CODEWITHSUNDEEP
pythonevaluation
monoceres
monoceres

Reputation: 4770

How does python evaluate this?

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?!

Upvotes: 1

Views: 203

Answers (4)

BrenBarn
BrenBarn

Reputation: 251378

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.

Upvotes: 9

Vivek S
Vivek S

Reputation: 5540

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, e.g., 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).

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1122032

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.

Upvotes: 2

Fedor Gogolev
Fedor Gogolev

Reputation: 10541

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.

Upvotes: 4

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