Lecture III - Building Reusable Functions

Programming with Python

Author
Affiliation

Dr. Tobias Vlćek

Kühne Logistics University Hamburg - Fall 2026

Checkpoint 1

The quarterly board review. The first 40 minutes are the checkpoint. It starts now.

  • Individual work: no AI, no neighbors, no chat
  • The link is on Moodle. Open it and start
  • ~6 short tasks: write code, trace code, fix a bug, answer a multiple choice
  • It sweeps Sessions I-II: variables, types, round and f-strings, if/elif, for loops

When you’re done

Menu → Download → Download Python code → upload the .py to the “Checkpoint 1” assignment on Moodle, before the 40 minutes are up. No retakes: one sitting.

. . .

The green live checks are provisional. The final grading runs on my side. Everything in it was rehearsed in the labs.

Episode 3: The Copy-Paste Soup

After the board review

On to Episode 3.

. . .

Tobi has been “reusing” code the only way he knows how: he pasted the same receipt block 14 times, once per menu item. A ten-cent price change yesterday cost him a whole afternoon of hunting down copies.

. . .

Today the code learns to reuse itself. We meet the function, and build a first class.

Writing Functions

The copy-paste pain

Tobi totals each receipt line by hand, the same shape, over and over:

print(round(2 * 4.50, 2))    # 2 wraps
print(round(1 * 12.00, 2))   # 1 bowl
print(round(5 * 3.20, 2))    # 5 fries
9.0
12.0
16.0

. . .

Same calculation, retyped every time. Change the rule once and you must fix it everywhere. There is a better way.

def: name the calculation once

Write the calculation once, give it a name, and call it whenever you need it:

def line_total(qty, price):     # def NAME(inputs):
    return round(qty * price, 2)  # indented body, hands a value back

print(line_total(2, 4.50))
9.0

. . .

def starts the definition · the name is line_total · the block is indented · return hands the answer back.

Parameters vs arguments

  • Parameters are the names in the definition, the function’s inputs: qty, price
  • Arguments are the actual values you pass when you call it: 2, 4.50
def line_total(qty, price):      # qty and price are PARAMETERS
    return round(qty * price, 2)

print(line_total(3, 3.20))       # 3 and 3.20 are ARGUMENTS
9.6

. . .

Same function, different arguments, different result. No retyping.

Predict: which number is the bill?

Tobi wants a 40 % tip on a 10 EUR bill… no wait, a 10 % tip on 40 EUR. He types it in a hurry. What prints?

def with_tip(bill, percent):
    return round(bill * (1 + percent / 100), 2)

print(with_tip(10, 40))

a) 44.0 b) 14.0 c) Error

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: position decides, not intent

b) 14.0: arguments land on parameters by position. 10 becomes bill, 40 becomes percent: a 40 % tip on 10 EUR. Python cannot read what Tobi meant, only where he put it.

def with_tip(bill, percent):
    return round(bill * (1 + percent / 100), 2)

print(with_tip(10, 40))   # bill=10, percent=40
print(with_tip(40, 10))   # bill=40, percent=10
14.0
44.0

return: hand the value back

return sends a value back to whoever called the function, so you can store it and use it later:

def line_total(qty, price):
    return round(qty * price, 2)

subtotal = line_total(3, 3.20)   # catch what came back
print(subtotal)
print(subtotal + 1.50)           # ...and keep using it
9.6
11.1

Predict: print or return?

label_price prints but has no return. What does the last line show?

def label_price(price):
    print(f"{price:.2f} EUR")

result = label_price(8.50)
print(result)

a) 8.50 EUR b) Error c) None

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: print shows, return hands back

c) None: label_price prints its line while running, but with no return it hands back None. That None is what lands in result. Printing is not returning.

def label_price(price):
    print(f"{price:.2f} EUR")

result = label_price(8.50)   # prints while running...
print(result)                # ...but the value handed back is None
8.50 EUR
None

Your turn: 5-10 minutes

Open the exercise

Scan the QR or type the link:

python.tobiasvlcek.com/notebooks/ex_03_a/

First predict what happens, then run it.

Default arguments

A parameter can carry a default, used when the caller leaves it out:

def service_fee(total, rate=0.05):   # rate defaults to 5 %
    return round(total * rate, 2)

print(service_fee(80))         # default rate → 4.0
print(service_fee(80, 0.10))   # override it → 8.0
4.0
8.0

. . .

Defaults let one function cover the common case and the special case.

Functions can call functions

A function can use another function you already wrote (reuse builds on reuse):

def bill(qty, price):
    items = line_total(qty, price)        # reuse line_total
    return round(items + service_fee(items), 2)  # ...and service_fee

print(bill(2, 4.50))
9.45

. . .

line_total and service_fee do their jobs; bill just orchestrates. That’s how small pieces become a program.

