Comparing string representation of object to string representation of each object in a list

Question

Is there a simple way of comparing the string representation of an object to each object in a list?

The example code below (Python 2.7) works as intended, but I assume there's a much nicer way of doing this in Python!

class url(object):
    def __init__(self, address):
       self.address = address

    def __str__(self):
        return self.address

list_of_urls = [url('http://www.example.foo'), url('http://www.example.bar')]
test_url = url('http://www.example.foobar')

test_url_listed = False
for link in list_of_urls:
    if str(test_url) == str(link):
        test_url_listed = True

if not test_url_listed:
    list_of_urls.append(test_url)

Is it possible to make it closer in structure to the following?

if test_url not in list_of_urls:
    list_of_urls.append(test_url)

(As is, this fails since it compares the objects and not the strings they represent.)

John Kugelman · Accepted Answer · 2012-10-20 21:41:55Z

1

if str(test_url) not in [str(url) for url in list_of_urls]

Or better yet, implement __cmp__ and/or __eq__ + __ne__ on the url class. (And probably also __hash__.) Then you could create a set of urls which would automatically ensure there are no duplicate urls.

edited Oct 20, 2012 at 21:41

answered Oct 20, 2012 at 21:36

John Kugelman

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1 Comment

Steven Maude Over a year ago

I was thinking more of how to implement it within a class, which the latter part of this solution does. Got my code working using __eq__.

Keith · Accepted Answer · 2012-10-20 23:42:22Z

1

The quickest way would be to implement the __cmp__ method.

class url(object):
    def __init__(self, address):
       self.address = address

    def __str__(self):
        return self.address

    def __cmp__(self, other):
        return cmp(self.address, other.address)


list_of_urls = [url('http://www.example.foo'), url('http://www.example.bar')]
test_url = url('http://www.example.foobar')

This in:

if test_url not in list_of_urls:
    print("Not in")

prints "Not in".

Alternatively you can use "rich" comparisons, but that's a little more effort. The total_ordering class method can help.

from functools import total_ordering

@total_ordering
class url(object):
    def __init__(self, address):
       self.address = address

    def __str__(self):
        return self.address

    def __eq__(self, other):
        return self.address == other.address

    def __lt__(self, other):
        return self.address < other.address

This makes all comparisons work with the string attribute named "address". This will fail if you compare other objects, however.

edited Oct 20, 2012 at 23:42

answered Oct 20, 2012 at 21:56

Keith

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2 Comments

user395760 Over a year ago

__cmp__ is obsolete, and does work in Python 3. In 3.2+ functools.total_ordering is an easy way to get those all comparisons for free. In previous versions, it's still only a one-liner each (only one oneliner if you only want equality and inequality).

Keith Over a year ago

@delnan True. Edited by adding that alternative.

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