BUG: `isinstance(array, SupportsFloat)` is `True` but `float(array)` raises

[d-perl]

Describe the issue:

Even when an array cannot be converted to a float, it still passes an isinstance(_, SupportsFloat) check.

>>> a = np.array([1.0,2.0,3.0])
>>> float(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: only length-1 arrays can be converted to Python scalars

>>> a = np.array([1.0])
>>> float(a)
<stdin>:1: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
1.0

Reproduce the code example:

import numpy as np
from typing import SupportsFloat

a = np.array([1.0, 2.0, 3.0])
isinstance(a, SupportsFloat) # True
float(a) # raises

Error message:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: only length-1 arrays can be converted to Python scalars

Python and NumPy Versions:

2.2.6
3.12.10 (main, May 21 2025, 10:38:42) [GCC 15.1.1 20250425]

Runtime Environment:

No response

Context for the issue:

Just affects tidiness, easy enough to work around, not high priority. I just thought it should be documented. I suppose this will be resolved anyway whenever this deprecated feature is removed and array doesn't have __float__ any longer.

[jorenham]

It only works for 0d arrays

>>> import numpy as np
>>> float(np.array(3.14))
3.14