Why is a NumPy int not an instance of a Python int, but a NumPy float is an instance of a Python float?

Question

Consider the following:

>>> import numbers
>>> import numpy
>>> a = numpy.int_(0)
>>> isinstance(a, int)
False
>>> isinstance(a, numbers.Integral)
True
>>> b = numpy.float_(0)
>>> isinstance(b, float)
True
>>> isinstance(b, numbers.Real)
True

NumPy's numpy.int_ and numpy.float_ types are both in Python's numeric abstract base class hierarchy, but it is strange to me that a np.int_ object is not an instance of the built-in int class, while a np.float_ object is an instance of the built-in float type.

Why is this the case?

Answer 1

Python integers can be arbitrary length: type(10**1000) is still int, and will print out a one and then a thousand zeros on your screen if you output it.

Numpy int64 (which is what int_ is on my machine) are integers represented by 8 bytes (64 bits), and anything over that cannot be represented. For example, np.int_(10)**1000 will give you a wrong answer - but quickly ;).

Thus, they are different kinds of numbers; subclassing one under the other makes as much sense as subclassing int under float would, is what I assume numpy people thought. It is best to keep them separate, so that no-one is confused about the fact that it would be unwise to confuse them.

The split is done because arbitrary-size integers are slow, while numpy tries to speed up computation by sticking to machine-friendly types.

On the other hand, floating point is the standard IEEE floating point, both in Python and in numpy, supported out-of-the-box by our processors.