pythonnumbersdecimalisinstance

Why isn't 'decimal.Decimal(1)' an instance of 'numbers.Real'?


I am trying to check if an object is an instance of a number of any type (int, float, Fraction, Decimal, etc.).

I came across this question and its answer: How to properly use python's isinstance() to check if a variable is a number?

However, I would like to exclude complex numbers such as 1j.

The class numbers.Real looked perfect but it returns False for Decimal numbers.

from numbers Real
from decimal import Decimal

print(isinstance(Decimal(1), Real))
# False

In contradiction, it works fine with Fraction(1), for example.

The documentation describes some operations which should work with the number. I tested them without any error on a decimal instance. Moreover, decimal objects cannot contain complex numbers.

So, why should isinstance(Decimal(1), Real) return False?


Solution

  • So, I found the answer directly in the source code of cpython/numbers.py:

    ## Notes on Decimal
    ## ----------------
    ## Decimal has all of the methods specified by the Real abc, but it should
    ## not be registered as a Real because decimals do not interoperate with
    ## binary floats (i.e.  Decimal('3.14') + 2.71828 is undefined).  But,
    ## abstract reals are expected to interoperate (i.e. R1 + R2 should be
    ## expected to work if R1 and R2 are both Reals).
    

    Indeed, adding Decimal to float would raise a TypeError.

    In my point of view, it violates the principle of least astonishment, but it does not matter much.

    As a workaround, I use:

    import numbers
    import decimal
    
    Real = (numbers.Real, decimal.Decimal)
    
    print(isinstance(decimal.Decimal(1), Real))
    # True