What is the **params syntax in a Python method definition?
What is the **params syntax in a Python method definition?
Consider a function with default arguments:
def func(foo=3):
print(foo)
The structure of the arguments is (in principle) very similar to a dictionary. The function foo has (essentially) a dictionary of default arguments (in this case {foo:3}). Now, lets say that you dont want to use the keyword in the function call, but you want to use a dictionary instead — then you can call foo as:
d = {foo:8}
func(**d)
This allows you to dynamically change what arguments you are passing to the function func.
This become a little more interesting if you try the following:
d = {foo:8, bar:12}
func(**d)
This doesnt work (it is equivalent to foo(foo=8, bar=12), but since bar isnt a valid argument, it fails).
You can get around that problem by giving those extra arguments a place to go inside the definition of foo.
def func( foo=3, **kwargs ):
print(foo,kwargs)
Now, try:
d = {foo:8, bar:12}
func(**d) #prints (8, {bar:12})
All the extra keyword arguments go into the kwargs dictionary inside the function.
This can also be called as:
func(foo=8, bar=12)
with the same result.
This is often useful if funcA calls funcB and you want funcA to accept all of the keywords of funcB (plus a few extra) which is a very common thing when dealing with classes and inheritance:
def funcA(newkey=None,**kwargs):
funcB(**kwargs)
Finally, here is a link to the documentation
The **params parameter represents all the keyword arguments passed to the function as a dictionary.