Functions

Write once, use anywhere

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📖 Concept Recap

def defines a function. return sends a value back to the caller.
Default parameters make arguments optional: def greet(name, greeting="Hello")
Multiple return values: return a, b returns a tuple — unpack with x, y = func()
Good functions do ONE thing and do it well. Name them with verbs: calculate_total(), format_name().
Docstrings (optional but good): describe what a function does with a string on the first line inside.

👀 Worked Example

A BMI calculator with default parameters and multiple return values:

def calculate_bmi(weight_kg, height_m, unit="metric"): if unit == "imperial": weight_kg = weight_kg * 0.453592 height_m = height_m * 0.0254 bmi = weight_kg / (height_m ** 2) if bmi < 18.5: category = "Underweight" elif bmi < 25: category = "Normal weight" elif bmi < 30: category = "Overweight" else: category = "Obese" return round(bmi, 1), category bmi, category = calculate_bmi(70, 1.75) print(f"BMI: {bmi} — {category}") bmi2, cat2 = calculate_bmi(154, 69, unit="imperial") print(f"BMI: {bmi2} — {cat2}")

✏️ Guided

Exercise 1 — Temperature Converter

Complete the temperature converter by filling in the blanks. The formulas are:

F to C: (F - 32) × 5/9
C to F: C × 9/5 + 32
C to K: C + 273.15
K to C: K - 273.15

def convert_temperature(value, from_unit, to_unit):
    # Convert to Celsius first
    if from_unit == "F":
        celsius = (value - 32) * ___    # fill in: 5/9
    elif from_unit == "K":
        celsius = value - ___           # fill in: 273.15
    else:
        celsius = value

    # Convert from Celsius to target unit
    if to_unit == "F":
        return celsius * ___ + ___      # fill in: 9/5 and 32
    elif to_unit == "K":
        return celsius + ___            # fill in: 273.15
    else:
        return celsius

# Expected outputs:
print(convert_temperature(100, "C", "F"))   # 212.0
print(convert_temperature(32, "F", "C"))    # 0.0
print(convert_temperature(0, "C", "K"))     # 273.15
print(convert_temperature(300, "K", "C"))   # 26.85

Output will appear here...

💡 Hint: In Python, 5/9 gives a float automatically. You can write 5/9 directly.

💪 Independent

Exercise 2 — List Statistics

Write a function summarize_list(numbers) that takes a list of numbers and returns a dictionary with these keys: min, max, sum, average, count, range (max - min).

Test it with: [4, 8, 15, 16, 23, 42] and print all the results neatly.

# Your code here

Output will appear here...

💡 Hint: Use Python’s built-ins: min(), max(), sum(), len(). Return a dict: return {"min": ..., "max": ..., ...}

🔥 Challenge

Exercise 3 — Table Formatter

Write a function format_table(headers, rows) that prints data as a formatted table with | separators and a header divider line. Example output:

| Name        | Score | Grade |
|-------------|-------|-------|
| Alice Smith |    95 | A     |
| Bob Jones   |    82 | B     |
| Carol Wu    |    74 | C     |

# Your format_table function here

# Test it:
headers = ['Name', 'Score', 'Grade']
rows = [
    ['Alice Smith', 95, 'A'],
    ['Bob Jones', 82, 'B'],
    ['Carol Wu', 74, 'C']
]
format_table(headers, rows)

Output will appear here...

💡 Hint: Find the max width for each column. Use str(val).ljust(width) or rjust(width) for alignment. Join columns with " | ".join(...).

🏆 Mini Project

Mini Project — Unit Converter Toolkit

Build a toolkit of 4 converter functions. Then write a master function that runs all four on any input value and prints a neat summary. Test with value = 100.

miles_to_km(miles) — 1 mile = 1.60934 km
kg_to_lbs(kg) — 1 kg = 2.20462 lbs
celsius_to_fahrenheit(c) — standard formula
usd_to_eur(usd, rate=0.92) — with a default exchange rate
run_conversions(value) — runs all four and prints a formatted summary

# Build your converter toolkit here

# Test it:
run_conversions(100)

Output will appear here...

💡 Hint: Each converter is 1-2 lines. The run_conversions function calls all four and uses f-strings to print results. E.g.: print(f"100 miles = {miles_to_km(100):.2f} km")

✅ Lab 4 Complete!

You’ve built functions with default parameters, multiple return values, and created a reusable toolkit. Functions are the key to writing code that’s clean, testable, and reusable.

Continue to Lab 5: Lists & Dictionaries →

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