For loop vs List Comprehension gives me None as answer

Question

I want to append words to a list. If I conduct a for loop the answers returned are correct.

However, if I conduct a list comprehension method I get None for the answers. What am I doing wrong?

x=[]
for i in range(0,10):
    x.append('hi'+str(i))
print(x)

Answer : ['hi0', 'hi1', 'hi2', 'hi3', 'hi4', 'hi5', 'hi6', 'hi7', 'hi8', 'hi9']

x= [x.append('hi'+str(i)) for i in range(0,10)]

print(x)

Answer : [None, None, None, None, None, None, None, None, None, None]

Does this answer your question? [What does "list comprehension" mean? How does it work and how can I use it?](https://stackoverflow.com/questions/34835951/what-does-list-comprehension-mean-how-does-it-work-and-how-can-i-use-it) — ThePyGuy, May 17 '21 at 15:21
Don't use `x.append` in the list comprehension, just use `"hi"+str(i)`. You just spell out what element makes up the list. The `append` returns nothing (i.e., `None`) and hence you end up with a list full of `None`s. — Robert, May 17 '21 at 15:57

score 1 · Accepted Answer · answered May 17 '21 at 15:21

1

You don't need to use append as you are inside the list and append does return None in all cases.

x= ['hi'+str(i) for i in range(0,10)]

print(x)

answered May 17 '21 at 15:21

Mohamed Sayed

491
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Ahh yes, because I am within the list already. Thank you. – zlk2000 May 17 '21 at 15:29

score 1 · Answer 2 · answered May 17 '21 at 15:22

1

Your x = [x.append('hi'+str(i)) for i in range(0,10)], build a list with the result of each x.append call, which is None, as it is done inplace, then you erase x with that new list

You want

x = [f"hi{i}" for i in range(10)]

answered May 17 '21 at 15:22

azro

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7
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For loop vs List Comprehension gives me None as answer

2 Answers2