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Python 3: An interactive deep dive

List Comprehensions

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A list comprehension provides a compact way of mapping a list into another list by applying a function to each of the elements of the list.

Python 3.5
a_list = [1, 9, 8, 4]
print ([elem * 2 for elem in a_list] )          #①
#[2, 18, 16, 8]
print (a_list)                                  #②
#[1, 9, 8, 4]
a_list = [elem * 2 for elem in a_list]          #③
print (a_list)
#[2, 18, 16, 8]

① To make sense of this, look at it from right to left. a_list is the list you’re mapping. The Python interpreter loops through a_list one element at a time, temporarily assigning the value of each element to the variable elem. Python then applies the function elem * 2 and appends that result to the returned list.

② A list comprehension creates a new list; it ...