quick beginners List comprehension question

[MRAB]

Diez B. Roggisch wrote:
> Dr Mephesto wrote:
> 
>> Hi,
>> Im new to python, and OOP, and am trying to get a handle on list
>> comprehension.
>>
>> Say I have a class Foo with a property called bar:
>>
>> class Foo:
>>     def __init__(self):
>>         self.bar = random.randint(1,100)
>>
>> and then I make a list of these objects:
>>
>> Newlist = []
>> for x in range(10):
>>     Newlist.append(Foo())
>>
>> Now, suppose I wanted to triple the value of 'bar', I could always do:
>>
>> for x in range(10):
>> Newlist[x].bar = Newlist[x].bar * 3
>>
>> but can I do this using list comprehension?  Thanks in Advance!
> 
> No, as such, because list-comprehensions require you to have an *expression*
> in front of the iteration:
> 
> resultlist = [<expr> for <variable(s)> in <iterable>]
> 
> Now what you of course can do is this:
> 
> def multiply(item):
>     item.bar = item.bar * 3
> 
> [multiply(i) for i in items]
> 
> However, doing this will make python produce a list of None-references -
> which is a waste. It's up to you if you care about that, but generally it
> is frowned upon because of that, and the fact that the conciseness of the
> list-comp here isn't really helping with the readability.
> 
If you had:

def multiply(item):
     item.bar = item.bar * 3
     return item

then:

[multiply(i) for i in items]

would return items. Still a bad idea, though, because you're using a 
list comprehension for its side-effect.