python如何评估呢？

Question

Can someone explain how these results are possible (python 2.6):

>>> 1<3>2
True
>>> (1<3)>2
False
>>> 1<(3>2)
False

I would think that one of the last two would match the first one, but apparently the operators in the first statement is somehow linked?!

Answer 1

Your first example shows comparison chaining . 1<3>2 means 1<3 and 3>2 (except each expression is evaluated only once). This applies to all comparison operators in Python.

Your second two examples force one comparison to be evaluated first, resulting in a boolean value which is then compared with the remaining integer.

Answer 2

In your first case 1<3>2 1 is actually lesser than 3 and 3 is greater than 2 , so True .

In your second case (1<3)>2 (1<3) evaluates as True that represented as 1 , so 1 is not greater than 2 .

In your third case 1<(3>2) , 1 is not lesser than True that represented as 1 .

Answer 3

The last two statements compare booleans against an integer:

>>> True > 2
False
>>> 1 < True
False

The first statement is comparison chaining , which works for all boolean comparisons in Python. Note from the documentation:

Comparisons yield boolean values: True or False.

By placing parts of your expression in brackets, those parts get evaluated first and you end up with comparing integers and booleans.

Answer 4

As per docs ,

Unlike C, all comparison operations in Python have the same priority, which is lower than that of any arithmetic, shifting or bitwise operation. Also unlike C, expressions like a < b < c have the interpretation that is conventional in mathematics:

comparison ::= or_expr ( comp_operator or_expr )*

comp_operator ::= "<" | ">" | "==" | ">=" | "<=" | "<>" | "!=" | "is" ["not"] | ["not"] "in"

Comparisons yield boolean values: True or False .

Comparisons can be chained arbitrarily, eg, x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).