Your turn: 5-10 minutes

Open the exercise

Scan the QR or type the link:

python.tobiasvlcek.com/notebooks/ex_03_b/

First predict what happens, then run it.

Scope, then a First Class

Local names stay local

A parameter is the function’s own private copy. What happens inside stays inside:

def bump(n):
    n = n + 10       # changes the function's OWN copy of n
    return n

print(bump(3))       # 13, the returned value
13

. . .

Names created inside a function live in its scope. They vanish when the function ends.

Predict: does the global move?

bump adds ten to its parameter. We call it, then print stock. What does the last line print?

def bump(n):
    n = n + 10
    return n

stock = 3
bump(stock)
print(stock)

a) 3 b) 13 c) Error

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: the outside variable is untouched

a) 3: bump changes only its own copy n. We never stored what it returned, so stock out here never moves. The function cannot reach out and rewrite your variables.

def bump(n):
    n = n + 10
    return n

stock = 3
bump(stock)      # the return value is thrown away
print(stock)     # still 3
3

Why that’s a feature, not a limit

  • A function can’t secretly change your variables, no spooky action at a distance
  • You can call it a hundred times and trust your data stays put
  • To use a result, you catch the return (stock = bump(stock)). Deliberate, visible

. . .

Isolation is what makes functions safe to reuse. That is the whole point of this episode.

A class bundles data with behavior

Sometimes data and the things you do with it belong together. A class is a blueprint that bundles both:

  • data: the facts about one thing (a courier’s name, their distance)
  • behavior: what you can compute from it (their fee)

. . .

A method is just a function that lives inside a class and can read the object’s own data.

class, __init__, and self

class Delivery:
    def __init__(self, courier, distance_km):  # runs when you build one
        self.courier = courier          # store data ON the object
        self.distance_km = distance_km
        self.rate_per_km = 1.20         # every delivery carries its rate

    def fee(self):                       # a method: note self
        return round(self.distance_km * self.rate_per_km, 2)

. . .

__init__ sets up a new object · self is this object · attributes are stored on self and read back through self. fee() multiplies two stored attributes.

Build one, call its method

Instantiate the class to make an object, then use its data and its methods:

trip = Delivery("Nadia", 4)   # build one, __init__ runs

print(trip.courier)           # read stored data
print(trip.fee())             # call the method
Nadia
4.8

. . .

trip is one Delivery object. trip.fee() computes from its own stored distance: data and behavior, traveling together.

Predict: does the second delivery overwrite the first?

Two deliveries get built from the same class, one after the other. What does the last line print?

trip = Delivery("Nadia", 4)
second = Delivery("Omar", 10)

print(trip.fee())

a) 12.0 b) Error c) 4.8

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: every object carries its own data

c) 4.8: __init__ ran twice and stored the data on two different objects. self inside trip.fee() is trip, so it reads Nadia’s 4 km, not Omar’s 10. Instances do not share attributes.

trip = Delivery("Nadia", 4)
second = Delivery("Omar", 10)

print(trip.fee())      # self is trip: 4 km
print(second.fee())    # self is second: 10 km
4.8
12.0

How far we go with classes

  • Today you write one small class with an __init__ and one method. That’s it
  • Classes get tricky fast (inheritance, self confusion), and that is not this course
  • We use them only to bundle data with behavior, nothing deeper

. . .

If the self keyword feels odd right now, copy the shape from the worked example. It gets clearer after you have written a few.

Your turn: 5-10 minutes

Open the exercise

Scan the QR or type the link:

python.tobiasvlcek.com/notebooks/ex_03_c/

First predict what happens, then run it.

To the Lab

After the break: the lab

  • Head to the lab notebook: Episode 3: The Copy-Paste Soup
  • You’ll write the fee and tip functions, fix a function that forgets to return, give a parameter a default, and build the startup’s first Order class
  • It runs entirely in your browser: no setup, just click and code

. . .

Download your .py before you leave. Closing the tab without downloading loses your work, and downloading is exactly how you just handed in the checkpoint.

Wrap-up

Three things to remember

  1. A function is written once with def, takes parameters, and hands a value back with return: print shows, return gives back (no return → None)
  2. Names inside a function are local: it can’t quietly change your variables, which is exactly what makes it safe to reuse
  3. A class bundles data (on self, via __init__) with behavior (methods). You’ll write just one small one today

. . .

Next time, Episode 4: the menu outgrows Tobi’s seventeen loose variables (price1, price2, price_final_FINAL2), and nobody can find anything. We give the data a shape: lists and dictionaries.

Literature

Books to start with

  • Downey, A. B. (2024). Think Python: How to think like a computer scientist (Third edition). O’Reilly. Link to free online version
  • Elter, S. (2021). Schrödinger programmiert Python: Das etwas andere Fachbuch (1. Auflage). Rheinwerk Verlag.

. . .

Nothing new here, but these are still great books to start with!

. . .

For more, see the literature list of this course.