Summer of code 2017: Python, Day 42 args and kwargs

As explained in my Summer of code 2017: Python post I decided to pick up Python

This is officially day 42.  Today I looked at args and kwargs, these notes are mostly for me but who knows, they might be helpful for someone else in the future as well


Args and kwargs? WTF is that? I had the same thought, it turns out these are magic variables  :-)

From the docs:

args

A tuple of positional arguments values. Dynamically computed from the arguments attribute.

kwargs

A dict of keyword arguments values. Dynamically computed from the arguments attribute.

Basically this is a way to pass in a unknown number of variables into a function.

Args are prefixed with an asterisk, kwargs are prefixed with 2 asterisks

The name of these  variables does not have to be *args and **kwargs, you can name it anything

For example, here is an args named bars

def test_args(foo, *bars):

And here is a kwargs also named bars

def test_kwargs(foo,**bars):

The only difference between these two is the single and double asterisk



Let's make a very simple function, that will loop through the *args and print them out. This function accepts a normal variable foo and an args variable named *bars

def test_args(foo, *bars):
    print ('first normal argument:', foo)
    for bar in bars:
        print ("another looping through all the *bars :", bar)
 
    print ('all *bars on one line', bars)

Now let's call this function like this

>>> test_args('args','Denis','likes','playing','with','Python')

Here is the output

>>> test_args('args','Denis','likes','playing','with','Python')
first normal argument: args
another looping through all the *bars : Denis
another looping through all the *bars : likes
another looping through all the *bars : playing
another looping through all the *bars : with
another looping through all the *bars : Python
all *bars on one line ('Denis', 'likes', 'playing', 'with', 'Python')
>>> 

Let's now call this function like this

>>> test_args('args','enough','Python')

Here is the output of that call

>>> test_args('args','enough','Python')
first normal argument: args
another looping through all the *bars : enough
another looping through all the *bars : Python
all *bars on one line ('enough', 'Python')
>>> 

As you can see, you can pass a variable number of values into the function by using args

Now let's take a look at kwargs, here is our function, we now named the variable **bars to denote that this is a variable of type kwargs

def test_kwargs(foo,**bars):
    print ('first normal argument:', foo)
    for bar in bars:
        print ("another looping through all the **bars :", bar)
 
    print ('all **bars on one line', bars)

Calling this function requires a change

If you try calling it like we did with args you will get an error

>>> test_kwargs('args','enough','Python')
Traceback (most recent call last):
  File "", line 1, in <module>
TypeError: test_kwargs() takes 1 positional argument but 3 were given
>>> 

What you have to do instead is use named arguments

If we change it to this it will work fine

>>> test_kwargs('kwargs',name ='Denis1', age = 200)

Here is the output

>>> test_kwargs('kwargs',name ='Denis1', age = 200)
first normal argument: kwargs
another looping through all the **bars : name
another looping through all the **bars : age
all **bars on one line {'name': 'Denis1', 'age': 200}
>>> 

Here is another example

>>> test_kwargs('kwargs',month ='July', day = 29)

Here is the output

>>> test_kwargs('kwargs',month ='July', day = 29)
first normal argument: kwargs
another looping through all the **bars : month
another looping through all the **bars : day
all **bars on one line {'month': 'July', 'day': 29}
>>> 

You might have noticed that we didn't print the value of the month or the value of the date. Let's change our function so it looks a little different, now we will print both the key and the value

def test_kwargs(foo, **bars):
    print ('first normal argument:', foo)
    if bars is not None:
        for key, value in bars.items():
            print ("%s : %s" %(key,value))
 
    print ('all *bars on one line', bars)

Now we can make the same call

test_kwargs('kwargs',name ='Denis1', age = 200)

And here is the output

>>> test_kwargs('kwargs',name ='Denis1', age = 200)
first normal argument: kwargs
name : Denis1
age : 200
all *bars on one line {'name': 'Denis1', 'age': 200}
>>> 

As you can see we now have the name as well as the value of the key printed


If you want to use args and kwargs in a function,  you need to have the args before the kwargs

def test_args(foo, **bars, *namedbars):
                               ^
SyntaxError: invalid syntax

This is an error because we have kwargs before args

def test_args(**bars, foo, *namedbars):
                            ^
SyntaxError: invalid syntax

This is also an error because we still have kwargs before args

This is how it should be, forst normal variables, then args and finally kwargs

def test_args(foo, *bars, **namedbars):

Here is an example of such a signature

def test_args(foo, *bars, **namedbars):
    print ('first normal argument:', foo)
    for bar in bars:
        print ("another looping through all the *bars :", bar)
 
    print ('all *bars on one line', bars)
    print ('all **namedbars on one line', namedbars)

Calling the function above gives us the following output

>>> test_args('args','Denis','likes','playing','with','Python')
first normal argument: args
another looping through all the *bars : Denis
another looping through all the *bars : likes
another looping through all the *bars : playing
another looping through all the *bars : with
another looping through all the *bars : Python
all *bars on one line ('Denis', 'likes', 'playing', 'with', 'Python')
all **namedbars on one line {}

As you can see, there is nothing printed for kwargs, this is because we did not pass anything in.
Let's make a change and add a value for the kwargs

Here is the output from that call

>>> test_args('args','enough','Python', namedbars='Denis')
first normal argument: args
another looping through all the *bars : enough
another looping through all the *bars : Python
all *bars on one line ('enough', 'Python')
all **namedbars on one line {'namedbars': 'Denis'}
>>> 

There you have it.. a rather simple blog post that that explains the difference between args and kwargs