CODEWITHSUNDEEP
pythonoperators
dio
dio

Reputation: 122

Universal Comparison Operator In Python?

With Crystal, I can compare two numbers using the <==> operator. Example:

p! 1 <=> 1

Running this prints:

1 <=> 1 # => 0

The zero signifies that both numbers are equal. If the value is higher, it would return a positive number. If the value is lower, it returns a negative number. I'd like to know if such an operator exists with Python. Trying to use the <==> operator gives a syntax error:

>>> 1 <==> 1
File "<stdin>", line 1
1 <==> 1
    ^
SyntaxError: invalid syntax

I can obviously use something like this:

if 1 == 1:
   #equal
elif 1 < 1:
   #less than
else:
    #greater than

But I think it would be simpler to use a universal operator for comparing.

Upvotes: 0

Views: 1184

Answers (2)

ti7
ti7

Reputation: 18792

To compare numbers and find out if they're greater or smaller in one go, you may be able to just use subtraction!

>>> 1-1
0
>>> 1-5
-4

However, if you wanted rich comparison, define __lt__ on a custom object

class Foo():
    def __init__(self, name, primary, secondary):
        self.name      = name
        self.primary   = primary
        self.secondary = secondary
    def __str__(self):
        return self.name
    def __repr__(self):
        return f"Foo({self})"
    def __lt__(self, other):
        base       = self.primary * 100 + self.secondary
        comparison = other.primary * 100 + other.secondary
        return base < comparison

>>> a = Foo("some A", 3,4)
>>> b = Foo("some other B", 2, 99)
>>> test = (a,b)
>>> test
(Foo(some A), Foo(some other B))
>>> sorted(test)
[Foo(some other B), Foo(some A)]

Upvotes: 0

Barmar
Barmar

Reputation: 780673

Even if you had such an operator, you still end up with the 3-way if:

if a <=> b == 0:
    # equal
elif a <=> b < 0:
    # less than
else:
    # greater than

so it doesn't really buy you much. It does mean you can avoid writing the comparison expression if it's complex:

compare = a <=> b
if compare == 0:
    ...

But that was apparently not useful enough for the Python designers to keep the built-in cmp() function that was in Python 2.x.

If you really want it, there are a number of implementations in How to use cmp() in Python 3?

Upvotes: 2

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