bpo-34049: classify abs() argument type by Windsooon · Pull Request #8103 · python/cpython
Just like callable() method in https://docs.python.org/3/library/functions.html#callable
...instances are callable if their class has a call() method.
I think we should also mention this in abs(),
abs(x)
Return the absolute value of a number. The argument may be an integer or a floating point number. If the argument is a complex number, its magnitude is returned. If x defines abs, abs(x) returns x.abs().