CODEWITHSUNDEEP
pythonlistsortingcomparison-operators
idailylife
idailylife

Reputation: 184

Python: how to compare custom class member to builtin type

New to python so forgive me if this is obvious. Say I've created a custom class Foo, and make a list of Foo() instances mixed with ints, for example:

foo_list = [Foo() for _ in range(10)]
for i in range(10):
  foo_list.append(i)
# then the foo_list is mixed with Foo()s and ints

How can a Foo instance be comparable with an int? Basically my target is to make it possible to sort() the above list.

Upvotes: 1

Views: 781

Answers (2)

Iain Shelvington
Iain Shelvington

Reputation: 32294

You can add __lt__ and __gt__ methods to the Foo class (less than and greater than) that evaluate Foo objects as being less than or greater than integers, this is used by sort when a key function is not provided

class Foo():

    def __lt__(self, other):
        if isinstance(other, int):
            return self._id < other
        elif isinstance(other, self.__class__):
            return self._id < other._id

    def __gt__(self, other):
        if isinstance(other, int):
            return self._id > other
        elif isinstance(other, self.__class__):
            return self._id > other._id

Upvotes: 1

pmelanson
pmelanson

Reputation: 330

You can specify a custom function for the python sort() function, see the "key" function in the docs for the sorting function.

As an example, if Foo has an integer-like field bar, you could sort each instance of a Foo by the value of its bar with this:

def custom_key_function(element):
    if isinstance(element, Foo):
        return element.bar
    else:
        return element  # element is just an int

foo_list.sort(key=custom_key_function)

Note the use of isinstance, which will tell you whether something is an instance of a Foo.

I don't know what fields your Foo has, but as long as your custom_key_function transforms a Foo into something that can be compared with an int you should be good.

Upvotes: 1

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