Use of *args and **kwargs [duplicate]

Question

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• What does ** (double star/asterisk) and * (star/asterisk) do for parameters? 18 answers

So I have difficulty with the concept of *args and **kwargs.

So far I have learned that:

• *args = list of arguments - as positional arguments
• **kwargs = dictionary - whose keys become separate keyword arguments and the values become values of these arguments.

I don't understand what programming task this would be helpful for.

Maybe:

I think to enter lists and dictionaries as arguments of a function AND at the same time as a wildcard, so I can pass ANY argument?

Is there a simple example to explain how *args and **kwargs are used?

Also the tutorial I found used just the "*" and a variable name.

Are *args and **kwargs just placeholders or do you use exactly *args and **kwargs in the code?

Answer 1

You can have a look at python docs (docs.python.org in the FAQ), but more specifically for a good explanation the mysterious miss args and mister kwargs (courtesy of archive.org) (the original, dead link is here).

In a nutshell, both are used when optional parameters to a function or method are used. As Dave says, *args is used when you don't know how many arguments may be passed, and **kwargs when you want to handle parameters specified by name and value as in:

myfunction(myarg=1)

Answer 2

These parameters are typically used for proxy functions, so the proxy can pass any input parameter to the target function.

def foo(bar=2, baz=5):
    print bar, baz

def proxy(x, *args, **kwargs): # reqire parameter x and accept any number of additional arguments
    print x
    foo(*args, **kwargs) # applies the "non-x" parameter to foo

proxy(23, 5, baz='foo') # calls foo with bar=5 and baz=foo
proxy(6)# calls foo with its default arguments
proxy(7, bar='asdas') # calls foo with bar='asdas' and leave baz default argument

But since these parameters hide the actual parameter names, it is better to avoid them.

Answer 3

Here's one of my favorite places to use the ** syntax as in Dave Webb's final example:

mynum = 1000
mystr = 'Hello World!'
print "{mystr} New-style formatting is {mynum}x more fun!".format(**locals())

I'm not sure if it's terribly fast when compared to just using the names themselves, but it's a lot easier to type!

Answer 4

Here's an example that uses 3 different types of parameters.

def func(required_arg, *args, **kwargs):
    # required_arg is a positional-only parameter.
    print required_arg

    # args is a tuple of positional arguments,
    # because the parameter name has * prepended.
    if args: # If args is not empty.
        print args

    # kwargs is a dictionary of keyword arguments,
    # because the parameter name has ** prepended.
    if kwargs: # If kwargs is not empty.
        print kwargs

>>> func()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: func() takes at least 1 argument (0 given)

>>> func("required argument")
required argument

>>> func("required argument", 1, 2, '3')
required argument
(1, 2, '3')

>>> func("required argument", 1, 2, '3', keyword1=4, keyword2="foo")
required argument
(1, 2, '3')
{'keyword2': 'foo', 'keyword1': 4}

Answer 5

*args and **kwargs are special-magic features of Python. Think of a function that could have an unknown number of arguments. For example, for whatever reasons, you want to have function that sums an unknown number of numbers (and you don't want to use the built-in sum function). So you write this function:

def sumFunction(*args):
  result = 0
  for x in args:
    result += x
  return result

and use it like: sumFunction(3,4,6,3,6,8,9).

**kwargs has a diffrent function. With **kwargs you can give arbitrary keyword arguments to a function and you can access them as a dictonary.

def someFunction(**kwargs):
  if 'text' in kwargs:
    print kwargs['text']

Calling someFunction(text="foo") will print foo.