N

Comprehensions

A typical task in Python is to iterate over a list, run some function on each value, and save the results into a new list.

Click here to view code image

# create a list
l = [1, 2, 3, 4, 5]

# list of newly calculated results
r = []

# iterate over the list
for i in l:
  # square each number and add the new value to a new list
  r.append(i ** 2)

print(r)

[1, 4, 9, 16, 25]

Unfortunately, this approach requires a few lines of code to do a relatively simple task. One way to rewrite this loop more compactly is by using a Python list comprehension. This shortcut offers a concise way of performing the same action.

Click here to view code image

# note the square brackets around on the right-hand side
# this saves the final results as a list
rc = [i ** 2 for i in l]
print(rc)

[1, 4, 9, 16, 25]

print(type(rc))

<class 'list'>

Our final results will be a list, so the right-hand side will have a pair of square brackets. From there, we write what looks very similar to a for loop. Starting from the center and moving toward the right side, we write for i in l, which is very similar to the first line of our original for loop. On the right side, we write i ** 2, which is similar to the body of the for loop. Since we are using a list comprehension, we no longer need to specify the list to which we want to append our new values.