List filtering: list comprehension vs. lambda + filter

Question

I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items.

My code looked like this:

my_list = [i for i in my_list if i.attribute == value]

But then i thought, wouldn't it be better to write it like this?

filter(lambda x: x.attribute == value, my_list)

It's more readable, and if needed for performance the lambda could be taken out to gain something.

Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way™ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)?

Thanks in advance

Answer 1

generally filter is slightly faster if using a builtin function.

I would expect the list comprehension to be slightly faster in your case

Answer 2

I find the second way more readable. It tells you exactly what the intention is: filter the list.
PS: do not use 'list' as a variable name

Answer 3

Although filter may be the "faster way", the "Pythonic way" would be not to care about such things unless performance is absolutely critical (in which case you wouldn't be using Python!).

Answer 4

It is strange how much beauty varies for different people. I find the list comprehension much clearer than the ugly filter+lambda, but use whichever you find easier. However, do stop giving your variables names already used for builtins, that's just ugly, and not open for discussion.

There are two things that may slow down your use of filter.

The first is the function call overhead: as soon as you use a Python function (whether created by def or lambda) it is likely that filter will be slower than the list comprehension. It almost certainly is not enough to matter, and you shouldn't think much about performance until you've timed your code and found it to be a bottleneck, but the difference will be there.

The other overhead that might apply is that the lambda is being forced to access a scoped variable (value). That is slower than accessing a local variable and in Python 2.x the list comprehension only accesses local variables. If you are using Python 3.x the list comprehension runs in a separate function so it will also be accessing value through a closure and this difference won't apply.

The other option to consider is to use a generator instead of a list comprehension:

def filterbyvalue(seq, value):
   for el in seq:
       if el.attribute==value: yield el

Then in your main code (which is where readability really matters) you've replaced both list comprehension and filter with a hopefully meaningful function name.

Answer 5

This is a somewhat religious issue in Python. Even though Guido considered removing map, filter and reduce from Python 3, there was enough of a backlash that in the end only reduce was moved from built-ins to functools.reduce.

Personally I find list comprehensions easier to read. It is more explicit what is happening from the expression [i for i in list if i.attribute == value] as all the behaviour is on the surface not inside the filter function.

I would not worry too much about the performance difference between the two approaches as it is marginal. I would really only optimise this if it proved to be the bottleneck in your application which is unlikely.

Also since the BDFL wanted filter gone from the language then surely that automatically makes list comprehensions more Pythonic ;-)