5 Types of Python Function Arguments

In Python, a function is defined with def. This is followed by the name of the function and a set of formal parameters. The actual parameters, or arguments, are passed during a function call. We can define a function with a variable number of arguments.

Here’s what you need to know about the five common types of arguments in Python function definition.

1. Default Arguments in Python

• Default arguments are values that are provided while defining functions.
• The assignment operator = is used to assign a default value to the argument.
• Default arguments become optional during the function calls.
• If we provide a value to the default arguments during function calls, it overrides the default value.
• The function can have any number of default arguments.
• Default arguments should follow non-default arguments.

In the below example, the default value is given to argument b and c.

def add(a,b=5,c=10):
    return (a+b+c)

This function can be called in one of three ways:

Giving Only the Mandatory Argument

print(add(3))
#Output:18

Giving One of the Optional Arguments

3 is assigned to a, 4 is assigned to b.

print(add(3,4))
#Output:17

Giving All the Arguments

print(add(2,3,4))
#Output:9

Default values are evaluated only once at the point of the function definition in the defining scope. So, it makes a difference when we pass mutable objects like a list or dictionary as default values.

More on Python: 13 Python Code Snippets You Need to Know

2. Keyword Arguments in Python

Functions can also be called using keyword arguments of the form kwarg=value.

During a function call, values passed through arguments don’t need to be in the order of parameters in the function definition. This can be achieved by keyword arguments. But all the keyword arguments should match the parameters in the function definition.

def add(a,b=5,c=10):
    return (a+b+c)

Calling the function add by giving keyword arguments

All parameters are given as keyword arguments, so there’s no need to maintain the same order.

print (add(b=10,c=15,a=20))
#Output:45

During a function call, only giving a mandatory argument as a keyword argument. Optional default arguments are skipped.

print (add(a=10))
#Output:25

More on Python:Function Wrappers in Python: Model Runtime and Debugging

3. Positional Arguments in Python

During a function call, values passed through arguments should be in the order of parameters in the function definition. This is called positional arguments.

Keyword arguments should follow positional arguments only.

def add(a,b,c):
    return (a+b+c)

The above function can be called in two ways:

First, during the function call, all arguments are given as positional arguments. Values passed through arguments are passed to parameters by their position. 10 is assigned to a, 20 is assigned to b and 30 is assigned to c.

print (add(10,20,30))
#Output:60

The second way is by giving a mix of positional and keyword arguments. Keyword arguments should always follow positional arguments.

print (add(10,c=30,b=20))
#Output:60

Default vs. Keyword vs. Positional Arguments

Important Points to Remember for Default, Keyword and Positional Arguments

Default Arguments Should Follow Non-Default Arguments

def add(a=5,b,c):
    return (a+b+c)

#Output:SyntaxError: non-default argument follows default argument

Keyword Arguments Should Follow Positional Arguments

def add(a,b,c):
    return (a+b+c)

print (add(a=10,3,4))
#Output:SyntaxError: positional argument follows keyword argument

All Keyword Arguments Passed Must Match One of the Arguments Accepted by the Function, and Their Order Isn’t Important

def add(a,b,c):
    return (a+b+c)

print (add(a=10,b1=5,c=12))
#Output:TypeError: add() got an unexpected keyword argument 'b1'

No Argument Should Receive a Value More Than Once

def add(a,b,c):
    return (a+b+c)

print (add(a=10,b=5,b=10,c=12))
#Output:SyntaxError: keyword argument repeated

Default Arguments Are Optional Arguments

Giving only the mandatory arguments:

def add(a,b=5,c=10):
    return (a+b+c)

print (add(2))
#Output:17

Giving all arguments (optional and mandatory arguments)

def add(a,b=5,c=10):
    return (a+b+c)

print (add(2,3,4))
#Output:9

4. Arbitrary Positional Arguments in Python

Variable-length arguments are also known as arbitrary arguments. If we don’t know the number of arguments needed for the function in advance, we can use arbitrary arguments. Arbitrary arguments come in two types: arbitrary positional arguments and arbitrary keyword arguments.

For arbitrary positional argument, an asterisk (*) is placed before a parameter in function definition which can hold non-keyword variable-length arguments. These arguments will be wrapped up in a tuple. Before the variable number of arguments, zero or more normal arguments may occur.

def add(*b):
    result=0
    for i in b:
         result=result+i
    return result

print (add(1,2,3,4,5))
#Output:15
print (add(10,20))
#Output:30

5. Arbitrary Keyword Arguments in Python

For arbitrary keyword argument, a double asterisk (**) is placed before a parameter in a function which can hold keyword variable-length arguments.

def fn(**a):
    for i in a.items():
        print (i)
fn(numbers=5,colors="blue",fruits="apple")
'''
Output:
('numbers', 5)
('colors', 'blue')
('fruits', 'apple')
'''

More on Python: Introduction to Priority Queues in Python

Understanding Special Parameters in Python

According to Python Documentation:

“By default, arguments may be passed to a Python function either by position or explicitly by keyword. For readability and performance, it makes sense to restrict the way arguments can be passed so that a developer need only look at the function definition to determine if items are passed by position, by position or keyword, or by keyword.”

As a result, a function definition may look like this:

Where / and * are optional. If used, these symbols indicate the kind of parameter by how the arguments may be passed to the function, including: 

1. Positional or keyword arguments.
2. Positional only parameters.
3. Keyword-only arguments.

Positional or Keyword Arguments

If / and * are not present in the function definition, arguments may be passed to a function by position or by keyword.

def add(a,b,c):
    return a+b+c

print (add(3,4,5))
#Output:12

print (add(3,c=1,b=2))
#Output:6

Positional Only Parameters

Positional-only parameters are placed before a / (forward-slash) in the function definition. The / is used to logically separate the positional-only parameters from the rest of the parameters. Parameters following the / may be positional-or-keyword or keyword-only.

def add(a,b,/,c,d):
    return a+b+c+d

print (add(3,4,5,6))
#Output:12

print (add(3,4,c=1,d=2))
#Output:6

If we specify keyword arguments for positional only arguments, it will raise TypeError.

def add(a,b,/,c,d):
    return a+b+c+d

print (add(3,b=4,c=1,d=2))
#Output:TypeError: add() got some positional-only arguments passed as keyword arguments: 'b'

Keyword Only Arguments

To mark parameters as keyword-only, place an * in the arguments list just before the first keyword-only parameter.

def add(a,b,*,c,d):
    return a+b+c+d

print (add(3,4,c=1,d=2))
#Output:10

If we specify positional arguments for keyword-only arguments it will raise TypeError.

def add(a,b,*,c,d):
    return a+b+c+d

print (add(3,4,1,d=2))
#Output:TypeError: add() takes 2 positional arguments but 3 positional arguments (and 1 keyword-only argument) were given

All three calling conventions are used in the same function. In the example below, the function add contains all three arguments:

• a,b: Positional only arguments.
• c: Positional or keyword arguments.
• d: Keyword-only arguments.

def add(a,b,/,c,*,d):
    return a+b+c+d

print (add(3,4,1,d=2))
#Output:10

Below are some important points to remember for special parameters in Python:

1. Use positional-only if you want the name of the parameters to not be available to the user. This is useful when parameter names have no real meaning.
2. Use positional-only if you want to enforce the order of the arguments when the function is called.
3. Use keyword-only when names have meaning and the function definition is more understandable by being explicit with names.
4. Use keyword-only when you want to prevent users from relying on the position of the argument being passed.   

Frequently Asked Questions