CODEWITHSUNDEEP
pythonpython-3.xreturn
Taylor
Taylor

Reputation: 2085

Python3.5 function that calls object's method returns None instead of object

I have a class and a function that tries to update and return instances of this class:

class Foo:
    def __init__(self):
        self.datum = 'a'
    def update(self):
        self.datum = 'b' if self.datum == 'a' else 'a'

def updater(foo_obj):
    return( foo_obj.update() )

but calling updater() returns Nones and not instances of class Foo. Why is this? What can I change to get the desired result?

type(updater(Foo())) #NoneType

Upvotes: 0

Views: 315

Answers (3)

Andrew
Andrew

Reputation: 388

If you'd like update to return instances of the Foo class, you have to explicitly have it do so with a return statement, otherwise it will implicitly return None (i.e. it's a void method, even though Python doesn't use these terms because duck-typing). Do this like so:

def update(self):
    self.datum = 'b' if self.datum == 'a' else 'a'
    return self

You could also return the Foo object in updater if you don't want your class to do it, like so:

def updater(foo_obj):
    foo_obj.update()
    return foo_obj

If you'd like to return just self.datum and not the whole object, then do that like:

def update(self):
    self.datum = 'b' if self.datum == 'a' else 'a'
    return self.datum

or similarly:

def updater(foo_obj):
    foo_obj.update()
    return foo_obj.datum

Upvotes: 0

Dipak
Dipak

Reputation: 6970

Update method updater.

def updater(foo_obj):
    foo_obj.update()
    return( foo_obj )

OR return self from class function:

def update(self):
        self.datum = 'b' if self.datum == 'a' else 'a'
        return self

Upvotes: 1

Mureinik
Mureinik

Reputation: 311723

Your update method does not return anything, which in Python means that it implicitly returns None. If you want it to have a meaningful return value, you'd have to do so explicitly. E.g.:

def update(self):
    self.datum = 'b' if self.datum == 'a' else 'a'
    return self.datum

Upvotes: 1

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