comparing two lists

[Ladislav Andel]

Peter Otten wrote:
> Ladislav Andel wrote:
>
>   
>> Peter Otten wrote:
>>     
>>> Ladislav Andel wrote:
>>>
>>>   
>>>       
>>>> what would be the most efficient way to do following?
>>>>
>>>> I have a list of dictionaries  taken from DB e.g.
>>>> dblist = [{'id:1, 'host':'google.com','ip_address':'1.2.3.4'},
>>>> {'id:3, 'host':'yahoo.com','ip_address':'5.6.7.8'},
>>>> {'id:9, 'host':'msn.com','ip_address':'11.3.2.3'}]
>>>>
>>>> and list of object instances in memory(it's just for example)
>>>> which are looping within itself and testing particular hosts
>>>>
>>>> memlist = [<instance 1>,<instance 2>]
>>>> memlist[0].id  is 1 and  memlist[0].host is google.com etc.
>>>> memlist[1].id  is 9 and  memlist[1].host is msn.com etc.
>>>>
>>>> Now I want to add a new instance to memlist since id=3(in dblist) is not
>>>> in memlist.
>>>> How would you iterate through it and insert a new instance?
>>>>
>>>> The result should be:
>>>> memlist = [<instance 1>,<instance 2>, <instance 3>]
>>>> memlist[0].id  is 1 and  memlist[0].host is google.com etc.
>>>> memlist[1].id  is 3 and  memlist[1].host is yahoo.com etc.
>>>> memlist[2].id  is 9 and  memlist[2].host is msn.com etc.
>>>>     
>>>>         
>>> You should replace the memlist with a dictionary using (host, id) tuples
>>> as the keys. Here's an example that uses a set but requires you to modify
>>> the <instance N> class:
>>>
>>> dblist = [{'id':1, 'host':'google.com','ip_address':'1.2.3.4'},
>>> {'id':3, 'host':'yahoo.com','ip_address':'5.6.7.8'},
>>> {'id':9, 'host':'msn.com','ip_address':'11.3.2.3'}]
>>>
>>> class Item(object):
>>>     def __init__(self, id, host, **discarded):
>>>         self._tuple = (id, host)
>>>     def __hash__(self):
>>>         return hash(self._tuple)
>>>     def __eq__(self, other):
>>>         return self._tuple == other._tuple
>>>     def __repr__(self):
>>>         return "Item(id=%r, host=%r)" % self._tuple
>>>
>>> items = set([Item(1, "google.com")])
>>> for d in dblist:
>>>     item = Item(**d)
>>>     if item not in items:
>>>         print "adding", item
>>>         items.add(item)
>>>     else:
>>>         print item, "already there"
>>>
>>>   
>>>       
>> Thank you for this nice solution. I wouldn't be able to write it this
>> way at all
>>     
>
> Then think twice before you use it. The dictionary approach should be
> straightforward.
>
>   
>> but what about removing from memlist if there is less items in dblist
>> than in items (instances)?
>> I will have to iterate over items(instances) and remove that one which
>> is not in dblist I guess.
>>     
>
> Yes, but again, if you use a dictionary instead of a list the lookup will be
> efficient.
>
> To follow up on my previous post: with sets there is a concise spelling:
>
> items &= set(Item(**d) for d in dblist)
>
> or even
>
> items.intersection_update(Item(**d) for d in dblist)
>
> if you don't mind object identity.
>
> Peter
>
>   

Thank you very much for this. I think everybody should ignore my 
previous email.. Shame on me.