3
ADD my_script.py /
ADD user.conf /srv/config/conf.d/
RUN pip3 install <some-package>
CMD [ "python3", "./my_script.py" ]
In this, we have RUN command which install required packages. Lets say if we have deleted the image for some reason and want to build it again, so is there any way we can skip this RUN step to save some time because I think this is already installed.
Also in my code I am using a file user.conf which is in other directory. So for that I am including this in DOckerfile and also saving a copy of it in current directory. Is there a way in docker where I can define my working directory so that docker image searches for the file inside those directories.
Thanks

Yes you cannot remove the RUN or other statements in dockerfile, if you want to build the docker image again after deleteing.
You use the command WORKDIR in your dockerfile but its scope will be within the docker images, i.e when you create the container from the image workdir will be set to that metioned in WORKDIR
For ex :
WORKDIR /srv/config/conf.d/
This /srv/config/conf.d/ will set as workingdir, but you have to use below in dockerfile while building in-order to copy that file in specified location
ADD user.conf /srv/config/conf.d/

Answering your first question: A docker image holds everything related to your python environment including the packages you install. When you delete the image then the packages are also deleted from the image. Therefore, no you cannot skip that step.
Now on to your second question, you can bind a direectory while starting the container by:
docker run -v /directory-you-want-to-mount:/src/config/ customdocker
You can also set the working directory with -w flag.
docker run -w /path/to/dir/ -i -t customdocker
https://docs.docker.com/v1.10/engine/reference/commandline/run/

How to run my python script on docker?

I am trying to run my python script on docker. I tried different ways to do it but not able to run it on docker. My python script is given below:
import os
print ('hello')
I have already installed docker on my mac. But i want to know how i can make images and then push it to docker after that i wanna pull and run my script on docker itself.

Going by question title, and if one doesn't want to create docker image but just want to run a script using standard python docker images, it can run using below command
docker run -it --rm --name my-running-script -v "$PWD":/usr/src/myapp -w /usr/src/myapp python:3.7-alpine python script_to_run.py

Alright, first create a specific project directory for your docker image. For example:
mkdir /home/pi/Desktop/teasr/capturing
Copy your dockerfile and script in there and change the current context to this directory.
cp /home/pi/Desktop/teasr/capturing.py /home/pi/Desktop/teasr/dockerfile /home/pi/Desktop/teasr/capturing/
cd /home/pi/Desktop/teasr/capturing
This is for best practice, as the first thing the docker-engine does on build, is read the whole current context.
Next we'll take a look at your dockerfile. It should look something like this now:
FROM python:latest
WORKDIR /usr/local/bin
COPY capturing.py .
CMD ["capturing.py", "-OPTIONAL_FLAG"]
The next thing you need to do is build it with a smart name. Using dots is generally discouraged.
docker build -t pulkit/capturing:1.0 .
Next thing is to just run the image like you've done.
docker run -ti --name capturing pulkit/capturing:1.0
The script now get executed inside the container and will probably exit upon completion.
Edit after finding the problem that created the following error:
standard_init_linux.go:195: exec user process caused "exec format error"
There's a different architecture beneath raspberry pi's (ARM instead of x86_64), which COULD'VE BEEN the problem, but wasn't. If that would've been the problem, a switch of the parent image to FROM armhf/python would've been enough.
Source
BUT! The error kept occurring.
So the solution to this problem is a simple missing Sha-Bang on top of the python script. The first line in the script needs to be #!/usr/bin/env python and that should solve the problem.
Source

You need to create a dockerfile in the directory your script is in.
You can take this template:
FROM python:latest
COPY scriptname.py /usr/local/share/
CMD ["scriptname.py", "-flag"]
Then simply execute docker build -t pulkit/scriptname:1.0 . and your image should be created.
Your image should be visible under docker images. If you want to execute it on your local computer, use docker run.
If you want it to upload to the DockerHub, you need to log into the DockerHub with docker login, then upload the image with docker push.

I Followed #samprog (most accepted) answer on my machine running on UBUNTU VERSION="14.04.6".
and was getting "standard_init_linux.go:195: exec user process caused "exec format error"
None of the solution worked for me mentioned above.
Fixed the error after changing my Dockerfile as follows
FROM python:latest
COPY capturing.py ./capturing.py
CMD ["python","capturing.py"]
Note: If your script import some other module then you need to modify COPY statement in your Dockerfile as follows - COPY *.py ./
Hope this will be useful for others.

Another way to run python script on docker can be:
copy the local python script to docker:
docker cp yourlocalscript.path container_id:/dst_path/
container id can be found using:
docker ps
run the python script on docker:
docker exec -it python /container_script_path.py

its very simple
1- go to your Python script directory and create a file with this title without any extension
Dockerfile
2-now open the docker file and write your script name instead of sci.py
( content of Dockerfile )
FROM python:slim #i choice slim version you can choose another tag for example python:3
WORKDIR /usr/local/bin
COPY sci.py . #replace you scrip name with sci.py
CMD [ "python", "sci.py" ] #replace you scrip name with sci.py
save it and now you should create image file from this dockerfile and script py
and next run it
3-in path address folder write CMD and press Enter key :
4-When the cmd window opens for you, type in it :
docker build -t my-python-app . #this create image in docker by this title my-python-app
5- and findly run image:
docker run -it --rm --name my-running-app my-python-app

I've encountered this problem recently, this dependency HELL between python2 and python3 got me. Here is the solution.
Bind your current working directory to a Docker container with python2 and pip2 running.
Pull the docker image.
docker pull frolvlad/alpine-python2
Add this alias into /home/user/.zshrc or /home/user/.bashrc
alias python2='docker run -it --rm --name python2 -v "$PWD":"$PWD" -w
"$PWD" frolvlad/alpine-python2'
Once you type python2 into your CMD you'll be thrown into the Docker instance.