Python's Insert Returning None

Python's insert returning None?

Mutating-methods on lists tend to return None, not the modified list as you expect -- such metods perform their effect by altering the list in-place, not by building and returning a new one. So, print numbers instead of print clean will show you the altered list.

If you need to keep numbers intact, first you make a copy, then you alter the copy:

clean = list(numbers)
clean.insert(3, 'four')

this has the overall effect you appear to desire: numbers is unchanged, clean is the changed list.

Why does insert change my list to None type?

It's an in-place insertion with no return value. Use this, instead.

animals.insert(2, "cobra")
print(animals) 

The insert function is returning None

insert() modifies the list in place but does not return a value. Typically with functional programming you avoid side effects and treat most data structures as immutable, so insert() is probably the wrong choice here. Instead of mututaing the returned list, you can simply return the result of recursion along with the new value:

def dectob(num):
    if num==0:
        return []
    hnum = int(num/2)
    if num - hnum == hnum:
        asn="0"
    else:
        asn="1"
    return dectob(hnum) + [asn]

int(''.join(dectob(6666))) == 1101000001010
# True

Why does append() always return None in Python?

append is a mutating (destructive) operation (it modifies the list in place instead of of returning a new list). The idiomatic way to do the non-destructive equivalent of append would be

>>> l = [1,2,3]
>>> l + [4]
[1,2,3,4]
>>> l
[1,2,3]

to answer your question, my guess is that if append returned the newly modified list, users might think that it was non-destructive, ie they might write code like

m = l.append("a")
n = l.append("b")

and expect n to be [1,2,3,"b"]

Add an element to a list, the output is none

Adding to the replies that you got, if it is the list b that needs to be updated but not the list a, you should proceed as such:

a =[1,2,3]
b = list(a)
b.insert(1,20)
print(a, b)

Trouble inserting in Python (None vs. empty list)

list.insert returns None, not the modified list.

This requires helper to be changed to read

def helper(list1, list2, result):
  if not list1:
    return result + list2
  elif not list2:
    return result + list1
  elif list1[0] < list2[0]:
    return helper(list1[1:], list2, result + [list1[0]])
  else:
    return helper(list1, list2[1:], result + [list2[0]])

Note the changes to the two base cases. None and the empty list [] are not the same thing. The pythonic way of testing if a list is empty is to treat the list as a Boolean value: empty lists are False, all others are True.

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And as the others have noticed before me, you need to explicitly return the return value of helper in linear_merge.

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