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} _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
