How can python find decorated functions within it?

Question

In the following code how does app.run() find that hello() exists? I browsed the code and couldn't find the answer. I think the hello() doesn't get added to a list of routes until it is called, but how does it get called?

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello World!"

if __name__ == "__main__":
    app.run()

I'm not interested in Flask in particular. I'd just like to implement something similar myself.

What is this design pattern called?

Please see the define of the `route` function https://github.com/pallets/flask/blob/master/flask/app.py#L1224 — stamaimer, Oct 27 '18 at 07:14
read this [_short_ intro on decorators](https://stackoverflow.com/a/1594484/7505395) — Patrick Artner, Oct 27 '18 at 07:47

score 4 · Accepted Answer · answered Oct 27 '18 at 07:54

4

Your assumption is wrong. Decorators are themselves executable code, and they are called when the function they decorate is defined, ie at import time. The code within the decorator can then add the route to its registry.

answered Oct 27 '18 at 07:54

Daniel Roseman

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How can python find decorated functions within it?

1 Answers1