The docs describe the attributes available on the request object (from flask import request) during a request. In most common cases request.data will be empty because it's used as a fallback:
from flask import request
@app.route('/', methods=['GET', 'POST'])
def parse_request():
# data is usuall empty
data = request.data
# Contains the incoming request data as string in case it came with a mimetype Flask does not handle.
print(request.data)
# the key/value pairs in the URL query string
print(request.args)
# the key/value pairs in the body, from a HTML post form, or JavaScript request that isn't JSON encoded
print(request.form)
# the files in the body, which Flask keeps separate from form. HTML forms must use enctype=multipart/form-data or files will not be uploaded.
print(request.files)
# combined args and form, preferring args if keys overlap
# parsed JSON data. The request must have the application/json content type, or use request.get_json(force=True) to ignore the content
print(request.values)
# Json of the request (only if Content type is Application/Json
print(request.json)