List comprehension vs map

Is there a reason to prefer using map() over list comprehension or vice versa? Is either of them generally more efficient or considered generally more pythonic than the other?

Solution 1:

map may be microscopically faster in some cases (when you're NOT making a lambda for the purpose, but using the same function in map and a listcomp). List comprehensions may be faster in other cases and most (not all) pythonistas consider them more direct and clearer.

An example of the tiny speed advantage of map when using exactly the same function:

$ python -m timeit -s'xs=range(10)' 'map(hex, xs)'
100000 loops, best of 3: 4.86 usec per loop
$ python -m timeit -s'xs=range(10)' '[hex(x) for x in xs]'
100000 loops, best of 3: 5.58 usec per loop

An example of how performance comparison gets completely reversed when map needs a lambda:

$ python -m timeit -s'xs=range(10)' 'map(lambda x: x+2, xs)'
100000 loops, best of 3: 4.24 usec per loop
$ python -m timeit -s'xs=range(10)' '[x+2 for x in xs]'
100000 loops, best of 3: 2.32 usec per loop

Solution 2:

Cases

• Common case: Almost always, you will want to use a list comprehension in python because it will be more obvious what you're doing to novice programmers reading your code. (This does not apply to other languages, where other idioms may apply.) It will even be more obvious what you're doing to python programmers, since list comprehensions are the de-facto standard in python for iteration; they are expected.
• Less-common case: However if you already have a function defined, it is often reasonable to use map, though it is considered 'unpythonic'. For example, map(sum, myLists) is more elegant/terse than [sum(x) for x in myLists]. You gain the elegance of not having to make up a dummy variable (e.g. sum(x) for x... or sum(_) for _... or sum(readableName) for readableName...) which you have to type twice, just to iterate. The same argument holds for filter and reduce and anything from the itertools module: if you already have a function handy, you could go ahead and do some functional programming. This gains readability in some situations, and loses it in others (e.g. novice programmers, multiple arguments)... but the readability of your code highly depends on your comments anyway.
• Almost never: You may want to use the map function as a pure abstract function while doing functional programming, where you're mapping map, or currying map, or otherwise benefit from talking about map as a function. In Haskell for example, a functor interface called fmap generalizes mapping over any data structure. This is very uncommon in python because the python grammar compels you to use generator-style to talk about iteration; you can't generalize it easily. (This is sometimes good and sometimes bad.) You can probably come up with rare python examples where map(f, *lists) is a reasonable thing to do. The closest example I can come up with would be sumEach = partial(map,sum), which is a one-liner that is very roughly equivalent to:

def sumEach(myLists):
    return [sum(_) for _ in myLists]

• Just using a for-loop: You can also of course just use a for-loop. While not as elegant from a functional-programming viewpoint, sometimes non-local variables make code clearer in imperative programming languages such as python, because people are very used to reading code that way. For-loops are also, generally, the most efficient when you are merely doing any complex operation that is not building a list like list-comprehensions and map are optimized for (e.g. summing, or making a tree, etc.) -- at least efficient in terms of memory (not necessarily in terms of time, where I'd expect at worst a constant factor, barring some rare pathological garbage-collection hiccuping).

"Pythonism"

I dislike the word "pythonic" because I don't find that pythonic is always elegant in my eyes. Nevertheless, map and filter and similar functions (like the very useful itertools module) are probably considered unpythonic in terms of style.

Laziness

In terms of efficiency, like most functional programming constructs, MAP CAN BE LAZY, and in fact is lazy in python. That means you can do this (in python3) and your computer will not run out of memory and lose all your unsaved data:

>>> map(str, range(10**100))
<map object at 0x2201d50>

Try doing that with a list comprehension:

>>> [str(n) for n in range(10**100)]
# DO NOT TRY THIS AT HOME OR YOU WILL BE SAD #

Do note that list comprehensions are also inherently lazy, but python has chosen to implement them as non-lazy. Nevertheless, python does support lazy list comprehensions in the form of generator expressions, as follows:

>>> (str(n) for n in range(10**100))
<generator object <genexpr> at 0xacbdef>

You can basically think of the [...] syntax as passing in a generator expression to the list constructor, like list(x for x in range(5)).

