为什么'decimal.Decimal(1)'不是'numbers.Real'的实例? [英] Why is not 'decimal.Decimal(1)' an instance of 'numbers.Real'?

问题描述

我尝试检查变量是否是任何类型(int，float，Fraction，Decimal等)的实例.

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，我想排除诸如1j的复数.

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

类 numbers.Real 看起来很完美，但返回了False用于 Decimal 数字...

from numbers Real
from decimal import Decimal

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

矛盾的是，它与 Fraction(1) 正常工作例如.

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

文档描述了一些应与数字，我在十进制实例上对它们进行了测试，没有任何错误. 此外，小数对象不能包含复数.

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)将返回False?

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

推荐答案

因此，我直接在

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).

实际上，在float上添加Decimal会产生一个TypeError.

Indeed, adding Decimal to float would raise a TypeError.

在我看来，它违反了最小惊讶原则，但没什么大不了.

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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