if-else behavior returning tuple

Question

def pair_with_target_sum(nums: Sequence[int], target: int) -> Tuple[int, int]:
    x = _pair_with_target_sum(nums, target, 0, len(nums) - 1)
    return (nums[x[0]], nums[x[1]]) if x else x

Given that _pair_with_target_sum may return None or a Tuple[int, int], the above code works as expected. However, if I remove the parentheses around the tuple in the return statement, and when x is None, the code fails with TypeError: 'NoneType' object is not subscriptable.

Why?.

Other relevant questions:

Python tuple unpacking in return statement

NoneType has no attribute IF-ELSE solution

You can see how Python breaks down the code with https://github.com/alexmojaki/birdseye — Alex Hall, Mar 01 '21 at 07:52

score 2 · Accepted Answer · answered Mar 01 '21 at 07:50

nums[x[0]], nums[x[1]] if x else x will be treated as:

nums[x[0]], (nums[x[1]] if x else x)

Proof:

>>> import ast
>>> root = ast.parse("nums[x[0]], nums[x[1]] if x else x")
>>> print(ast.dump(root, indent=1))
Module(
 body=[
  Expr(
   value=Tuple(
    elts=[
     Subscript(
      value=Name(id='nums', ctx=Load()),
      slice=Subscript(
       value=Name(id='x', ctx=Load()),
       slice=Constant(value=0),
       ctx=Load()),
      ctx=Load()),
     IfExp(
      test=Name(id='x', ctx=Load()),
      body=Subscript(
       value=Name(id='nums', ctx=Load()),
       slice=Subscript(
        value=Name(id='x', ctx=Load()),
        slice=Constant(value=1),
        ctx=Load()),
       ctx=Load()),
      orelse=Name(id='x', ctx=Load()))],
    ctx=Load()))],
 type_ignores=[])
>>>

As you can see, the elements of the tuple are Subscript (nums[x[0]]) and IfExp.

Apparently, x is None, but nums[x[0]] will attempt to index into it anyway.

In case of operator associativity and precedence, I can understand, but why does it associate this way? — Abhijit Sarkar, Mar 01 '21 at 07:55
@AbhijitSarkar, you can have a look at the [operator precedence table](https://docs.python.org/3/reference/expressions.html#operator-precedence). — ForceBru, Mar 01 '21 at 07:59
I have, there's nothing about comma or tuple in it. I'll accept your answer since it explains the behavior, but I'm curious as to why this behavior occurs. — Abhijit Sarkar, Mar 01 '21 at 08:00

if-else behavior returning tuple

1 Answers1