Create requirements.txt if it doesn't exist

[lloydchang]

This resolves:
Could not open requirements file: [Errno 2] No such file or directory: 'requirements.txt'
The command '/bin/sh -c python3 -m pip install -r requirements.txt' returned a non-zero code: 1

[StephenWeatherford]

Thanks, will take a look soon. Can you recommend a simple python project that I could base a test off of?

[lloydchang]

You can use my hello-world at https://github.com/lloydchang/exercism-solutions/tree/master/python/hello-world

Here are my steps to reproduce the error:
A. In background, run: Docker daemon (e.g. Run Docker Desktop on a Mac)
B. From Terminal, load the hello-world into Visual Studio Code (e.g. cd hello-world && code .)
C. In Visual Studio Code, run:

1. Command Palette (⇧⌘P)
2. Docker: Add Docker Files to Workspace
3. Select Application Platform: Python
4. Which port does your app listen on: 3000
5. Command Palette (⇧⌘P)
6. Docker: Build Image
7. helloworld:latest
8. In Visual Studio TERMINAL: 1: Docker, it should display the error as follows:

docker build --rm -f "Dockerfile" -t hello-world:latest .
$ docker build --rm -f "Dockerfile" -t hello-world:latest .
Sending build context to Docker daemon 18.43kB
Step 1/7 : FROM python:alpine
---> aadc3feb2b19
Step 2/7 : LABEL Name=hello-world Version=0.0.1
---> Running in dc7ac6dedae8
Removing intermediate container dc7ac6dedae8
---> 6d35511ab82e
Step 3/7 : EXPOSE 3000
---> Running in 0dc87111fcc3
Removing intermediate container 0dc87111fcc3
---> 4a2d4bc74829
Step 4/7 : WORKDIR /app
---> Running in 60f8aeaca7e7
Removing intermediate container 60f8aeaca7e7
---> 145ec4b03cb2
Step 5/7 : ADD . /app
---> ebac5a2b6232
Step 6/7 : RUN python3 -m pip install -r requirements.txt
---> Running in 57c035381f7e
Could not open requirements file: [Errno 2] No such file or directory: 'requirements.txt'
The command '/bin/sh -c python3 -m pip install -r requirements.txt' returned a non-zero code: 1
$

Initial Context: I ran into the error while following Generating Docker files from https://code.visualstudio.com/docs/azure/docker

Writing Docker and docker-compose files by hand can be tricky and time consuming. To help you, VS Code can generate the necessary Docker files for your project. From the Command Palette (⇧⌘P), run the Docker: Add Docker files to Workspace command to generate Dockerfile, docker-compose.yml, and docker-compose.debug.yml files for your workspace type:

Additional Contexts: In Visual Studio Python tutorials, Create a requirements.txt file for the environment is an optional activity; please see:
• https://code.visualstudio.com/docs/python/tutorial-django#_optional-activities
• https://code.visualstudio.com/docs/python/tutorial-flask#_optional-activities

Considering requirements.txt may (be optionally created manually) or may not exist (in a hello-world simple python project), I amended my pull request to create requirements.txt if it doesn't exist:
RUN if [ ! -f requirements.txt ]; then pip3 freeze > requirements.txt; fi

[StephenWeatherford]

Should this be added to the commented out sections for pipenv and miniconda as well?

[StephenWeatherford]

When I try to run the built image, I get this:

docker run --rm -it -p 3000:3000/tcp hello-world:latest
/usr/local/bin/python3: No module named hello-world

[StephenWeatherford]

Okay, so it looks like we're picking up the module name from the name of the folder the project is in (in this case hello-world). So I guess it's by design for the current implementation, although we do a better job here on other languages. Is there an official way to properly pick this up (e.g. from a file in the project folder), for possible use later? Thx.

[StephenWeatherford]

@brettcannon Brett, other than the questions above, this change looks good to me. Do you have any reservations? Thanks!

[StephenWeatherford]

@lloydchang I'm going to go ahead and merge this so we can get it into the next release. If you'd like to make updates or provide more info according to above comments, just let me know. Thanks!

[brettcannon]

Sorry for not getting back sooner but I was travelling last week.

So I actually disagree with this change as freezing should be an explicit step the developer takes, otherwise you aren't making a conscious decision of what dependencies you do (or do not) want to end up in your image.

And if the decision to keep this line stands then it should be updated to follow the line it precedes, e.g. python3 -m pip freeze.

[StephenWeatherford]

No problem, thanks for letting me know. I would rather show (or document) best practice first, and have it be convenient second. Does it make sense to only supply the requirements.txt file to install if it exists? Or is it better to supply it always and document that users should create it? Or something else? Thanks!

[StephenWeatherford]

@brettcannon, @lloydchang Moved the discussion to #693. Reverting the change for the time being.