指数为 Python 的负数

Question

为什么 Python 会给我 2 个不同的答案？

print(-1**2)   # prints -1 
print((-1)**2) # prints 1

Answer 1

Python 按以下顺序解析 print(-1**2)：

所以就变成了这样：

>>> -(1 ** 2)
-1
>>>

因为 -1 转换为 print(0 - 1 ** 2)。但在运算顺序上table，**（指数）比-.

排名靠前

因此，Python数学遵循常规数学中的运算顺序，**更强大（优先于）-。

如 Wikipedia 所述：

The acronym PEMDAS is common. It stands for Parentheses, Exponents, Multiplication/Division, Addition/Subtraction.

Python Docs上也提到：

Operator	Description
(expressions...), [expressions...], {key: value...}, {expressions...}	Binding or parenthesized expression, list display, dictionary display, set display
x[index], x[index:index], x(arguments...), x.attribute	Subscription, slicing, call, attribute reference
await x	Await expression
**	Exponentiation
+x, -x, ~x	Positive, negative, bitwise NOT
*, @, /, //, %	Multiplication, matrix multiplication, division, floor division, remainder 6

Answer 2

** 运算符在 - 上有 precedence，所以 -1**2 实际上是 -(1**2)

python 中的运算符优先级列表，从最高优先级（顶部）到最低优先级（底部）：

Operator	Function
()	Parentheses (grouping)
f(args...)	Function call
x[index:index]	Slicing
x[index]	Subscription
x.attribute	Attribute reference
**	Exponentiation
~x	Bitwise not
+x, -x	Positive, negative
*, /, %	Multiplication, division, remainder
+, -	Addition, subtraction
<<, >>	Bitwise shifts
&	Bitwise AND
^	Bitwise XOR
\|	Bitwise OR
in, not in, is, is not, <, <=, >, >=, <>, !=, ==	Comparisons, membership, identity
not x	Boolean NOT
and	Boolean AND
or	Boolean OR
lambda	Lambda expression

Negative numbers with exponents in Python