How to apply function to elements of a list without creating a new list?

Question

I want to apply a function to all elements in the list, but I want to actually change the elements (which are objects), not view results. I think this is the problem with using map() or list comprehensions.

class Thing(object):
    pass

# some collection of things
my_things

# they are all big...

# produces SyntaxError: invalid syntax
[i.size = "big" for i in my_things]

# produces SyntaxError: lambda cannot contain assignment
map(lambda i: i.size="big", [i for i in my_things]) 

# no error, but is it the preferred way?
for i in my_things:
    i.size="big"

What is the way to do this?

Asked By: cammil

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Source

Answer 1

I think this is the problem with using map() or list comprehensions.

Not entirely.

my_things[:] = map(...)

Answered By: Ignacio Vazquez-Abrams

Answer 2

And what’s wrong with

for i in my_things:
    i.size = "big"

You don’t want to use neither map nor list comprehansion because they actually create new lists. And you don’t need that overhead, do you?

Answered By: freakish

Answer 3

You could use the __setattr__ method to assign the attribute in a list comprehension. Although, according to some SO threads, using list comprehensions without using the output is not pythonic.

[i.__setattr__('size', 'big') for i in my_things]

Answered By: garnertb

Answer 4

While I agree that there is nothing wrong with

for i in my_things:
    i.size = "big"

some people are hot into python one-liners. 😉

One option is to add a set method to your class, which can then be called from lambda (essentially hiding the assignment in a function):

class Thing(object):
    def setSize(self, size):
        self.size = size

map(lambda i: i.setSize("big"), my_things)

Answered By: DirkR

Answer 5

maybe like this on python3:

[dict(**i.__dict__, size='big') for i in my_things]

or

map(lambda x: dict(**x.__dict__, size='big'), my_things)

if i is dict not object

[dict(**i, size='big') for i in my_things]

on python2

[dict(i.copy(), size='big') for i in my_things]  # isinstance(i, dict)
[dict(i.__dict__.copy(), size='big') for i in my_things]  # isinstance(i, object)

Answered By: mackjoner

How to apply function to elements of a list without creating a new list?

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