Special Parameters

Normally, we can pass the arguments to function either by position and Keyword. But we can also restrict the user with optional (/ and *) symbols which decides how the argument should be passed to function

Function Definition

As per python documentation, please find below the function definition

def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):
    -----------    ----------     ----------
        |             |                  |
        |        Positional or keyword   |
        |                                - Keyword only
        -- Positional only

Types of Special Parameters

• positional-only

• positional-or-keyword

• keyword-only

Positional Only

Below function indicates that it must call with first two parameters are positional and last two parameter are either keyword or positional

#Positional Only
def sum(num1, num2, /, num3, num4):
    result = num1 + num2 + num3 + num4
    print(result)

#Four positional
sum(10, 20, 30, 40)

#Three Positional and One Keyword
sum(10, 20, 30, num4=40)

#Two Positional and Two Keyword
sum(10, 20, num3=30, num4=40)

Output

100
100
100

If we call function with more than two keywords arguments raises exception

#Positional Only
def sum(num1, num2, /, num3, num4):
    result = num1 + num2 + num3 + num4
    print(result)

#One Positional and Three Keyword - Raises Exception
sum(10, num2=20, num3=30, num4=40)

Output

Traceback (most recent call last):
File "<string>", line 7, in <module>
TypeError: sum() got some positional-only arguments passed as keyword arguments: 'num2'

Positional or Keyword

Below function indicates we can pass all arguments either positional-or-keyword

#Positional or Keyword
def sum(num1, num2,  num3, num4):
    result = num1 + num2 + num3 + num4
    print(result)

#Four positional
sum(10, 20, 30, 40)

#Three Positional and One Keyword
sum(10, 20, 30, num4=40)

#Two Positional and Two Keyword
sum(10, 20, num3=30, num4=40)

#One Positional and Three Keyword
sum(10, num2=20, num3=30, num4=40)

#Four Keyword
sum(num1=10, num2=20, num3=30, num4=40)

Output

100
100
100
100
100

Keyword Only

Below function indicates that it must call with first two arguments as positional and last two arguments as keyword

#Positional or Keyword
def sum(num1, num2, *, num3, num4):
    result = num1 + num2 + num3 + num4
    print(result)

#Two Positional and Two Keyword
sum(10, 20, num3=30, num4=40)

#One Positional and Three Keyword
sum(10, num2=20, num3=30, num4=40)

#Four Keyword
sum(num1=10, num2=20, num3=30, num4=40)

Output

100
100
100

If we call function with more than two positional arguments raises exception

#Positional Only
def sum(num1, num2, /, num3, num4):
    result = num1 + num2 + num3 + num4
    print(result)

#Four positional
sum(10, 20, 30, 40)

#Three Positional and One Keyword
sum(10, 20, 30, num4=40)

#Two Positional and Two Keyword
sum(10, 20, num3=30, num4=40)

Output

Traceback (most recent call last):
File "<string>", line 7, in <module>
TypeError: sum() takes 2 positional arguments but 4 were given