List comprehension vs map

Question

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

Answer 1

So since Python 3, map() is an iterator, you need to keep in mind what do you need: an iterator or list object.

As @AlexMartelli already mentioned, map() is faster than list comprehension only if you don't use lambda function.

I will present you some time comparisons.

_{Python 3.5.2 and CPython
I've used Jupiter notebook and especially %timeit built-in magic command
Measurements: s == 1000 ms == 1000 * 1000 µs = 1000 * 1000 * 1000 ns}

Setup:

x_list = [(i, i+1, i+2, i*2, i-9) for i in range(1000)]
i_list = list(range(1000))

Built-in function:

%timeit map(sum, x_list)  # creating iterator object
# Output: The slowest run took 9.91 times longer than the fastest. 
# This could mean that an intermediate result is being cached.
# 1000000 loops, best of 3: 277 ns per loop

%timeit list(map(sum, x_list))  # creating list with map
# Output: 1000 loops, best of 3: 214 µs per loop

%timeit [sum(x) for x in x_list]  # creating list with list comprehension
# Output: 1000 loops, best of 3: 290 µs per loop

lambda function:

%timeit map(lambda i: i+1, i_list)
# Output: The slowest run took 8.64 times longer than the fastest. 
# This could mean that an intermediate result is being cached.
# 1000000 loops, best of 3: 325 ns per loop

%timeit list(map(lambda i: i+1, i_list))
# Output: 1000 loops, best of 3: 183 µs per loop

%timeit [i+1 for i in i_list]
# Output: 10000 loops, best of 3: 84.2 µs per loop

There is also such thing as generator expression, see PEP-0289. So i thought it would be useful to add it to comparison

%timeit (sum(i) for i in x_list)
# Output: The slowest run took 6.66 times longer than the fastest. 
# This could mean that an intermediate result is being cached.
# 1000000 loops, best of 3: 495 ns per loop

%timeit list((sum(x) for x in x_list))
# Output: 1000 loops, best of 3: 319 µs per loop

%timeit (i+1 for i in i_list)
# Output: The slowest run took 6.83 times longer than the fastest. 
# This could mean that an intermediate result is being cached.
# 1000000 loops, best of 3: 506 ns per loop

%timeit list((i+1 for i in i_list))
# Output: 10000 loops, best of 3: 125 µs per loop

You need list object:

Use list comprehension if it's custom function, use list(map()) if there is builtin function

You don't need list object, you just need iterable one:

Always use map()!