What does the slash(/) in the parameter list of a function mean in Python?

The slash (/) in a function's parameter list marks the boundary between positional-only parameters and other parameters. Parameters before the slash can only be passed by position, not as keyword arguments.

Basic Function Example

Let's first understand a regular Python function with parameters ?

# Creating a Function
def demo(car_name):
   print("Car:", car_name)

# Function calls
demo("BMW")
demo("Tesla")

Car: BMW
Car: Tesla

Positional-Only Parameters with Slash

The slash (/) denotes that parameters before it are positional-only. Arguments are mapped to parameters based solely on their position.

Example with Custom Function

def calculate(x, y, /):
    return x + y

# Valid calls - positional arguments
result1 = calculate(10, 5)
result2 = calculate(20, 15)

print("Result 1:", result1)
print("Result 2:", result2)

Result 1: 15
Result 2: 35

The divmod() Function

The built-in divmod() function is a perfect example of positional-only parameters ?

# Check divmod documentation
help(divmod)

Help on built-in function divmod in module builtins:

divmod(x, y, /)
    Return the tuple (x//y, x%y).  Invariant: div*y + mod == x.

Using divmod() with Positional Arguments

# Valid usage - positional arguments
quotient_remainder = divmod(17, 5)
print("17 divided by 5:", quotient_remainder)

# Another example
result = divmod(20, 3)
print("20 divided by 3:", result)

17 divided by 5: (3, 2)
20 divided by 3: (6, 2)

What Happens with Keyword Arguments?

Since parameters before the slash are positional-only, using keyword arguments will raise a TypeError ?

# This will raise an error
try:
    divmod(x=10, y=3)
except TypeError as e:
    print("Error:", e)

Error: divmod() takes no keyword arguments

Function Parameter Types

Conclusion

The slash (/) enforces positional-only parameters, preventing keyword argument usage. This design choice improves function call performance and maintains cleaner APIs for certain built-in functions like divmod().