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):

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 kw
example(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. – 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:
print(1, 2, 3, keyword_required=4)
first positional arg: 1
another positional arg: 2
another positional arg: 3
keyword_required value: 4
keyword_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 arguments
print("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)

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])

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)]


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

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