Function returns a list in Python

Question

Hi everyone! I'm new in Python, i'm trying to create a function that return a list and this is my code:
def fi(n):
list1=[]
for i in range (n+1):
if i==0 or i==1:
a=1
if i>1:
a[i]=a[i-1]+a[i-2]
list1=list1.append(a)
return list1

fi(5)

But when i run it, it returns error "'NoneType' object has no attribute 'append'". Why i got this error while i created a variable list1 that's a list?
Thanks everyone!

Answer 1

hi there are aome issues in your code:

def fi(n):
list1=[]
for i in range (n+1):
if i==0 or i==1:
a=1
if i>1:
#a[i]=a[i-1]+a[i-2] # you have to rework this ***1
a =99
list1.append(a) # <-changed!!
return list1

fi(5) #***2 use: result = fi(5)

***1 you have an integer var "a" and you try to to use an index on it. this is not possible => if you can explain what you ar trying to do here, i cold help you.
***2 you should use a call like this, otherwise you could not catch the returned list

Answer 2

append() does add the element to the list but does not have a return value (it returns None). So you don't need to reassign it to the list1 variable. Just change that line to this:
list1.append(a)
And it should work

Answer 3

Catalin, the reason why the expression

<a[i]=a[i-1]+a[i-2]>

does not create an error (at this time) is the behaviour of the interpreter, which doesn't execute this line of code because of var i is not greater than 1. So the first error in the logical flow of code sequence is

<list1=list1.append(a)>. (This should be: <list1.append(a)>)

You can trace this in debug mode. If you change condition from

<if i >1:> (temporarily) to <if True:>

the interpreter will try to execute expression <a[i]=a[i-1]+a[i-2]> which leads to an error like <int object is not subscriptable...>
In compiler languages this will not happen, because types will be checked all in advance (except user does input an unproper value at runtime.

Answer 4

Thanks very much,i understand the reason and i'm working on it!!
i'm trying to create a list of Fibonacci Sequence

Answer 5

ok, just to give you something that is essy to understand:

def fi(n):
list1=[0, 1] # this has to be pre-set
for i in range (n-2):
list1.append(list1[-1]+list1[-2]) # -1 is the last element, -2 the penultimate element in list1
return list1

print(fi(10))

Answer 6

Oh indeed i did not saw that method call fi(5). I was assuming that the iteration starts with i = 1