Q.What is the difference between default parameters and positional parameters in Python ? Also give an example of a function header which uses both.

The key idea is how arguments are assigned to parameters when a function is called.

Positional parameters must be supplied in the exact order they appear in the function definition. Default parameters have a default value assigned in the header; if the caller omits that argument, the default is used. Default parameters must always come after all positional parameters in the definition.

Example function header using both:

def create_profile(name, age, city="Unknown"):
``` …

The core idea

When you define a function in Python, you decide which inputs are mandatory and which are optional. Positional parameters are the mandatory ones — the caller must supply a value for each, in the exact order they appear. Default parameters are optional — they already have a value baked in, so the caller can skip them if the default works.

Think of it like ordering at a restaurant. The positional parameter is the dish you must choose (e.g., "pasta"). The default parameter is like asking for extra cheese — if you say nothing, you get no extra cheese; but you can override it by saying "yes, please."

Step-by-step breakdown

1. Positional parameters

   These are the parameters listed first in the function definition. They get their values from the arguments passed in the same left-to-right order. Every positional parameter must be supplied an argument — there is no fallback value.

   Example:

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

   Here, a and b are positional. Calling add(5) would raise a TypeError because b is missing.

2. Default parameters

   These are assigned a default value in the function header using the = operator. If the caller omits that argument, the default is used. If they do pass a value, it overrides the default.

   Example:

   def power(base, exp=2):
       return base ** exp
   

   exp has default 2. So power(3) returns 3 ** 2, which is 9, while power(3, 3) returns 3 ** 3, which is 27.

3. The key difference

   • Positional: required, no default, order matters.
   • Default: optional, has a default, can be omitted. Python enforces that all positional parameters come before any default parameters in the function header. This is not just convention — it's a syntax rule. If you write def f(a=1, b):, Python will raise a SyntaxError.

def append_to(item, lst=[]):
    lst.append(item)
    return lst
``` …