"'NoneType' object is not subscriptable" error

Question

I am new to python and I have a problem with this code. I dont understand why I can not run the code: print(m[1][1])

I always get this error message: TypeError: 'NoneType' object is not subscriptable

    edges = [(1,2), (2,7), (1,3), (2,4), (4,7), (3,5), (4,5), (5,6), (6,7), (1,8), (5,8), (6,9), (7,9), (9,10), (5,10), (8,10)]`

    def generateAdjMatrix(edges):
    
        max_knoten = max(max(edges))
        
        matrix = [[0 for i in range(max_knoten)] for j in range(max_knoten)]
        
        for kante in edges:
            matrix[kante[0]-1][kante[1]-1] = 1
    pass
    
    if __name__ == '__main__':
        m = generateAdjMatrix(edges)
        print(m[1][1])
    
    pass

Answer 1

m = generateAdjMatrix(edges)

You are calling this function, and this function was declared with this body:

max_knoten = max(max(edges))
    
matrix = [[0 for i in range(max_knoten)] for j in range(max_knoten)]
    
for kante in edges:
    matrix[kante[0]-1][kante[1]-1] = 1

Do you see any return? No, because your function is the equivalent to a C/C++ void function.

So what is being put into m? Nothing, which in Python is None, an object of type NoneType.

---

In general, while programming in Python, if you get an error message like this:

TypeError: 'NoneType' object is not subscriptable

you have to check where does the variable you are subscripting come from.

Answer 2

m is None after m = generateAdjMatrix(edges) because generateAdjMatrix doesn't explicitly return anything.

See Defining Functions in the documentation:

Coming from other languages, you might object that fib is not a function but a procedure since it doesn’t return a value. In fact, even functions without a return statement do return a value, albeit a rather boring one. This value is called None (it’s a built-in name).