虚假解析/评估复杂文字 [英] Bogus parsing/eval of complex literals

问题描述

在评估复数时，python喜欢摆弄符号.

When evaluating complex numbers, python likes to fiddle the signs.

>>> -0j
(-0-0j)
>>> (-0-0j)
0j

为什么?

nb:在阅读这个问题时，我注意到了.

nb: I noticed it when reading this question.

推荐答案

这里的问题是Python不会将诸如(-0-0j)之类的复数解析为文字，它们实际上被解析为一个表达式:

The issue here is that Python doesn't parse complex numbers such as (-0-0j) as literals, they are actually parsed as an expression:

>>> import ast
>>> ast.dump(ast.parse('(-0-0j)'))
'Module(body=[Expr(value=BinOp(left=UnaryOp(op=USub(), operand=Num(n=0)), op=Sub(), right=Num(n=0j)))])'

所以，这不是一个复杂的文字，而是一个反映的减去一个复杂和一个整数.

So, this is not a complex literal but a reflected subtraction of a complex and an integer.

>>> -0-0j
0j
>>> (0j).__rsub__((0).__neg__())
0j

将int部分强制为具有0j复杂分量，然后我们丢失预期的签名零，因为减去了复杂的分量. 0j - 0j的结果应带有正号，例如 IEEE 754-2008 规定.

The int part is coerced to having a 0j complex component, and then we lose the expected signed zero from the result because of the subtraction of the complex components. The result of 0j - 0j should have positive sign, as IEEE 754-2008 dictates.

这可以说是解析器问题，因为零的符号会影响方程的解.但是，问题一直是重复 已关闭为'不是错误"，因此看起来该行为不会很快消失.当您关心带符号的零时，初始化复数的可靠方法是调用

This is arguably a parser issue, because the sign of the zero can influence the solutions of equations. However, the issue has been repeatedly raised and closed on the python tracker as 'not a bug', so it doesn't look like that behaviour will be going away any time soon. The reliable way to initialize complex numbers when you care about signed zeros is by calling the complex built-in:

>>> 0-0j
0j
>>> 0+0j
0j
>>> complex(0., -0.)
-0j
>>> complex(0., +0.)
0j

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