Why is cmp( ) useful?

Question

According to the doc and this tutorial,

cmp() returns -1 if x < y

and

cmp() returns 0 if x == y

and

cmp() returns 1 if x > y

The tutorial also said that

cmp() returns the sign of the difference of two numbers

I don't really get what sign of the difference of two numbers means. Doesn't that mean that it returns a value when the sign of numbers aren't equal? Since...

cmp(80, 100) :  -1      # both have positive sign. cmp(180, 100) :  1      # both also have positive sign. cmp(-80, 100) :  -1 cmp(80, -100) :  1 

**Note: code from the tutorial.*

Despite my confusion in sign differences, I can't really think of why do we need a built-in function to return a value of -1 when x < y.

Isn't the function cmp( ) easily implemented ? Is there any reason why Python creators keep cmp( ) function, or is there any hidden usage of this Python's cmp( ) function ?

Answer 1

Why cmp( ) is useful?

It isn't very useful, which is why it was deprecated (the builtin cmp is gone and builtin sorts no longer accept one in Python 3). Rich comparison methods supplanted it:

object.__lt__(self, other) object.__le__(self, other) object.__eq__(self, other) object.__ne__(self, other) object.__gt__(self, other) object.__ge__(self, other) 

This allows the < symbol (and other symbols) to be overloaded comparison operators, enabling, for example, subset and superset comparisons of set objects.

>>> set('abc') < set('cba') False >>> set('abc') <= set('cba') True >>> set('abc') == set('cba') True >>> set('abc') >= set('cba') True >>> set('abc') > set('cba') False 

while it could enable the above, cmp wouldn't allow the following:

>>> set('abc') == set('bcd') False >>> set('abc') >= set('bcd') False >>> set('abc') <= set('bcd') False 

Toy usage for cmp

Here's an interesting usage which uses its result as an index (it returns -1 if the first is less than the second, 0 if equal, and 1 if greater than):

def cmp_to_symbol(val, other_val):     '''returns the symbol representing the relationship between two values'''     return '=><'[cmp(val, other_val)]  >>> cmp_to_symbol(0, 1) '<' >>> cmp_to_symbol(1, 1) '=' >>> cmp_to_symbol(1, 0) '>' 

According to the docs, you should treat cmp as if it wasn't there:

https://docs.python.org/3/whatsnew/3.0.html#ordering-comparisons

cmp removed, equivalent operation

But you can use this as the equivalent:

(a > b) - (a < b) 

in our little toy function, that's this:

def cmp_to_symbol(val, other_val):     '''returns the symbol representing the relationship between two values'''     return '=><'[(val > other_val) - (val < other_val)] 

Answer 2

I don't really get what does it mean sign of the difference of two numbers.

This means: take the difference, and then the sign of that difference. For example, if x and y are two numbers:

• x < y => x - y < 0 and the function returns -1.
• x == y => x - y == 0 and the function returns 0.
• x > y => x - y > 0 and the function returns 1.

For more information on three-way comparisons, see Wikipedia.