Default and Keyword Arguments

🎯 Learning Objective

After this lesson, you will be able to:

• Define optional parameters with safe default values.
• Call the same function with omitted defaults or explicit overrides.
• Use keyword arguments to make multi-parameter calls easier to read.
• Order required and default parameters correctly in a function signature.

Short Explanation

A default argument gives a parameter a value when the caller omits that argument:

def format_status(service, status="unknown"):
    return f"{service}: {status}"

format_status("worker") uses "unknown", while format_status("worker", "healthy") overrides it.

Parameters without defaults must come before parameters with defaults:

def create_job(name, retries=3):
    ...

Python also lets callers pass arguments by name:

create_job(name="backup", retries=5)

Keyword arguments make calls clearer when several values have similar types or when only one optional setting needs an override. Their order can differ from the parameter order, as long as every name is valid and no argument is supplied twice.

Function tests can verify default behavior by calling a function with fewer arguments. The current function-task runner calls functions positionally, so this lesson practices keyword-call syntax through focused code tasks while using real function tests for the function behavior.

Advanced topics such as *args, **kwargs, keyword-only parameters, and mutable default pitfalls belong in the later Functions Deep Dive unit.

💡 Tip: Defaults should represent a safe, unsurprising common case—not a value chosen only to make one example shorter.

Clean Code Examples

1. Use a default when a value is omitted

def cache_label(service, ttl=300):
    return f"{service}: {ttl}s"

print(cache_label("api"))

Output:

api: 300s

The caller supplies only the required argument.

2. Override the default

def retry_label(job, retries=3):
    return f"{job}: {retries} retries"

print(retry_label("sync", 5))

Output:

sync: 5 retries

The second argument replaces the default value.

3. Use keyword arguments for clarity

def build_window(name, width=800, height=600):
    return f"{name}: {width}x{height}"

print(build_window(name="Dashboard", height=720, width=1280))

Output:

Dashboard: 1280x720

Keyword arguments may be supplied in a different order.

4. Keep required parameters first

def create_alert(message, level="info", channel="console"):
    return f"[{level}] {channel}: {message}"

print(create_alert("Queue delayed", channel="slack"))

Output:

[info] slack: Queue delayed

Only the channel is overridden; the level keeps its default.

⚠️ Common Mistakes

1. Do not place a required parameter after a default parameter.
2. Do not provide the same argument both positionally and by keyword.
3. Do not misspell a keyword argument; it must match the parameter name.
4. Do not use a confusing default that changes the function's meaning unexpectedly.