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Python Object-Oriented Python Dice Roller Compare and Contrast

Posted by Duane Smith
Duane Smith
1,919 Points

How do I compare instances from a given class?

I'm trying to create methods that return a true/false Boolean value that will compare two different instances in terms of their length. I feel like I'm either using the wrong variable or I'm not using the right method to compare instances, but either way I'm fairly lost as to where I should go.

Any help will be appreciated!

Thanks in advance!

songs.py 
class Song:
    def __init__(self, artist, title, length):
        self.artist = artist
        self.title = title
        self.length = length

   def __eq__(self, new):
    if not isinstance(new, Song):
        return NotImplemented
    return self.title == new.title and self.artist == new.artist

def __lt__(self, new):
    if not isinstance(new, Song):
        return NotImplemented
    return self.title > new.title and self.artist > new.artist

def __gt__(self, new):
    if not isinstance(new, Song):
        return NotImplemented
    return self.title < new.title and self.artist < new.artist

def __le__(self, new):
    if not isinstance(new, Song):
        return NotImplemented
    return self.title <= new.title and self.artist <= new.artist

def __ge__(self, new):
    if not isinstance(new, Song):
        return NotImplemented
    return new.title >= self.title and new.artist >= self.artist

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

The question is precisely how do you wish to compare objects by length? Standard comparison operators map directly to the dunder methods already defined. If you now wish to use the standard comparison operators the above methods would need to be redefined to use self.length instead of name and title. 

def __gt__(self, new):
    if not isinstance(new, Song):
        return NotImplemented
    return self.length < new.length

If you do not wish to use the standard comparison operators, there are a few options:

  1. use direct comparisons of length: song1.length > song2.length

  2. add a __len__ method that returns the self.length. Then, you could compare len(song1) > len(song2)

  3. you could create a new "regular" method, such as gt_len, using the same form as above as the other dunder methods. The new method could be called as instance1.gt_len(instance2)

  4. a really cheeky alternative: override the modulo operator %. which calls the __mod__ dunder method. This would be very non-standard and not recommended.

def __mod__(self, new):
    """returns True if two instance lengths equal
        Usage:
        if song1 % song2:
            print(f"{song1} and {song2} have equal lengths")
    """
    if not isinstance(new, Song):
        return NotImplemented
    return self.length == new.length

post back if you wish more help. Good luck!!

Duane Smith
Duane Smith
1,919 Points

Wow.. Chris Freeman Thank You.

These examples and explanations are amazing!! On my own, I was trying to go more something more like option 1 or possibly option 3. I felt the code would just be more self-explanatory or readable to any programmer that would look at it down the road. I'm trying to train myself as if I'm writing programs for a team that may need to get reviewed or revisited.

This is awesome and exactly what I was looking for in getting help!

I truly appreciate it.

Best,

Duane Smith