Apparently your API requires JSON-encoded and not form-encoded data. When you pass a dict in as the data parameter, the data is form-encoded. When you pass a string (like the result of json.dumps), the data is not form-encoded.
Consider this quote from the requests documentation:
Typically, you want to send some form-encoded data — much like an HTML form. To do this, simply pass a dictionary to the data argument. Your dictionary of data will automatically be form-encoded when the request is made.
There are many times that you want to send data that is not form-encoded. If you pass in a string instead of a dict, that data will be posted directly.
For example, the GitHub API v3 accepts JSON-Encoded POST/PATCH data:
>>> import json
>>> url="https://api.github.com/some/endpoint"
>>> payload = {'some': 'data'}
>>> r = requests.post(url, data=json.dumps(payload))
Refs:
- http://www.w3.org/TR/html401/interact/forms.html#h-17.13.3.4
- http://docs.python-requests.org/en/latest/user/quickstart/#more-complicated-post-requests