map#

Function map applies function to each element of sequence and returns iterator with result.

For example, map can be used to perform element transformations. Convert all strings to uppercase:

In [1]: list_of_words = ['one', 'two', 'list', '', 'dict']

In [2]: map(str.upper, list_of_words)
Out[2]: <map at 0xb45eb7ec>

In [3]: list(map(str.upper, list_of_words))
Out[3]: ['ONE', 'TWO', 'LIST', '', 'DICT']

Converting to numbers:

In [3]: list_of_str = ['1', '2', '5', '10']

In [4]: list(map(int, list_of_str))
Out[4]: [1, 2, 5, 10]

With map it is convenient to use lambda expressions:

In [5]: vlans = [100, 110, 150, 200, 201, 202]

In [6]: list(map(lambda x: 'vlan {}'.format(x), vlans))
Out[6]: ['vlan 100', 'vlan 110', 'vlan 150', 'vlan 200', 'vlan 201', 'vlan 202']

If map function expects two arguments, two lists are passed:

In [7]: nums = [1, 2, 3, 4, 5]

In [8]: nums2 = [100, 200, 300, 400, 500]

In [9]: list(map(lambda x, y: x*y, nums, nums2))
Out[9]: [100, 400, 900, 1600, 2500]

List comprehension instead of map#

As a rule, you can use list comprehension instead of map. The list comprehension option is often clearer, and in some cases even faster.

Alex Martelli response with comparison of map and list comprehension

But map can be more effective when you have to generate a large number of elements because map is an iterator and list comprehension generates a list.

Examples similar to those above in list comprehension version.

Convert all strings to uppercase:

In [48]: list_of_words = ['one', 'two', 'list', '', 'dict']

In [49]: [str.upper(word) for word in list_of_words]
Out[49]: ['ONE', 'TWO', 'LIST', '', 'DICT']

Converting to numbers:

In [50]: list_of_str = ['1', '2', '5', '10']

In [51]: [int(i) for i in list_of_str]
Out[51]: [1, 2, 5, 10]

String formatting:

In [52]:  vlans = [100, 110, 150, 200, 201, 202]

In [53]: ['vlan {}'.format(x) for x in vlans]
Out[53]: ['vlan 100', 'vlan 110', 'vlan 150', 'vlan 200', 'vlan 201', 'vlan 202']

Use zip to get pairs of elements:

In [54]: nums = [1, 2, 3, 4, 5]

In [55]: nums2 = [100, 200, 300, 400, 500]

In [56]: [x * y for x, y in zip(nums, nums2)]
Out[56]: [100, 400, 900, 1600, 2500]