You (Probably) Don’t Need For-Loops

Daw-Ran Liou
Nov 17, 2016 · 5 min read

What you gain by not writing for-loops everywhere

with ...:
for ...:
if ...:
try:
except:
else:

“Flat is better than nested” — The Zen of Python

Tools you can use to avoid using for-loops

1. List Comprehension / Generator Expression

result = []
for item in item_list:
new_item = do_something_with(item)
result.append(new_item)
result = [do_something_with(item) for item in item_list]
result = (do_something_with(item) for item in item_list)

2. Functions

doubled_list = map(lambda x: x * 2, old_list)
from functools import reduce
summation = reduce(lambda x, y: x + y, numbers)
>>> a = list(range(10))
>>> a
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> all(a)
False
>>> any(a)
True
>>> max(a)
9
>>> min(a)
0
>>> list(filter(bool, a))
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> set(a)
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
>>> dict(zip(a,a))
{0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8, 9: 9}
>>> sorted(a, reverse=True)
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> str(a)
‘[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]’
>>> sum(a)
45

3. Extract Functions or Generators

results = []
for item in item_list:
# setups
# condition
# processing
# calculation
results.append(result)
def process_item(item):
# setups
# condition
# processing
# calculation
return result
results = [process_item(item) for item in item_list]
results = []
for i in range(10):
for j in range(i):
results.append((i, j))
results = [(i, j)
for i in range(10)
for j in range(i)]
# finding the max prior to the current item
a = [3, 4, 6, 2, 1, 9, 0, 7, 5, 8]
results = []
current_max = 0
for i in a:
current_max = max(i, current_max)
results.append(current_max)
# results = [3, 4, 6, 6, 6, 9, 9, 9, 9, 9]
def max_generator(numbers):
current_max = 0
for i in numbers:
current_max = max(i, current_max)
yield current_max
a = [3, 4, 6, 2, 1, 9, 0, 7, 5, 8]
results = list(max_generator(a))

4. Don’t write it yourself. itertools got you covered

from itertools import accumulate
a = [3, 4, 6, 2, 1, 9, 0, 7, 5, 8]
results = list(accumulate(a, max))

Conclusion

Action

Python Pandemonium

A place to read and write about all things Python. If you want to become a writer for this publication then let me know.

Daw-Ran Liou

Written by

Developer

Python Pandemonium

A place to read and write about all things Python. If you want to become a writer for this publication then let me know.