The syntax is the * and **. The names *args and **kwargs are only by convention but there's no hard requirement to use them.
You would use *args when you're not sure how many arguments might be passed to your function, i.e. it allows you pass an arbitrary number of arguments to your function. For example:
>>> def print_everything(*args):
for count, thing in enumerate(args):
... print( '{0}. {1}'.format(count, thing))
...
>>> print_everything('apple', 'banana', 'cabbage')
0. apple
1. banana
2. cabbage
Similarly, **kwargs allows you to handle named arguments that you have not defined in advance:
>>> def table_things(**kwargs):
... for name, value in kwargs.items():
... print( '{0} = {1}'.format(name, value))
...
>>> table_things(apple = 'fruit', cabbage = 'vegetable')
cabbage = vegetable
apple = fruit
You can use these along with named arguments too. The explicit arguments get values first and then everything else is passed to *args and **kwargs. The named arguments come first in the list. For example:
def table_things(titlestring, **kwargs)
You can also use both in the same function definition but *args must occur before **kwargs.
You can also use the * and ** syntax when calling a function. For example:
>>> def print_three_things(a, b, c):
... print( 'a = {0}, b = {1}, c = {2}'.format(a,b,c))
...
>>> mylist = ['aardvark', 'baboon', 'cat']
>>> print_three_things(*mylist)
a = aardvark, b = baboon, c = cat
As you can see in this case it takes the list (or tuple) of items and unpacks it. By this it matches them to the arguments in the function. Of course, you could have a * both in the function definition and in the function call.
python - Use of *args and **kwargs - Stack Overflow
default value - Proper way to use **kwargs in Python - Stack Overflow
What are args** and kwargs** and __somethinghere__ in python?
What are *args and **kwargs ?
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The syntax is the * and **. The names *args and **kwargs are only by convention but there's no hard requirement to use them.
You would use *args when you're not sure how many arguments might be passed to your function, i.e. it allows you pass an arbitrary number of arguments to your function. For example:
>>> def print_everything(*args):
for count, thing in enumerate(args):
... print( '{0}. {1}'.format(count, thing))
...
>>> print_everything('apple', 'banana', 'cabbage')
0. apple
1. banana
2. cabbage
Similarly, **kwargs allows you to handle named arguments that you have not defined in advance:
>>> def table_things(**kwargs):
... for name, value in kwargs.items():
... print( '{0} = {1}'.format(name, value))
...
>>> table_things(apple = 'fruit', cabbage = 'vegetable')
cabbage = vegetable
apple = fruit
You can use these along with named arguments too. The explicit arguments get values first and then everything else is passed to *args and **kwargs. The named arguments come first in the list. For example:
def table_things(titlestring, **kwargs)
You can also use both in the same function definition but *args must occur before **kwargs.
You can also use the * and ** syntax when calling a function. For example:
>>> def print_three_things(a, b, c):
... print( 'a = {0}, b = {1}, c = {2}'.format(a,b,c))
...
>>> mylist = ['aardvark', 'baboon', 'cat']
>>> print_three_things(*mylist)
a = aardvark, b = baboon, c = cat
As you can see in this case it takes the list (or tuple) of items and unpacks it. By this it matches them to the arguments in the function. Of course, you could have a * both in the function definition and in the function call.
One place where the use of *args and **kwargs is quite useful is for subclassing.
class Foo(object):
def __init__(self, value1, value2):
# do something with the values
print value1, value2
class MyFoo(Foo):
def __init__(self, *args, **kwargs):
# do something else, don't care about the args
print 'myfoo'
super(MyFoo, self).__init__(*args, **kwargs)
This way you can extend the behaviour of the Foo class, without having to know too much about Foo. This can be quite convenient if you are programming to an API which might change. MyFoo just passes all arguments to the Foo class.
You can pass a default value to get() for keys that are not in the dictionary:
self.val2 = kwargs.get('val2',"default value")
However, if you plan on using a particular argument with a particular default value, why not use named arguments in the first place?
def __init__(self, val2="default value", **kwargs):
While most answers are saying that, e.g.,
def f(**kwargs):
foo = kwargs.pop('foo')
bar = kwargs.pop('bar')
...etc...
is "the same as"
def f(foo=None, bar=None, **kwargs):
...etc...
this is not true. In the latter case, f can be called as f(23, 42), while the former case accepts named arguments only -- no positional calls. Often you want to allow the caller maximum flexibility and therefore the second form, as most answers assert, is preferable: but that is not always the case. When you accept many optional parameters of which typically only a few are passed, it may be an excellent idea (avoiding accidents and unreadable code at your call sites!) to force the use of named arguments -- threading.Thread is an example. The first form is how you implement that in Python 2.
The idiom is so important that in Python 3 it now has special supporting syntax: every argument after a single * in the def signature is keyword-only, that is, cannot be passed as a positional argument, but only as a named one. So in Python 3 you could code the above as:
def f(*, foo=None, bar=None, **kwargs):
...etc...
Indeed, in Python 3 you can even have keyword-only arguments that aren't optional (ones without a default value).
However, Python 2 still has long years of productive life ahead, so it's better to not forget the techniques and idioms that let you implement in Python 2 important design ideas that are directly supported in the language in Python 3!
Hello everyone, I hope you all are doing well. I’m confused about these keywords in Python and what they do and where I can use them, since am new to python.
Anyone?
I read few explaination but I just understood they are for variable arguments other than this I am confused
You can do that in Python 3.
def func(a,b,*args,kw1=None,**kwargs):
The bare * is only used when you want to specify keyword only arguments without accepting a variable number of positional arguments with *args. You don't use two *s.
To quote from the grammar, in Python 2, you have
parameter_list ::= (defparameter ",")*
( "*" identifier [, "**" identifier]
| "**" identifier
| defparameter [","] )
while in Python 3, you have
parameter_list ::= (defparameter ",")*
( "*" [parameter] ("," defparameter)*
[, "**" parameter]
| "**" parameter
| defparameter [","] )
which includes a provision for additional parameters after the * parameter.
UPDATE:
Latest Python 3 documentation here.
If you want to do a mixture of both remember that *args and **kwargs must be the last parameters specified.
def func(arg1,arg2,*args,kw1=None,kw2=None,**kwargs): #Invalid
def func(arg1,arg2,kw1=None,kw2=None,*args,**kwargs): #Valid
The comments seem to be based on mixing up how a function definition is constructed compared to how the arguments provided are assigned back to the parameters specified in the definition.
This is the definition of this function which has 6 parameters. It is called by passing named and unnamed arguments to it in a function call.
For this example... When an argument is named when calling the function it can be provided out of order. arg1 and arg2 are mandatory parameters and if not passed to the function as named arguments, then they must be assigned in order from the provided unnamed arguments. kw1 and kw2 have default values provided in the function definition so they are not mandatory, but if not provided for as named arguments they will take any available values from the remaining provided unnamed arguments. Any unnamed arguments left over are provided to the function in an array called args Any named arguments that do not have a corresponding parameter name in the function definition are provided to the function call in a dictionary called kwargs.