CODEWITHSUNDEEP
pythonlistsorting
PresumablyZach
PresumablyZach

Reputation: 99

Custom __lt__ method for comparisons

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!

Upvotes: 4

Views: 4295

Answers (2)

Robᵩ
Robᵩ

Reputation: 168736

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:

Upvotes: 5

mechanical_meat
mechanical_meat

Reputation: 169444

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>]

Upvotes: 1

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