How to use map for class method within another class method in Python

Question

I want to define a class method which applies a different class method to a list - I thought the below would work but it doesn't seem to, am I missing something obvious?

class Foo():

    def __init__(self):
          pass

    def method1(self, data):
        do some stuff

    def method2(self, iterable):
        map(self.method1, iterable)
        do some more stuff

As a concrete example, method2 is applying method1 to each element of a list but the actual content of method1 (printing) doesn't seem to be executing:

class Foo():
    
    def __init__(self):
        self.factor = 2
        
    def method1(self, num):
        print(num*self.factor)
        
    
    def method2(self, ls):
        map(self.method1, ls)

f = Foo()
f.method2([1,2,3])

I would expect this to print 2, 4, 6 but nothing is printed.

I think you are making confusion with class methods and (instance) methods... I think you mean the latter one — cards
– cards, Commented Nov 26, 2021 at 9:21
Could you expand on "doesn't seem to [work]"? Give a minimal reproducible example. — jonrsharpe
– jonrsharpe, Commented Nov 26, 2021 at 9:23
@jonrsharpe sorry for ambiguity, have added dumb toy example - I would expect f.method2([1,2,3]) to print [2,4,6] but although the code runs nothing is printed — addawaddawoo
– addawaddawoo, Commented Nov 26, 2021 at 9:44

jonrsharpe · Accepted Answer · 2021-11-26 09:46:25Z

1

map returns a generator (in Python 3), so you still need to iterate over it:

def method2(self, iterable):
    for val in map(self.method1, iterable):
        do some stuff with val

If you don't care about the return values, you could always just wrap it in a list: list(map(self.method1, iterable)).

edited Nov 26, 2021 at 9:46

jonrsharpe

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answered Nov 26, 2021 at 9:18

Dominic Price

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1 Comment

jonrsharpe Over a year ago

A list comprehension would be e.g. [self.method1(value) for value in iterable], but using a list/list comprehension for side effects isn't idiomatic - you're building a list you don't need.

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