为什么'decimal.Decimal(1)'不是'numbers.Real'的一个实例?
问题描述
我尝试检查一个变量是否是任何类型(int、float、Fraction、十进制等).
I try to check if a variable is an instance of a number of any type (int, float, Fraction, Decimal, etc.).
我遇到了这个问题及其答案:如何正确使用 python 的 isinstance() 检查变量是否为数字?
I cam accross this question and its answer: How to properly use python's isinstance() to check if a variable is a number?
但是，我想排除复数，例如 1j.
However, I would like to exclude complex numbers such as 1j.
类 numbers.Real 看起来很完美，但它为 <返回 Falsecode>十进制 数字...
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
相反，它适用于 Fraction(1) 例如.
In contradiction, it works fine with Fraction(1) for example.
文档描述了一些应该与数字，我在十进制实例上测试它们没有任何错误.此外，十进制对象不能包含复数.
The documentation describes some operations which should work with the number, I tested them without any error on a decimal instance. Decimal objects cannot contains complex numbers moreover.
那么，为什么 isinstance(Decimal(1), Real) 会返回 False?
So, why isinstance(Decimal(1), Real) would return False?
解决方案
所以，我直接在cpython/numbers.py:
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).
确实，将 Decimal 添加到 float 会引发 TypeError.
Indeed, adding Decimal to float would raise a TypeError.
在我看来，它违反了最小惊讶的原则，但没关系.
In my point of view, it violates the principle of least astonishment, but it does not matter much.
作为一种解决方法，我使用:
As a workaround, I use:
import numbers
import decimal
Real = (numbers.Real, decimal.Decimal)
print(isinstance(decimal.Decimal(1), Real))
# True
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