1

Lets assume the following object:

class Test:
    id1 = ""
    id2 = ""
    id3 = ""

    def __init__(self,arg1,arg2,arg3):
        self.id1 = arg1
        self.id2 = arg2
        self.id3 = arg3

As it can be seen, this class must contain 3 unique ids.

t = []
t.append(Test(200,201,193))
t.append(Test(403,221,213))
t.append(Test(3,523,2003))

Assuming the code above, what would be the easiest way for me to find the object with id1 = 403, id2 = 221, and id3 =213 in the list t?

Thanks in advance.

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5
  • Do you need to preserve a specific order for t, or can you sort it? Will you be looking up instances within t repeatedly? Commented Mar 1, 2012 at 11:02
  • I should have stated that in the question, yes the list t can be sorted. Also, yes I would be looking up for instance within t repeatedly. Commented Mar 1, 2012 at 13:14
  • What sorts of objects are these? If you already know the id1, id2 and id3 of the thing you're looking for, why do you need the actual object (i.e. what else is there)? Do you really want to associate several sorts of IDs with the underlying objects? I.e. are the ID values a fundamental part of the data, or just what you use to look them up? Commented Mar 2, 2012 at 12:03
  • Note that your lines like id1 = "" aren't necessary or helpful. Commented Mar 2, 2012 at 16:51
  • In new Python 2 code, it's best always to inherit object rather than nothing, which will make you use "new-style classes". There are various benefits to doing so, but you don't have to think about them much until you need them later on. Commented Mar 2, 2012 at 16:52

4 Answers 4

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2

Use iteration with comparison.

matches = [i for i in t if i.id1 == id1 and i.id2 == id2 and i.id3 == id3]

If you know it's there and there's only one, you can do it this way:

match = next(i for i in t if i.id1 == id1 and i.id2 == id2 and i.id3 == id3)

Note, though, that this will raise StopIteration if there is no such item. next can take a default value, though, so if you're not sure that it will exist, you could specify a default value:

match = next((i for i in t if i.id1 == id1 and i.id2 == id2 and i.id3 == id3), None)
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3 Comments

Nice -- but note that next takes an optional default value -- if you pass one, no StopIteration exception is raised.
@senderle: good point; I'd forgotten that briefly. Now integrated into answer.
An alternate code style to i.id1 == id1 and i.id2 == id2 and i.id3 == id3 is (i.id1, i.id2, i.id3) == (id1, id2, id3). Depending on what the values mean, you may find this more readable.
2

In case you have a lot of objects and you need to find a match many times, it may be more efficient to first pre-process them into a dictionary with a key of (id1,id2,id3). That way you'll be able to find the object in O(1).

To build the dict:

# In Python 2.7+
the_dict = {(o.id1, o.id2, o.id3) : o for o in objects}
# In Python 2.6-
the_dict = dict((o.id1, o.id2, o.id3),o) for o in objects)

Then to find the object in O(1):

the_dict[(id1,id2,id3)]

Note that placing the objects as dictionary values does not copy them (Python never copies implicitly), so you should not have to worry much about the memory impact.

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Comments

1 
>>> [x for x in t if x.id1==200 and x.id2==201 and x.id3==193]
[<__main__.Test object at 0x00BA3030>]
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1

If you need to store your objects in a list (i.e., if you can't use a dict and avoid the searching entirely), I would add a method to Test that returns the IDs as a tuple:

class Test(object):
    ...
    def getids(self):
        return (self.id1, self.id2, self.id3)

You can then simply loop and check against your tuple of ids to search for:

for obj in t:
    if obj.getids() == (403, 221, 213):
        return obj

This is "easy" as in cleaning up the interface. To speed it up, you should use a dictionary instead of a list and you can retrieve your objects in one step:

t = dict()  
obj = Test(403, 221, 213)
t[obj.getids()] = obj      # Store in dictionary, using ID as the key

You can then retrieve objects by ID, or check if they are present in t, in any of these ways:

found = t[(403, 221, 213)]       # returns object or raises KeyError
found = t.get((403, 221, 213))   # returns object or returns None
if (403, 221, 213) in t:         # True or False
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