Brief contrived example

from operator import neg
print({x:x**2 for x in map(neg,range(5))})

print({x:x**2 for x in [-y for y in range(5)]})

print({x:x**2 for x in (-y for y in range(5))})

List comprehensions are non-lazy, so may require more memory (unless you use generator comprehensions). The square brackets [...] often make things obvious, especially when in a mess of parentheses. On the other hand, sometimes you end up being verbose like typing [x for x in.... As long as you keep your iterator variables short, list comprehensions are usually clearer if you don't indent your code. But you could always indent your code.

print(
    {x:x**2 for x in (-y for y in range(5))}
)

or break things up:

rangeNeg5 = (-y for y in range(5))
print(
    {x:x**2 for x in rangeNeg5}
)

Efficiency comparison for python3

map is now lazy:

% python3 -mtimeit -s 'xs=range(1000)' 'f=lambda x:x' 'z=map(f,xs)'
1000000 loops, best of 3: 0.336 usec per loop            ^^^^^^^^^

Therefore if you will not be using all your data, or do not know ahead of time how much data you need, map in python3 (and generator expressions in python2 or python3) will avoid calculating their values until the last moment necessary. Usually this will usually outweigh any overhead from using map. The downside is that this is very limited in python as opposed to most functional languages: you only get this benefit if you access your data left-to-right "in order", because python generator expressions can only be evaluated the order x[0], x[1], x[2], ....

However let's say that we have a pre-made function f we'd like to map, and we ignore the laziness of map by immediately forcing evaluation with list(...). We get some very interesting results:

% python3 -mtimeit -s 'xs=range(1000)' 'f=lambda x:x' 'z=list(map(f,xs))'                                                                                                                                                
10000 loops, best of 3: 165/124/135 usec per loop        ^^^^^^^^^^^^^^^
                    for list(<map object>)

% python3 -mtimeit -s 'xs=range(1000)' 'f=lambda x:x' 'z=[f(x) for x in xs]'                                                                                                                                      
10000 loops, best of 3: 181/118/123 usec per loop        ^^^^^^^^^^^^^^^^^^
                    for list(<generator>), probably optimized

% python3 -mtimeit -s 'xs=range(1000)' 'f=lambda x:x' 'z=list(f(x) for x in xs)'                                                                                                                                    
1000 loops, best of 3: 215/150/150 usec per loop         ^^^^^^^^^^^^^^^^^^^^^^
                    for list(<generator>)

In results are in the form AAA/BBB/CCC where A was performed with on a circa-2010 Intel workstation with python 3.?.?, and B and C were performed with a circa-2013 AMD workstation with python 3.2.1, with extremely different hardware. The result seems to be that map and list comprehensions are comparable in performance, which is most strongly affected by other random factors. The only thing we can tell seems to be that, oddly, while we expect list comprehensions [...] to perform better than generator expressions (...), map is ALSO more efficient that generator expressions (again assuming that all values are evaluated/used).

It is important to realize that these tests assume a very simple function (the identity function); however this is fine because if the function were complicated, then performance overhead would be negligible compared to other factors in the program. (It may still be interesting to test with other simple things like f=lambda x:x+x)

If you're skilled at reading python assembly, you can use the dis module to see if that's actually what's going on behind the scenes:

>>> listComp = compile('[f(x) for x in xs]', 'listComp', 'eval')
>>> dis.dis(listComp)
  1           0 LOAD_CONST               0 (<code object <listcomp> at 0x2511a48, file "listComp", line 1>) 
              3 MAKE_FUNCTION            0 
              6 LOAD_NAME                0 (xs) 
              9 GET_ITER             
             10 CALL_FUNCTION            1 
             13 RETURN_VALUE         
>>> listComp.co_consts
(<code object <listcomp> at 0x2511a48, file "listComp", line 1>,)
>>> dis.dis(listComp.co_consts[0])
  1           0 BUILD_LIST               0 
              3 LOAD_FAST                0 (.0) 
        >>    6 FOR_ITER                18 (to 27) 
              9 STORE_FAST               1 (x) 
             12 LOAD_GLOBAL              0 (f) 
             15 LOAD_FAST                1 (x) 
             18 CALL_FUNCTION            1 
             21 LIST_APPEND              2 
             24 JUMP_ABSOLUTE            6 
        >>   27 RETURN_VALUE

