How to apply function to elements of a list?

问题

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?

回答1:

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?

回答2:

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)

回答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]

回答4:

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

Not entirely.

my_things[:] = map(...)

回答5:

maybe like this on python3:

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

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)

来源：https://stackoverflow.com/questions/10554470/how-to-apply-function-to-elements-of-a-list

标签

python

list

map

python-2.7