Type hint in Python tuples

Question

I have the code below, I understand that the function takes 2 integers, so (n : int, m : int), and it returns a tuple with integers of any length, therefore -> tuple[int,...]:

I think this also correct for an empty tuple T : tuple[()] since the tuple at some point in the function has only one value.

However I get this error

main.py:14: error: Incompatible types in assignment (expression has type "Tuple[int]", variable has type "Tuple[]")

What am I missing?


def vectorise(n : int, m : int) -> tuple[int,...]:
  '''creates a tuple of element n repeated m times'''
  T : tuple[()] = () #empty tuple
  counter : int = 0
  while counter < m:
    T = T + (n,) #in Python (n,) is a tuple with one element n
    counter+=1
  #as a result, T can be of arbitrary length
  return T

L = vectorise(100, 5)

Why not start as you mean to go on - T: tuple[int, ...] = .... — jonrsharpe
– jonrsharpe, Commented Nov 2, 2021 at 11:39
"What am I missing?": The type for T needs to at least be equivalent (able to encompass) the intended return type. As you have hinted T to be an EMPTY tuple, you cannot then return a different type - hence the type incompatibility error. You could instead add the OTHER hint for the return type e.g. tuple[int,...] | tuple[()] to cater for both eventualities. — Android Control
– Android Control, Commented Oct 22, 2023 at 16:42

Jörg W Mittag · Accepted Answer · 2021-11-02 11:41:36Z

2

The variable T has the type "empty tuple". You are trying to assign something to it which is not an empty tuple. Therefore, you get a type error.

You either need to make sure that you are only assigning empty tuples to T, or make sure that the type of T admits non-empty tuples.

answered Nov 2, 2021 at 11:41

Jörg W Mittag

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