Identify if a function is a nested function

Question

How can I identify whether or not a given function is:

A closure function
A nested function which isn't a closure
Not a nested function

I know I can say len(function.__closure__) > 0 but this would only answer whether or not this is a closure. What about a non-closure nested function? And is there a better way to do this?

All nested functions are closures, whether or not there is anything to actually close over. — chepner
– chepner, Commented Feb 12, 2019 at 17:59
Why do you need to know this? If the function doesn't close over any variables, the fact that it's nested makes no difference. — Barmar
– Barmar, Commented Feb 12, 2019 at 18:01
What do you need this for, what is the problem you are trying to solve? — Martijn Pieters
– Martijn Pieters, Commented Feb 12, 2019 at 18:01
@MartijnPieters I'm poking around with code injection into existing functions at runtime, and have realized I need to treat closures differently while doing so. — Aviv Cohn
– Aviv Cohn, Commented Feb 12, 2019 at 18:04
@AvivCohn: only if there are closed-over names. If you are patching such functions, see Can you patch *just* a nested function with closure, or must the whole outer function be repeated?. — Martijn Pieters
– Martijn Pieters, Commented Feb 12, 2019 at 18:06

Martijn Pieters · Accepted Answer · 2019-02-12 18:25:58Z

3

The only difference between a nested function with an empty closed-over set of names, and a function defined at the module level, is their __qualname__. All functions have a __closure__ attribute, which is empty when there is no closed-over set of names.

The __qualname__ attribute is writeable, so is not a reliable way of detecting a nested function, but you could look for the string .<locals>. in it:

>>> def foo():
...     def bar():
...         pass
...     return bar
...
>>> foo().__qualname__
'foo.<locals>.bar'

Nested functions that do use closed-over names, will have a non-empty __closure__ tuple.

edited Feb 12, 2019 at 18:25

answered Feb 12, 2019 at 18:05

Martijn Pieters

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