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Python *args and **kwargs 2023 - CodingCompiler
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# Python *args and **kwargs

Python *args and **kwargs are useful in writing functions and performing ther tasks by using Python programming language. Learn more about them here.

## Using Python **kwargs when writing functions

You can define a function that takes an arbitrary number of keyword (named) arguments by using the double star

``before a parameter name:``
`def print_kwargs(**kwargs):print(kwargs)`

When calling the method, Python will construct a dictionary of all keyword arguments and make it available in the function body:

`print_kwargs(a="two", b=3)`
##### prints: “{a: “two”, b=3}”

Note that the **kwargs parameter in the function definition must always be the last parameter, and it will only match the arguments that were passed in after the previous ones.

`def example(a, **kw):print kwexample(a=2, b=3, c=4) # => {'b': 3, 'c': 4}`

Inside the function body, kwargs is manipulated in the same way as a dictionary; in order to access individual elements in kwargs you just loop through them as you would with a normal dictionary:

`def print_kwargs(**kwargs):for key in kwargs:print("key = {0}, value = {1}".format(key, kwargs[key]))`

Now, calling print_kwargs(a=”two”, b=1) shows the following output:

`print_kwargs(a = "two", b = 1)key = a, value = "two"key = b, value = 1`

## Python *args and **kwargs: Using *args when writing functions

You can use the star * when writing a function to collect all positional (ie. unnamed) arguments in a tuple:

`def print_args(farg, *args):print("formal arg: %s" % farg)for arg in args:print("another positional arg: %s" % arg)`

Calling method:

`print_args(1, "two", 3)`

In that call, farg will be assigned as always, and the two others will be fed into the args tuple, in the order they were received.

GoalKicker.com – Python® Notes for Professionals 496

## Populating kwarg values with a dictionary

`def foobar(foo=None, bar=None):return "{}{}".format(foo, bar)values = {"foo": "foo", "bar": "bar"}foobar(**values) # "foobar"`

## Keyword-only and Keyword-required arguments

Python 3 allows you to define function arguments which can only be assigned by keyword, even without default values. This is done by using star * to consume additional positional parameters without setting the keyword parameters. All arguments after the * are keyword-only (i.e. non-positional) arguments. Note that if keyword-only arguments aren’t given a default, they are still required when calling the function.

`def print_args(arg1, *args, keyword_required, keyword_only=True):print("first positional arg: {}".format(arg1))for arg in args:print("another positional arg: {}".format(arg))print("keyword_required value: {}".format(keyword_required))print("keyword_only value: {}".format(keyword_only))print(1, 2, 3, 4) # TypeError: print_args() missing 1 required keyword-only argument:'keyword_required'print(1, 2, 3, keyword_required=4)first positional arg: 1another positional arg: 2another positional arg: 3keyword_required value: 4keyword_only value: True`

## Using **kwargs when calling functions

You can use a dictionary to assign values to the function’s parameters; using parameters name as keys in the dictionary and the value of these arguments bound to each key:

`def test_func(arg1, arg2, arg3): # Usual function with three argumentsprint("arg1: %s" % arg1)print("arg2: %s" % arg2)print("arg3: %s" % arg3)Note that dictionaries are unordered, so we can switch arg2 and arg3. Only the names matter. kwargs = {"arg3": 3, "arg2": "two"}Bind the first argument (ie. arg1) to 1, and use the kwargs dictionary to bind the others test_var_args_call(1, **kwargs)`

## **kwargs and default values

To use default values with **kwargs

`def fun(**kwargs):print kwargs.get('value', 0)`

fun()
print 0 fun(value=1)
print 1

## Python *args and **kwargs: Using *args when calling functions

The eﬀect of using the * operator on an argument when calling a function is that of unpacking the list or a tuple argument

`def print_args(arg1, arg2):print(str(arg1) + str(arg2))a = [1,2]b = tuple([3,4])print_args(*a)`
##### 12
`print_args(*b)`
##### 34

Note that the length of the starred argument need to be equal to the number of the function’s arguments.

A common python idiom is to use the unpacking operator * with the zip function to reverse its eﬀects:

`a = [1,3,5,7,9]b = [2,4,6,8,10]zipped = zip(a,b)`

#### [(1,2), (3,4), (5,6), (7,8), (9,10)]

`zip(*zipped)`