python *args and **kwargs

Posted by Jeffery Yee | 2:21 PM

Python *args and **kwargs

The * operator used when defining a function means that any extra positional arguments passed to the function end up in the variable prefaced with a *. So:
>>> def one(*args):
...     print args # 1
>>> one()
()
>>> one(1, 2, 3)
(1, 2, 3)
>>> def two(x, y, *args): # 2
...     print x, y, args
>>> two('a', 'b', 'c')
a b ('c',)
The first function one simply prints whatever (if any) positional arguments are passed to it. As you can see at point #1 we simply refer to the variable args inside the function - *args is only used in the function definition to indicate that positional arguments should be stored in the variable args. Python also allows us to specify some variables and catch any additional parameters in args as we can see at point #2.

The * operator can also be used when calling functions and here it means the analogous thing. A variable prefaced by * when calling a function means that the variable contents should be extracted and used as positional arguments. Again by example:
>>> def add(x, y):
...     return x + y
>>> lst = [1,2]
>>> add(lst[0], lst[1]) # 1
3
>>> add(*lst) # 2
3
The code at point #1 does exactly the same thing as the code at point #2 - Python is doing automatically for us at point #2 what we could do manually for ourselves. This isn’t too bad - *args means either extract positional variables from an iterable if calling a function or when defining a function accept any extra positional variables.
Things get only slightly more complicated when we introduce ** which does for dictionaries & key/value pairs exactly what * does for iterables and positional parameters. Simple, right?
>>> def foo(**kwargs):
...     print kwargs
>>> foo()
{}
>>> foo(x=1, y=2)
{'y': 2, 'x': 1}
When we define a function we can use **kwargs to indicate that all uncaptured keyword arguments should be stored in a dictionary called kwargs. As before neither the name args nor kwargs is part of Python syntax but it is convention to use these variable names when declaring functions. Just like * we can use ** when calling a function as well as when defining it.
>>> dct = {'x': 1, 'y': 2}
>>> def bar(x, y):
...     return x + y
>>> bar(**dct)
3

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