I am working on my first Flask application. Taking some code directly out of this, I am trying to make sure that a value is present in the user's cookies.
def after_this_request(f):
if not hasattr(g, 'after_request_callbacks'):
g.after_request_callbacks = []
g.after_request_callbacks.append(f)
return f
@app.after_request
def call_after_request_callbacks(response):
for callback in getattr(g, 'after_request_callbacks', ()):
response = callback(response)
return response
@app.before_request
def detect_unique_id():
unique_id = request.cookies.get('unique_id')
if unique_id is None:
unique_id = generate_unique_id()
@after_this_request
def remember_unique_id(response):
response.set_cookie('unique_id', unique_id)
g.unique_id = unique_id
I keep getting this error:
Traceback (most recent call last):
File "/..../env/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
return self.wsgi_app(environ, start_response)
File "/..../env/lib/python2.7/site-packages/flask/app.py", line 1690, in wsgi_app
return response(environ, start_response)
TypeError: 'NoneType' object is not callable
I am trying to understand the reason for this error. Please help.
remember_unique_id
does not return the response object, but call_after_request_callbacks
assigns the result of calling each callback added via the after_this_request
decorator to result
and then returns it. That is to say:
# This
for callback in getattr(g, 'after_request_callbacks', ()):
response = callback(response)
# translates to this
for callback in [remember_unique_id]:
response = callback(response)
# which translates to this
response = remember_unique_id(response)
# which translates to this
response = None
Either:
remember_unique_id
to return the modified response objectUpdate call_after_request_callbacks
to check the returned object and make sure it is not None:
for callback in getattr(g, 'after_request_callbacks', ()):
result = callback(response)
if result is not None:
response = result
Flask is a WSGI application under the covers and it expects response
to be a WSGI application (that is, a callable object). When it is handling responses from view templates it runs some checks to make sure that it is being something it can use as a response object and if the returned value is not a WSGI application it converts it to one. It does not check that the response object hasn't been altered by the after_request
decorators, and so when it tries to call the response object (which it assumes is a valid WSGI application at this point) you get the TypeError
.
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