TypeError: object of type 'int' has no len() in python

Question

Here is my code first:

res = []

for x in range(9):

    if x == 0:
        res.append([1])
    elif x == 1:
        res.append([1,1])
        print(res)
    else:
        l=[]
        for y in range(len(res[x-1])):
            if y == 0:
                l.append(1)
            else:
                l.append(res[x-1][y] + res[x-1][y-1])
    l.append(1)
    res.append(1)

and I meet the problem. Then I change a little bit of code like this:

res = [[1],[1,1]]

for i in range(2,9):
res.append([1])
for j in range(len(res[i-1])):
    if j > 0:
        res[i].append(int(res[i-1][j-1]+res[i-1][j]))
res[i].append(1)
print(res)

and the error has gone. I am quite wondering what is wrong inside?

Answer 1

Let's say you have my_var = 5 . By running len(my_var) you will get an error, because variable my_var is an int (short for an integer).

However, if you have my_var = [5] , your command len(my_var) will work because it is a list.

Similarly in your code, as khelwood says, if you have l.append(1) , your variable l will contain an integer 1. This will raise an error if you call len() on this element. If you want to append something to a list then find its length later, make sure it is not an int but a list, try l append([1]) .