Why does this code output a list of "None"?

Question

fibonacci = [0, 1]
fibonacci = [fibonacci.append(fibonacci[i - 1] + fibonacci[i - 2]) for i in range(2, 11)]
print(fibonacci)

This code is supposed to print a list of the first 10 fibonacci numbers. I copied it from another question, but it doesn't seem to work. Why is this?

Because you're putting the result of `append` in a list, and `append` returns None. Either use `append` _or_ use a list comprehension. — khelwood, Aug 01 '20 at 18:59
You can get it to work by not assigning fibonacci to the list comprehension i.e. just have `[fibonacci.append(fibonacci[i - 1] + fibonacci[i - 2]) for i in range(2, 11)]` though it's discouraged to use list comprehension just for side effects. — DarrylG, Aug 01 '20 at 19:03

score 2 · Answer 1 · answered Aug 01 '20 at 19:08

2

Unless you love one-liners, in my opinion it makes more sense to write it this way:

fibonacci = [0, 1]
for i in range(2, 11):
    fibonacci.append(fibonacci[i - 1] + fibonacci[i - 2])

answered Aug 01 '20 at 19:08

Jannes Carpentier

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score 1 · Accepted Answer · answered Aug 01 '20 at 19:02

Append will return None. So remove the assignment to the same variable. It will work.

In [11]: fibonacci = [0, 1] 
    ...: [fibonacci.append(fibonacci[i - 1] + fibonacci[i - 2]) for i in range(2, 11)]                                                                                                                      
Out[11]: [None, None, None, None, None, None, None, None, None]

In [12]: print(fibonacci)                                                                                                                                                                                   
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

Why does this code output a list of "None"?

2 Answers2