[Tutor] Dict operation question.

[Javier Ruere]

David Driver wrote:
> I have a function
> 
> def updateit(self,**mydict):
> 
> which is intended to update a dictionary self.somedict. 
> 
> If there are keys in mydict that are not in self.somedict I want them
> returned in a new dict.
> 
> here is what i am doing now:
> 
> errdict = dict([(a,b) for a,b in mydict.items() if not a in self.somedict])
> 
> I then test (if errdict) and raise an exception that contains errdict
> as an attribute. Else i update self.somedict.
> 
> Is there something other than a list comprehension in a dict function
> that can do this?
> 
> Thanks!

Hi,
	You could have posted the code but anyway, this is your algorithm as I understood it:

class Test:
    def __init__(self, dict):
        self.somedict = dict

    def updateit(self, **mydict):
        errdict = dict([(a,b) for a,b in mydict.items() if not a in self.somedict])

        if (not errdict):
            raise Exception(errdict)

        self.somedict.update(errdict)


The following implementation uses Sets to find the new keys and is slower (twice as slow).

from sets import Set

class Test:
    def __init__(self, dict):
        self.somedict = dict

    def updateit(self, **mydict):
        new = Set(mydict.iterkeys())
        cur = Set(self.somedict.iterkeys())
        diff = new - cur

        for k in diff:
            self.somedict[k] = mydict[k]

        if (not diff):
            raise Exception(errdict)

The last one uses dicts and is a bit faster than the original.

class Test:
    def __init__(self, dict):
        self.somedict = dict

    def updateit(self, **mydict):
        aux = mydict.copy()
        aux.update(self.somedict)

        size = len(self.somedict)
        self.somedict.update(aux)

        if (len(self.somedict) == size):
            raise Exception({}) # It would be empty.


Javier