This is me writing down my discussion with someone from work regarding for loop vs list comprehension in Python.

During a code review, I was commenting on a chunk of code to change from for loop to a list comprehension. We also talked about why I wanted it that way. One reason is list comprehension could be (arguably) more readable than a for loop. For a simple loop, that might be the case. Another reason is list comprehension is faster than the for loop. How fast? I have never tried comparing them before. So I made a few examples to showcase that. These are from the excellent For Loop vs List Comprehension article.

This is running on CPython 3.9.5

For loop

# for_loop_append_list.py          
output = []
MILLION_NUMBERS = list(range(1_000_000))

def for_loop():
    for element in MILLION_NUMBERS:
        output.append(element)

    return output

Then try with timeit module.

$ python -m timeit 'from for_loop_append_list import for_loop' 'for_loop()' 
5 loops, best of 5: 40.7 msec per loop  

List comprehension

# list_comprehension_append_list.py 
MILLION_NUMBERS = list(range(1_000_000))

def list_comprehension():
    return [number for number in MILLION_NUMBERS]

Running it

$ python -m timeit 'from list_comprehension_append_list import list_comprehension' 'list_comprehension()'                                  
20 loops, best of 5: 14.1 msec per loop

The numbers don’t lie. Of course, this is a very simplified example with a large number of items. It might not matter much when we are going through small numbers of n.

We continue about why someone would prefer to use for loop in some situation such as when the line is really long with if condition or when there are two list comprehensions. I agree with that. I would prefer (from the readability standpoint)

list_of_words = []
for sentence in text:
    for word in sentence:
       list_of_words.append(word)
return list_of_words

over

[word for sentence in text for word in sentence] 

With enough practice and usage, I think we can get used to the double list comprehension though so let’s see.

The same goes for the comprehension with a long line with if and else ). Maybe black will help if it’s really long.

Why is the list comprehension faster actually?

From StackOverflow

https://stackoverflow.com/questions/30245397/why-is-a-list-comprehension-so-much-faster-than-appending-to-a-list/30245465#30245465

• List comprehension is basically just a “syntactic sugar” for the regular for loop.
• In this case the reason that it performs better is because it doesn’t need to load the append attribute of the list and call it as a function at each iteration.
• In other words and in general, list comprehensions perform faster because suspending and resuming a function’s frame, or multiple functions in other cases, is slower than creating a list on demand.

Adding “learn more about python bytecodes” and dive deep into the (in)famous ceval code to my endless list of things to read!

One more thing…

Aside from all the things I wrote above, the excellent Mr @reuvenmlerner pointed out why use a list comprehension, when a “for” loop is just as good.

The basic idea is that

• We use for loop when we want to perform an action and don’t care much about the return value from the action.

for person in people:
    send_email(person)

• On the other hand, we use list comprehension when we want getting a list back, either for use directly as a list or as input to create a different data structure.

# Get usernames from Unix's /etc/passwd
[
    line.split(':')[0] for line in open('/etc/passwd') 
        if line.strip() and not line.startswith('#')
]

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