Custom lt method for comparisons

Question

I want to be able to sort a list of classes by one attribute, which happens to be another class Date. I have done some research and want to use the sorted(list, key=lambda x:date) method of sorting, but seeing as date is a class in itself how can I write an __lt__ function in date to allow me to sort chronologically?

I want something along the lines of:

if self.year!= other.year:
    return self.year < other.year
elif self.month != pther.month
...

and so on.

Here is my Date class:

class Date:
    def __init__(self, month, day, year, minute, hour, string):
        self.month = month
        self.day = day
        self.year = year
        self.minute = minute
        self.hour = hour
        self.string = string

I should probably mention that this is the first time I have ever used Python, so I'm not very good at this.

Thanks in advance!

Just curious: why are you storing dates like that instead of using the built-in dates? — mechanical_meat, Feb 18 '17 at 00:27
@bernie The assignment I am doing requires the date string (among other things) to be created in a C program and stored in a text file, but to be honest I had no idea about the built-in dates. Would they make this easier? — PresumablyZach, Feb 18 '17 at 00:31
If you really want something of the sort, I think you're on a good track. Why not write it? — Sebastian Wozny, Feb 18 '17 at 00:31
@SebastianWozny I was more having trouble with the syntax. Do you just continue writing what I had there into an `__lt__` function? and that will sort it when I call sorted? — PresumablyZach, Feb 18 '17 at 00:32
This question should really be closed. I'm glad you were able to solve your problem, however it is unlikely to ever help someone else. — Sebastian Wozny, Feb 18 '17 at 00:54

score 5 · Answer 1 · edited May 23 '17 at 12:17

5

The easy way to compare two complex data structures is to compare tuples of their attributes, in the proper sort order. Try this:

class Date:
    def __init__(self, month, day, year, minute, hour, string):
        self.month = month
        self.day = day
        self.year = year
        self.minute = minute
        self.hour = hour
        self.string = string
    def __lt__(self, other):
        return (self.year, self.month, self.day, self.hour, self.minute) < \
               (other.year, other.month, other.day, other.hour, other.minute)

assert Date(4, 15, 2016, 30, 12, '') < \
       Date(4, 16, 2016, 0, 0, '') < \
       Date (1, 1, 2017, 59, 23, '')

assert not (Date(4, 16, 2016, 0, 0, '') < Date(4, 15, 2016, 30, 12, ''))

Of course, this only implements <. Depending upon the nature of your code, you may want to implement all of the other comparison functions, >, ==, !=, etc. One convenient way to do this is to use the @functools.total_ordering class decorator.

Reference:

edited May 23 '17 at 12:17

Community

1
1

answered Feb 18 '17 at 00:42

Robᵩ

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I think you need parentheses there. – user2357112 Feb 18 '17 at 00:47
Where, precisely? And why do you think so? – Robᵩ Feb 18 '17 at 00:48
Around your tuples. `<` has higher precedence than the comma, so instead of a boolean, you're actually returning a 9-element tuple with a `self.minute < other.year` boolean in the middle. – user2357112 Feb 18 '17 at 00:49
Also, it'd be a good idea to implement `__eq__` and put [`functools.total_ordering`](https://docs.python.org/3/library/functools.html#functools.total_ordering) on this class to get the rest of the comparison methods, or at least mention that it's important to remember them in real code. – user2357112 Feb 18 '17 at 00:53
@user2357112 - Agreed. – Robᵩ Feb 18 '17 at 00:58

score 1 · Answer 2 · answered Feb 18 '17 at 00:50

I propose adding a datetime object to the class definition:

import datetime as dt, operator as op

class MyDate(object):
    def __init__(self, month, day, year, minute, hour, string):
        self.month = month
        self.day = day
        self.year = year
        self.minute = minute
        self.hour = hour
        self.string = string
        self.dt = dt.datetime(self.year,self.month,self.day,self.hour,self.minute)

l = []
l.append(MyDate(2,17,2017,33,16,'somestring')) # not sure what `string` should be
l.append(MyDate(2,17,2017,37,16,'anotherstring'))
l.append(MyDate(2,17,2017,38,16,'yetanotherstring'))

Then you can use operator.attgetter to sort, e.g.:

>>> sorted(l,key=op.attrgetter('dt'),reverse=True)
[<__main__.MyDate object at 0x000000000A4C84A8>, 
 <__main__.MyDate object at 0x000000000A4C8208>, 
 <__main__.MyDate object at 0x000000000A4C8978>]

Custom __lt__ method for comparisons

2 Answers2

Custom lt method for comparisons