[BUG][PYTHON] Do not set Content-Type for GET, HEAD or DELETE requests

[johnboyes]

This commit ensures that the Python generator no longer sets a default
Content-Type ofapplication/json for GET, HEAD and DELETE
requests.

Fixes #9831

Related issues/PRs: #476, #1061, #5941, #6167

Having the Content-Type set for these requests was causing issues with
other tools which insist that GET, HEAD and DELETE requests do not have
a Content-Type (as per the OpenAPI 3 specification).

An example of the problem that this commit fixes is when using
Prism as a validation proxy.

Prism rejects any GET request that has a Content-Type.

Here is an example of the problem manifesting itself.

To validate the fix in this commit:

1. Start with any OpenAPI3 spec e.g. the Petstore example at
   https://editor.swagger.io/
2. Generate Python client code for the spec
3. Look at the generated rest.py e.g. in the standard sample in this
   repo and see that the Content-Type defaults to application/json
   for all HTTP methods (including GET, HEAD and DELETE), rather than
   there being no Content-Type for GET, HEAD and DELETE.

cc @taxpon, @frol, @mbohlool, @cbornet, @kenjones-cisco, @tomplus, @Jyhess, @arun-nalla, @spacether

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[johnboyes]

Hi, is anyone able to take a look at this, please? 🙏🏻
@taxpon, @frol, @mbohlool, @cbornet, @kenjones-cisco, @tomplus, @Jyhess, @arun-nalla, @spacether

[wing328]

@johnboyes thanks for the PR. My understanding is that it shouldn't set a default content-type header at all. I think that's how other clients (e..g ruby, php, C#, etc) set the content-type at the moment: no default and set the content-type only if it's provided in the spec.

[johnboyes]

Hi @wing328, thanks for the comment. If that's the case, I propose to treat that (not setting a default content-type header for any type of request) as a subsequent separate issue, if you agree? Reasoning being that all of the related issues/PRs to this one (#476, #1061, #5941, #6167) all specifically mention GET, HEAD and DELETE requests only, and I'm wary of this PR having unexpected side-effects if it grows in scope - especially as the PR as is fixes a genuine, current issue. Thoughts?

[wing328]

Updated the samples (as mentioned in the 3rd item in the PR checklist). Let's see if the CI tests pass.

[wing328]

Travis CI (including the python tests) was not run since this project has run out of free build credits.

I tested it locally but got the following errors:

        try:
            # For `POST`, `PUT`, `PATCH`, `OPTIONS`, `DELETE`
            if method in ['POST', 'PUT', 'PATCH', 'OPTIONS', 'DELETE']:
                # Only set a default Content-Type for POST, PUT, PATCH and OPTIONS requests
                if (method != 'DELETE') and ('Content-Type' not in headers):
                    headers['Content-Type'] = 'application/json'
                if query_params:
                    url += '?' + urlencode(query_params)
>               if re.search('json', headers['Content-Type'], re.IGNORECASE):
E               KeyError: 'Content-Type'

petstore_api/rest.py:148: KeyError

To run the petstore server locally, please refer to https://github.com/OpenAPITools/openapi-generator/wiki/Integration-Tests#how-to-add-integration-tests-for-new-petstore-samples

[johnboyes]

I tested it locally but got the following errors:

Thanks @wing328, I was able to recreate the error locally too (thanks to your helpful comments, which helped me work out how to update the samples properly and to run the integration tests locally). I have fixed the error and also updated the samples in e846404 - very grateful if you can take another look, please.

[wing328]

Tested locally and the result is good:

============================= 159 passed in 4.65s ==============================
___________________________________ summary ____________________________________
  py3: commands succeeded
  congratulations :)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 02:22 min
[INFO] Finished at: 2021-07-07T11:01:49+08:00
[INFO] ------------------------------------------------------------------------