Thanks all! I thought update() would add an item even if it would be a duplicate, but apparently not. I also now better understand why I am always passing around *args and **kwargs when calling super(). Very interesting...
On 12/10/10, Hugo Arts <hugo.yo...@gmail.com> wrote: > On Fri, Dec 10, 2010 at 8:14 PM, Alex Hall <mehg...@gmail.com> wrote: >> Hi all, >> I was googling a way to do something like >> mydict=mydict.extend(additionaldict) >> > > mydict.update(additionaldict) > > see also: > http://docs.python.org/library/stdtypes.html#dict.update > >> and someone on a forum recommends this: >> mydict=dict(mydict, **additionaldict) >> What is the ** doing here? I tried to look it up, but Google seems to >> ignore it since it is punctuation. The poster on the forum says it >> will put all elements of additionaldict in mydict that are not already >> in mydict, which is the behavior I am looking for, but I am not sure >> why this would be so. TIA. >> > > The * and **, when calling functions, are what's called extended > calling syntax. It's a bit hard to explain, so I'm going to do a sort > of clarify by example here: > >>>> def f(a, b, c): > ... print a, b, c > ... >>>> f(1, 2, 3) > 1 2 3 >>>> a = [1, 2, 3] >>>> f(*a) > 1 2 3 >>>> b = {'a': 2, 'b': 1, 'c': 3} >>>> f(*b) > a c b >>>> f(**b) > 2 1 3 > > In short, if a = [1, 2, 3], f(*a) is the same as f(1, 2, 3). if b = > {'a': 2, 'b': 1, 'c': 3}, then f(**b) is the same as f(a=2, b=1, c=3). > > The * will iterate over its argument, and supply the resulting values > as arguments to the function (iterating over a dict will return its > keys, which is what you see with f(*b) here). ** requires a mapping > type, and supplies the keys and values to the function as keyword > arguments. > > * and ** are very flexible, and you can use them alongside with > regular arguments: > >>>> f(1, *[2, 3]) > 1 2 3 >>>> f(1, **{'b': 2, 'c': 3}) > 1 2 3 > > * and ** also have uses in function definition, where they sort of do > the reverse. They can capture any number of regular or keyword > arguments in a tuple or dictionary: > >>>> def f(*args, **kwargs): > ... print args, kwargs > ... >>>> f() > () {} >>>> f(1, 2, 3) > (1, 2, 3) {} >>>> f(1, 2, 3, a=4, b=5) > (1, 2, 3) {'a': 4, 'b': 5} >>>> f(a=4, b=5) > () {'a': 4, 'b': 5} > > Like with function calls, you can mix up regular arguments and */** > arguments, as long as the resulting function is unambiguous: > >>>> def f(a, *args, **kwargs): > ... print a, args, kwargs > ... >>>> f() > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: f() takes at least 1 argument (0 given) >>>> f(1) > 1 () {} >>>> f(1, 2, 3) > 1 (2, 3) {} >>>> f(1, 2, 3, b=4, c=5) > 1 (2, 3) {'c': 5, 'b': 4} > -- Have a great day, Alex (msg sent from GMail website) mehg...@gmail.com; http://www.facebook.com/mehgcap _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
