在 Python 中(我只检查了 Python 3.6,但我相信它也应该适用于许多以前的版本):
In Python (I checked only with Python 3.6 but I believe it should hold for many of the previous versions as well):
(0, 0) == 0, 0 # results in a two element tuple: (False, 0)
0, 0 == (0, 0) # results in a two element tuple: (0, False)
(0, 0) == (0, 0) # results in a boolean True
但是:
a = 0, 0
b = (0, 0)
a == b # results in a boolean True
为什么两种方法的结果不同?相等运算符是否以不同方式处理元组?
Why does the result differ between the two approaches? Does the equality operator handle tuples differently?
前两个表达式都解析为元组:
The first two expressions both parse as tuples:
(0, 0) == 0
(即False
),后跟0
0
,然后是 0 == (0, 0)
(这样仍然是 False
).(0, 0) == 0
(which is False
), followed by 0
0
, followed by 0 == (0, 0)
(which is still False
that way around).由于逗号分隔符相对于相等运算符的相对优先级,表达式被拆分为:Python 看到一个包含两个表达式的元组,其中一个恰好是相等测试,而不是两个元组之间的相等测试.
The expressions are split that way because of the relative precedence of the comma separator compared to the equality operator: Python sees a tuple containing two expressions, one of which happens to be an equality test, instead of an equality test between two tuples.
但是在您的第二组语句中,a = 0, 0
不能 是一个元组.元组是值的集合,与相等测试不同,赋值在 Python 中没有值.赋值不是表达式,而是语句;它没有可以包含在元组或任何其他周围表达式中的值.如果您尝试使用 (a = 0), 0
之类的方法来强制解释为元组,则会出现语法错误.这就留下了将元组分配给一个变量——这可以通过写成 a = (0, 0)
来更明确——作为 a = 0, 0<的唯一有效解释/代码>.
But in your second set of statements, a = 0, 0
cannot be a tuple. A tuple is a collection of values, and unlike an equality test, assignment has no value in Python. An assignment is not an expression, but a statement; it does not have a value that can be included into a tuple or any other surrounding expression. If you tried something like (a = 0), 0
in order to force interpretation as a tuple, you would get a syntax error. That leaves the assignment of a tuple to a variable – which could be made more explicit by writing it a = (0, 0)
– as the only valid interpretation of a = 0, 0
.
所以即使在赋值给 a
时没有括号,它和 b
都被赋值为 (0,0)
,所以a == b
因此是 True
.
So even without the parentheses on the assignment to a
, both it and b
get assigned the value (0,0)
, so a == b
is therefore True
.
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