[Python] msrest does not enforce JSON Accept header

[lmazuel]

Hi,

msrest does no content negotiation currently and then does not fill the Accept header in any request.
Unfortunately, not all provider use JSON by default when the Accept header is not present (this leads to problem like this Azure/azure-sdk-for-python#553, solved by adding the right Accept header)

In addition, msrest cannot currently handle XML data. All requests have to be JSON.

I suggest to force Accept: application/json in all requests done using msrest. This can be made at several places:

• In msrest directly (here https://github.com/Azure/autorest/blob/master/ClientRuntimes/Python/msrest/msrest/service_client.py#L119)
• In each operation, between these two lines:

# Construct headers
header_parameters = {}
header_parameters['Content-Type'] = 'application/json; charset=utf-8'

What do you think?

I don't mind adding directly in msrest core code, since as I said msrest cannot parse XML anyway.

I can do the PR once you we agreed on the result.

@annatisch

[annatisch]

I would be keen to know how the other clients/languages support or enforce this.
The generated code will often set the Content-type header if the content-type is specified in the swagger spec so I'm not sure we should be simply overwriting it. Particularly for stream requests/responses.
Though perhaps we should set it where it doesn't already exist.

[devigned]

I believe we select the first accept type from the swagger that corresponds to application/json.

[lmazuel]

@devigned

Are you saying that if the Swagger file has:

  "produces": [
    "application/json"
  ],

We should add an Accept for application/json, if not leave the server decides?

[lmazuel]

Side effect: do we have to refuse generate Python code if "produces" does not contains application/json (since anyway we can't do anything with something else notJSON)?

[annatisch]

@lmazuel - I think it's okay to put this is msrest,
I can add:
session.headers['Accept'] = 'application/json'
at line 120 as you suggest.

[lmazuel]

Thank you @annatisch I will do a PR that way!

[annatisch]

@amarzavery
This can be closed now :)