So I was trying out the *args and **kwargs in Python. But I get an error that I cannot understand the reason of.
def new(a,b,c):
print a,b,c
a={'a': 7, 'b': 8, 'c': 9}
b={'1':7, '2':8, '3':9}
new(**a)
new(**b)
The new(**a) prints the results as expected, that is, the keys, 7,8,9. But, new(**b) gives the error:
new(**b)
TypeError: new() got an unexpected keyword argument '1'
Could anyone explain this? I am passing string as an argument in both cases, but 'a' works and '1' doesn't.
The reason is simple. When you pass a dictionary to a python function as kwargs it unpacks that and uses the keys in the dictionary as the names of the variables and you can **only use parameter names that you have specified in the function parameters list.
for a dictionary data={"key1":val1, "key2":val2, ..., "keyn":valn} when you do dictionary unpacking in a function call, fun(**data), is equivalent to doing fun(key1=val1, key2=val2,..., keyn=valn) and this will success as long as your function had arguments key1,...,keyn and/or accept keyword arguments.
So the problem with new(**b) is that is trying to do new(1=7,2=8,3=9) which is a nonsense as the function only had arguments named a,b,and c, therefore the error.
For your, your second example try with list instead
b=[7,8,9]
new(*b)
Also, you may want to play around with this function
def test(*argv, **kwarg):
print "positional arguments", argv
print "key word arguments", kwarg
like for instance
test(1,23,42)
test(w=3,r="buu")
test(1,2,3,q=23,ans=42)
a=[143,56,76]
b={'a': 7, 'b': 8, 'c': 9}
test(*a,**b)
This is related to a similar question, but in short passing **kwargs to a method is a way to pass in a number of key word arguments as a dictionary.
The reason your attempt fails is that your method new defines 3 named arguments - a,b,c so if you pass in a kwargs dictionary it must have these values as keys.
Its important to note that you can also catch kwargs in the method so you can call a method with numeric arguments, like so:
def foo(**kwargs):
print kwargs
a={'a': 7, 'b': 8, 'c': 9}
b={'1':7, '2':8, '3':9}
foo(**a)
foo(**b)
Gives:
{'a': 7, 'c': 9, 'b': 8}
{'1': 7, '3': 9, '2': 8}
new(**a)is essentiallynew(a=7, b=8, c=9), you can probably figure out why it does not work.1,2, or3.