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Python Basics 3 – Functions and ReuseDefault Parameters (5/6)
Chapter 5
Python Basics 3 – Functions and Reuse

Default Parameters

22 minutes read

Beginner friendly

Includes practice

Hi there! Today you’ll learn how to give your Python functions built-in backup values—so they still work even when the caller leaves something out. This feature is called default parameters.

Runtic's Learning Tips

  • Think of default parameters like polite defaults: your code keeps working even if someone forgets to pass a value.

  • Reading examples aloud helps you understand the behavior line by line—try explaining what greet() will print.

  • Even experienced coders use lookup hints for mutable defaults—don’t feel bad revisiting this!

The Big Idea

Sometimes a function has a typical value that should be used unless the caller says otherwise.

Instead of forcing the user to pass this value every time, we can bake it right into the function header.

Real-world analogy

  • Think of ordering coffee: if you say “Regular, please,” the barista automatically adds milk and sugar unless you specify a change.
  • In Python, you can set milk='yes', sugar='yes' as defaults.

Basic Syntax

Example Code
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  • The = goes in the function header, not in the call.
  • Parameters with defaults are optional when calling the function.

Worked Example

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Quick Practice

2 minutes

Test how well you understand the behavior of functions with default parameters.

What will `greet()` print if you don't pass an argument?

How do you override the default in `greet(name='friend')`?

Parameter Ordering Rule

Rule: Required parameters must come before optional (default) ones.

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A Heads-Up on Mutable Defaults

Watch out when using things like lists or dictionaries as default values.

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⚠ Default list bucket=[] is shared between calls!

✅ Safer Pattern:

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Why Mutable Defaults Are Risky

  • Default values like lists are created once during function definition
  • They're shared across all future calls
  • This can lead to unexpected behavior like growing lists across multiple uses

Practice Zone

Try filling in these function blanks below. Use default parameters where appropriate.

Remember: tax and discount should already have fallback values.

Practice Challenge

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📝 Your Task:

Write a function called final_price(price,tax_rate=0.05,discount=0.0)that returns the total price after adding tax and subtracting any discount. Then call it with a price of 100 to verify the result

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Return the price after adding tax and subtracting discount.

Practice Challenge

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📝 Your Task:

Write a function called letter_grade(score,passing=60)that returns "Pass"if the score is greater than or equal to the passing mark, and "Fail"otherwise. Test it with a score above 60 to verify the result.

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If score is greater than or equal to passing, return 'Pass'; otherwise return 'Fail'.

Quick Summary

  • Default parameters allow functions to run with optional inputs

  • Required parameters must go before defaults

  • Avoid using mutable values like [] or {} as defaults

Reflection Prompt

Ask yourself:

  • Which parameters did you choose to make optional, and why?
  • Did you accidentally place a required parameter after a default one?

Write a quick sentence to clarify your thinking before moving forward.

Mini Self-Quiz

Don't scroll back – see how much you retained:

Quick Practice

3 minutes

Check your understanding of default parameters.

Where must the `=` default assignment appear?

True or False: `def f(x=[])` is safe for lists.

Fill in the blank: Parameters with defaults are _______ when calling the function.

Lesson Complete!
100%
Progress
What You've Learned:
  • You learned how to set default values for function parameters
  • You practiced defining functions with both required and optional arguments
  • You discovered the pitfalls of using mutable default values like lists

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