Comparison function in Python using Lambdas

Question

I am trying to understand lambdas and I get the idea but how do I define multiple conditions for a Point2 [x,y] comparison, so something like:

if x1 < x2: -1
if x1 == x2: 0
if x1 > x2: 1

Answer 1

EDIT: Updated to be real Python according to PEP308 :) Note that the PEP has interesting information about how this should be parenthesized depending on which version of Python you're using. I won't attempt to reproduce it here - just read the PEP!

How about:

-1 if x1 < x2 else (0 if x1 == x2 else 1)

(That's without any knowledge of Python lambda expressions, but it's a fairly common way of expressing this logic in a single expression, which I guess is what you're after.)

EDIT: Others have suggested using cmp - I've been assuming that the questioner actually wants more complicated logic, such as providing their own comparisons, but wants the general form of "choose from a few conditions in a single expression".

Answer 2

The code above is equivalent to:

cmp(x1,x2)

or in a (ugly) lambda expression:

lambda x1,x2: 1 if x1>x2 else (0 if x1==x2 else -1)

(works only in Python 2.6 and above).

Normally you should use lambda expressions only for functions like

def fun(...):
    return ...

Answer 3

my_compare = lambda x1,x2 : cmp(x1, x2)
my_compare( -100, 100 )

Answer 4

In such a case, lambda expressions aren't usually the best thing. As Jon Skeet mentioned, you're gonna end with multiple if-else expressions:

lambda x1, x2: -1 if x1 < x2 else (0 if x1 == x2 else -1)

For your specific problem:

lambda x1, x2: cmp(x1, x2)

is the way to go.