Question

Apparently, Flask’s app.route / app.add_url_rule doesn’t work with closures. For example, creating a basic app with,

for name in ('/hi', '/bye'):
    app.add_url_rule(name, view_func=lambda: name)

and querying it,

dev:~/pg/yelp-main> curl localhost:9113/hi
/bye

shows that it doesn’t work with closures. What’s the easiest way to work around this? Can I force Python to actually create two functions?

Answer 1

You haven’t actually created a closure in your code that preserves the value of name:

view_func=lambda: name  # <- points at the *symbol*, which holds the *last* value
# in the case of a loop

To preserve the value you need to pass the value into the closure:

view_maker = lambda name: (lambda: name)

for name in ('/hi', '/bye'):
    endpoint = name.replace("/", "")
    app.add_url_rule(name, view_func=view_maker(name), endpoint=endpoint)

EDIT: In addition, you’ll need to ensure that each time you register a function using add_url_rule you either specify an endpoint or ensure each function has a unique __name__ (since Flask actually stores the routes in a dictionary keyed on the endpoint, which it derives from the function’s __name__ if no other is provided). Otherwise, your second view will overwrite your first one.

You may want to look into Flask’s class-based Views – they may make it easier to build the dynamics you are looking for (although closures and classes are quite similar [in that both are the poor man’s substitute for the other]).