Python closure's return statement

Asked Nov 12 '17 at 19:11

Active Nov 12 '17 at 19:11

Viewed 145 times

Anyone know why it is that its necessary that in a python first class function that embeds another, it is necessary to return the embedded function else an instance of the first class function is of type None?

Here's an example:

def logger(msg):
     def log_message():
         print(f'Log: {msg}')


log_hi = logger('Hi')
print(log_hi)
>>> None

type(log_hi)
>>> <class 'NoneType'>

log_hi()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 TypeError: 'NoneType' object is not callable

asked Nov 12 '17 at 19:11

allan e

3

Because in Python if you do not add an explicit `return` statement, it will implicitely return `None`. This has *nothing* to do with closures, this is how it *always works with every function* – juanpa.arrivillaga Nov 12 '17 at 19:13
2

If you don't return it, then you're just defining a local function that immediately goes out of scope. This seems to be the same question as asking why you have to `return` any value from a function. – Carcigenicate Nov 12 '17 at 19:13
Thanks guys, that clears up a lot ... and makes me feel kinda dumb. – allan e Nov 12 '17 at 19:19
To be fair, there are languages where `log_message` would still be a global name, despite being defined in a function. Python just isn't one of them. – chepner Nov 12 '17 at 19:35
@chepner there are also lots of languages that let you leave out `return` statements. – juanpa.arrivillaga Nov 12 '17 at 19:37
@juanpa.arrivillaga: Could you post answer as an answer, so others know this is answered? – Terry Jan Reedy Nov 12 '17 at 20:20
@TerryJanReedy I think it is actually a duplicate. – juanpa.arrivillaga Nov 12 '17 at 20:32

Python closure's return statement

0 Answers0