>>> listComp2 = compile('list(f(x) for x in xs)', 'listComp2', 'eval')
>>> dis.dis(listComp2)
  1           0 LOAD_NAME                0 (list) 
              3 LOAD_CONST               0 (<code object <genexpr> at 0x255bc68, file "listComp2", line 1>) 
              6 MAKE_FUNCTION            0 
              9 LOAD_NAME                1 (xs) 
             12 GET_ITER             
             13 CALL_FUNCTION            1 
             16 CALL_FUNCTION            1 
             19 RETURN_VALUE         
>>> listComp2.co_consts
(<code object <genexpr> at 0x255bc68, file "listComp2", line 1>,)
>>> dis.dis(listComp2.co_consts[0])
  1           0 LOAD_FAST                0 (.0) 
        >>    3 FOR_ITER                17 (to 23) 
              6 STORE_FAST               1 (x) 
              9 LOAD_GLOBAL              0 (f) 
             12 LOAD_FAST                1 (x) 
             15 CALL_FUNCTION            1 
             18 YIELD_VALUE          
             19 POP_TOP              
             20 JUMP_ABSOLUTE            3 
        >>   23 LOAD_CONST               0 (None) 
             26 RETURN_VALUE

>>> evalledMap = compile('list(map(f,xs))', 'evalledMap', 'eval')
>>> dis.dis(evalledMap)
  1           0 LOAD_NAME                0 (list) 
              3 LOAD_NAME                1 (map) 
              6 LOAD_NAME                2 (f) 
              9 LOAD_NAME                3 (xs) 
             12 CALL_FUNCTION            2 
             15 CALL_FUNCTION            1 
             18 RETURN_VALUE 

It seems it is better to use [...] syntax than list(...). Sadly the map class is a bit opaque to disassembly, but we can make due with our speed test.

Solution 3:

Python 2: You should use map and filter instead of list comprehensions.

An objective reason why you should prefer them even though they're not "Pythonic" is this:
They require functions/lambdas as arguments, which introduce a new scope.

I've gotten bitten by this more than once:

for x, y in somePoints:
    # (several lines of code here)
    squared = [x ** 2 for x in numbers]
    # Oops, x was silently overwritten!

but if instead I had said:

for x, y in somePoints:
    # (several lines of code here)
    squared = map(lambda x: x ** 2, numbers)

then everything would've been fine.

You could say I was being silly for using the same variable name in the same scope.

I wasn't. The code was fine originally -- the two xs weren't in the same scope.
It was only after I moved the inner block to a different section of the code that the problem came up (read: problem during maintenance, not development), and I didn't expect it.

Yes, if you never make this mistake then list comprehensions are more elegant.
But from personal experience (and from seeing others make the same mistake) I've seen it happen enough times that I think it's not worth the pain you have to go through when these bugs creep into your code.

Conclusion:

Use map and filter. They prevent subtle hard-to-diagnose scope-related bugs.

Side note:

Don't forget to consider using imap and ifilter (in itertools) if they are appropriate for your situation!

Solution 4:

Actually, map and list comprehensions behave quite differently in the Python 3 language. Take a look at the following Python 3 program:

def square(x):
    return x*x
squares = map(square, [1, 2, 3])
print(list(squares))
print(list(squares))

You might expect it to print the line "[1, 4, 9]" twice, but instead it prints "[1, 4, 9]" followed by "[]". The first time you look at squares it seems to behave as a sequence of three elements, but the second time as an empty one.

In the Python 2 language map returns a plain old list, just like list comprehensions do in both languages. The crux is that the return value of map in Python 3 (and imap in Python 2) is not a list - it's an iterator!

The elements are consumed when you iterate over an iterator unlike when you iterate over a list. This is why squares looks empty in the last print(list(squares)) line.

To summarize:

• When dealing with iterators you have to remember that they are stateful and that they mutate as you traverse them.
• Lists are more predictable since they only change when you explicitly mutate them; they are containers.
• And a bonus: numbers, strings, and tuples are even more predictable since they cannot change at all; they are values.

Solution 5:

If you plan on writing any asynchronous, parallel, or distributed code, you will probably prefer map over a list comprehension -- as most asynchronous, parallel, or distributed packages provide a map function to overload python's map. Then by passing the appropriate map function to the rest of your code, you may not have to modify your original serial code to have it run in parallel (etc).