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

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

**Positional parameters** are matched to arguments strictly by their position/order in the function call and must always be supplied by the caller, whereas **default parameters** are assigned a default value in the function header and become optional — if the caller omits them, the default value is used. **Example function header using both:** ```python def calculate_bill(item, qty, discount=10): ... ``` Here, `item` and `qty` are positional parameters (compulsory, matched by order), while `discount` is a default parameter (optional, defaults to 10 if not passed).

Working with Functions

Positional and Default Parameters

Official Answer

Positional parameters are matched to arguments strictly by their position/order in the function call and must always be supplied by the caller, whereas default parameters are assigned a default value in the function header and become optional — if the caller omits them, the default value is used.

Example function header using both:

``python


def calculate_bill(item, qty, discount=10):
    ...

Here, item and qty are positional parameters (compulsory, matched by order), while discount` is a default parameter (optional, defaults to 10 if not passed).

positional parametersdefault parametersfunction headerkeyword argumentsoptional argumentsparameter orderPython functions

Marking Scheme

11 mark for correctly distinguishing positional parameters (compulsory, order-based) from default parameters (optional, pre-assigned value).
21 mark for a syntactically valid function header example containing at least one positional and one default parameter, with defaults placed after positional parameters.

Hint

Positional = compulsory, matched by order; Default = optional, has a fallback value — and defaults must come last in the header.

Quick Oral Answer

Positional parameters must be supplied in order and have no default, while default parameters have a pre-assigned value in the header and can be skipped by the caller; defaults must always come after positional parameters.

Analysis & Explanation

This question checks conceptual clarity on Python's parameter-passing mechanisms, which is foundational to writing flexible functions.

Key distinction

Positional parameters have no default value; the caller MUST supply a value for each, and values are assigned strictly in the order the arguments appear.
Default parameters carry a default value specified in the function header (using =); the caller MAY omit the corresponding argument, in which case Python uses the default.

Rule students often miss

In a function header, ALL default parameters must appear AFTER all positional parameters — def f(a=1, b): is a SyntaxError because a non-default parameter cannot follow a default one.

Common Mistakes

1Writing a function header with a default parameter before a positional parameter, e.g. `def f(a=1, b):`, which is a SyntaxError.
2Confusing 'default parameters' with 'keyword arguments' passed at the call site — the default value is defined in the function's header, not at the call.

Interesting Facts

Python evaluates default parameter values only ONCE, at the time the function is defined — a classic pitfall occurs when a mutable default (like a list) is modified across multiple calls, retaining state unexpectedly.

Python also supports keyword-only parameters (placed after a `*` in the header), which must always be passed by name, offering a third parameter-passing style beyond positional and default.

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Frequently Asked Questions

Can a positional parameter appear after a default parameter in a function header?

No, this raises a SyntaxError. All positional (non-default) parameters must be listed before any default parameters in the function header.

Can a default parameter be overridden when calling the function?

Yes. If the caller explicitly passes a value for that parameter, it overrides the default; only if the argument is omitted does Python use the default value.