Is it un-pythonic to define a function inside of a class method?

Question

PyLint told me that one of my class methods didn't need to be a method, but could just be a function in a class since it didn't use any class attribute. That made me do things I thought were "bad," but maybe they are Pythonic. Is the following code what Python wants us to do?

class TestClass(ParentClass):
    def __init__(self):
        def callbackfunction(text):
            print("hello")
        ParentClass.map_event_to_callback(ParentClass.event, callbackfunction)

where ParentClass.event emits text to its callback, but we'll just ignore that print "hello" instead. Even simpler:

class TestClass():
    def __init__(self, text):
        def printhello(text):
            print("hello")
        printhello(text)

assuming I don't care about text or printhello after __init__.

A function within a method is really no different from a function within an function. Sometimes it makes sense, and when it does, there's no reason not to use it. — Tom Karzes
– Tom Karzes, Commented Mar 9, 2016 at 0:24
Thank you, @MartijnPieters and @TomKarzes! Putting a callback function inside of a class' __init__ made me feel dirty, but I didn't know if there was a technical reason to avoid it or not. @MartijnPieters, if you make your comment an answer I can select it. — Scott Howard
– Scott Howard, Commented Mar 9, 2016 at 18:00

Martijn Pieters · Accepted Answer · 2016-03-09 18:14:51Z

1

Creating a nested function for a callback is just fine. It even gives that function access to any locals in the parent function (as closures).

You can use a lambda if all you need to execute is one expression:

class TestClass(ParentClass):
    def __init__(self):
        ParentClass.map_event_to_callback(ParentClass.event, lambda text: print("hello"))

answered Mar 9, 2016 at 18:14

Martijn Pieters

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