Default Parameters

Set default values for function parameters

Default Parameters — Sensible Defaults for Functions

What Are Default Parameters?

Sometimes a parameter should have a default value that's used if the caller doesn't provide one. This makes functions more flexible — callers can use the common case with minimal code, but override when needed.

Syntax

python

def greet(name="Guest"):
    print(f"Hello, {name}!")

greet("Alice")    # Hello, Alice!
greet()           # Hello, Guest! (uses default)

Part	What it means
`name="Guest"`	Default parameter: if no argument, `name = "Guest"`
`greet()`	No argument → uses the default
`greet("Alice")`	Has argument → overrides the default

How It Works

python

def greet_full(greeting="Hello", name="World"):
    print(f"{greeting}, {name}!")

greet_full()                       # Hello, World!
greet_full("Hi")                   # Hi, World!
greet_full("Hey", "Python")        # Hey, Python!
greet_full(name="Alice")           # Hello, Alice! (keyword argument)

� **Positional vs Keyword Arguments:** `greet_full("Hi", "Python")` passes by position. `greet_full(name="Alice")` passes by keyword — you specify which parameter to set. Keyword args let you skip earlier defaults.

Rules for Default Parameters

python

# ✅ OK: non-default first, then defaults
def create_user(name, age=18, city="Unknown"):
    print(f"{name}, {age}, from {city}")

# ❌ Error: default before non-default
# def create_user(name="Guest", age):    # SyntaxError!

️ **Defaults go LAST.** Parameters with default values must come after parameters without defaults.

Practical Example

python

def send_email(to, subject="No Subject", cc=None):
    print(f"Sending to: {to}")
    print(f"Subject: {subject}")
    if cc:
        print(f"CC: {cc}")

send_email("[email protected]")                    # Uses both defaults
send_email("[email protected]", "Meeting Tomorrow")  # Custom subject
send_email("[email protected]", cc="[email protected]")  # Keyword arg

Why Use Default Parameters?

**API design** — Make common usage simple, advanced usage possible

**Backward compatibility** — Add new parameters without breaking existing code

**DRY principle** — Avoid overloading with multiple similar functions

**Configuration** — Sensible defaults for settings (e.g., `language="en"`)

Common Mistakes

❌ Wrong	Why	✅ Right
`def func(x=5, y):`	Default params must come after non-default	Swap: `def func(y, x=5):`
Default is a mutable value like an empty list	Same list shared across calls — unexpected behavior	Use `None` and create inside: `if x is None: x = []`
Assuming defaults are evaluated fresh each call	Defaults are evaluated once at definition time	For mutable defaults, use the `None` pattern

Your Turn!

Define a function power(base, exponent=2) that returns base ** exponent. Call it:

With just a base: `power(5)` — should return 25 (5 squared)

With base and exponent: `power(2, 10)` — should return 1024

💡Need a hint? (3 available)

#1Default params are used when no argument is passed

#2Defaults must come AFTER non-default parameters

#3Override defaults by passing an argument

🔑Show solution (try it yourself first!)

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

print(power(5))
print(power(2, 10))

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