Can not give an empty list to Python function throws "TypeError"

Question

The function is supposed to print out all the bit vetcors that are n bits long and contain k 1's. The problem is that I can not give an empty list as an input to the function.

I am working on Ubuntu and using Python 3.6. I have also tried to use a non empty list like [1], but this throws the same error. When trying to find a length of an empty list outside of the function, it works as it should: len([]) returns 0.

Here is my code:

def bitvector(n, k, vektor):
    if len(vektor) == n:
        if vektor.count(1) == k:
            print(vektor)
    bitvector(n, k, vektor.append(0))
    bitvector(n, k, vektor.append(1))


bitvector(1, 2 , [])

EDIT:

def bitvector(n, k, vektor):
    if len(vektor) > n:
        return
    if len(vektor) == n:
        print("test")
        if vektor.count(1) == k:
            print(vektor)
            return
    bitvector(n, k, vektor.append(1))
    bitvector(n, k, vektor.append(0))


bitvector(1, 2 , [])

It is supposed to get the length of the list, but instead it throws the following error:

TypeError: object of type 'NoneType' has no len()

Answer 1

Since I was not limited to use a certain Type, the I chose to use a string instead of the list.

def bitvector(n, k, vektor):
    vektor_list = list(vektor)
    if len(vektor) > n:
        return
    if len(vektor) == n:
        if vektor_list.count("1") == k:
            print(vektor)
            return
    bitvector(n, k, vektor + "0")
    bitvector(n, k, vektor + "1")

EDIT, optimized my code:

def bitvector(n, k, vektor):
vektor_list = list(vektor)
if vektor_list.count("1") > k:
    return
if len(vektor) == n:
    if vektor_list.count("1") == k:
        print(vektor)
        return
    else:
        return
bitvector(n, k, vektor + "0")
bitvector(n, k, vektor + "1")