C3,4.Lists & Conditional Control Flow

Lists & Conditional Control Flow
Session 2 — Lists · If Statements
SEC1302 · Python for Information Security
>>> ports = [22, 80, 443]
>>> if 22 in ports:
... print("SSH exposed")
SSH exposed
Agenda
Lists
What is a list (Accessing elements · Index positions · Using individual values)
1.Changing, adding, removing (Modifying · Adding · Removing)
2.Organizing a list (sort() · sorted() · Reverse order · Length)
3.Avoiding index errors
4.Looping through a list (Closer look · More work in a loop · After a loop · Indentation errors)
5.Numerical lists (range() · range() to list · Simple statistics · List comprehensions)
6.Working with part of a list (Slicing · Looping through a slice · Copying)
7.Tuples (Defining · Looping · Writing over a tuple)
8.
If Statements
A simple example
1.Conditional tests (Equality · Ignoring case · Inequality · Numerical comparisons · Multiple conditions, in / not in)
2.if statements (Simple if · if-else · if-elif-else · Multiple elif · Omitting else · Testing multiple conditions)
3.Using if statements with lists (Special items · Non-empty list · Multiple lists)
4.Exercises · Chapter summaries
What learners will be able to do after this session
Create, index, modify, sort and slice lists; explain the difference between a list and a tuple.
Iterate over lists with for loops and range(); build lists with list comprehensions.
Write conditional tests (==, !=, <, >, and, or, in, not in) and if / if-else / if-elif-else chains.
Combine lists and if statements to implement simple security checks (allow-lists, block-lists, input normalisation).
Mapped to CLO1 — apply Python to solve basic programming problems (C3-P2).
Lists
What Is a List?
A list is an ordered collection of items, written in square brackets [ ] with items separated by commas.
A list can hold any type of value, and items do not need to be related; names are usually plural (bicycles, ports, users).
Lists are mutable: items can be changed after creation.
bicycles = ['trek', 'cannondale', 'redline', 'specialized']
print(bicycles)['trek', 'cannondale', 'redline', 'specialized']
Security examples of lists
open ports [22, 80, 443]
banned IP addresses
usernames from a log file
Sources: docs.python.org/3/tutorial/introduction.html#lists
docs.python.org/3/library/stdtypes.html#lists
Accessing elements
bicycles = ['trek', 'cannondale', 'redline', 'specialized']
print(bicycles[0])
print(bicycles[0].title())
message = f"My first bicycle was a {bicycles[0].title()}."
print(message)trek
Trek
My first bicycle was a Trek.
Explanation
Write the list name followed by the index in square brackets.
The result is the item itself (here a str), so string methods from Chapter 2 such as title() apply directly.
Individual items can be used inside f-strings like any other variable.
Source: docs.python.org/3/tutorial/introduction.html#lists
Index positions start at 0, not 1
trek
positive: 0 | negative: -4
cannondale
positive: 1 | negative: -3
redline
positive: 2 | negative: -2
specialized
positive: 3 | negative: -1
bicycles = ['trek', 'cannondale', 'redline', 'specialized']
print(bicycles[1])
print(bicycles[3])
print(bicycles[-1])cannondale
specialized
specialized
The second item has index 1; the fourth item has index 3.
Negative indexes count from the end: -1 is the last item, -2 the second-to-last.
bicycles[-1] works for a list of any length; useful for "the most recent log entry".
Source: docs.python.org/3/library/stdtypes.html#common-sequence-operations
Modifying elements in a list
motorcycles = ['honda', 'yamaha', 'suzuki']
print(motorcycles)
motorcycles[0] = 'ducati'
print(motorcycles)['honda', 'yamaha', 'suzuki']
['ducati', 'yamaha', 'suzuki']Assigning to list[index] replaces the item at that position; other items are unchanged.
Contrast
Strings are immutable (Chapter 2), so "honda"[0] = "H" raises TypeError; lists are mutable.
Source: docs.python.org/3/library/stdtypes.html#mutable-sequence-types
Adding elements to a list
append() adds to the end
motorcycles = []
motorcycles.append('honda')
motorcycles.append('yamaha')
motorcycles.append('suzuki')
print(motorcycles)['honda', 'yamaha', 'suzuki']
insert() adds at a position
motorcycles = ['honda', 'yamaha', 'suzuki']
motorcycles.insert(0, 'ducati')
print(motorcycles)['ducati', 'honda', 'yamaha', 'suzuki']
Starting from an empty list and calling append() is the standard way to collect data whose values are not known in advance (user input, scan results, parsed log lines).
Source: docs.python.org/3/tutorial/datastructures.html#more-on-lists
Removing elements — del and pop()
del by position
motorcycles = ['honda', 'yamaha', 'suzuki']
del motorcycles[1]
print(motorcycles)['honda', 'suzuki']pop() removes the last item and returns it
motorcycles = ['honda', 'yamaha', 'suzuki']
popped_motorcycle = motorcycles.pop()
print(motorcycles)
print(popped_motorcycle)['honda', 'yamaha']
suzuki
pop(index) removes any position
motorcycles = ['honda', 'yamaha', 'suzuki']
first_owned = motorcycles.pop(0)
print(f"The first motorcycle I owned was a {first_owned.title()}.")The first motorcycle I owned was a Honda.Visual: "stack" diagram for pop() — the top of the stack is the end of the list.
Sources: docs.python.org — del statement
docs.python.org — using lists as stacks
Removing by value — remove(), and which method to choose
motorcycles = ['honda', 'yamaha', 'suzuki', 'ducati']
too_expensive = 'ducati'
motorcycles.remove(too_expensive)
print(motorcycles)
print(f"\nA {too_expensive.title()} is too expensive for me.")['honda', 'yamaha', 'suzuki']
A Ducati is too expensive for me.remove() deletes only the FIRST occurrence; removing a missing value raises ValueError: list.remove(x): x not in list
Decision table
Rule of thumb (coursebook): use del when the item is no longer needed; use pop() when the item is still needed.
Sorting a list permanently with sort()
cars = ['bmw', 'audi', 'toyota', 'subaru']
cars.sort()
print(cars)
cars.sort(reverse=True)
print(cars)['audi', 'bmw', 'subaru', 'toyota']
['toyota', 'subaru', 'bmw', 'audi']
Notes
sort() changes the list in place and returns None (a common bug: cars = cars.sort() sets cars to None).
Python's sort is guaranteed to be stable: items that compare equal keep their original order.
Sources: docs.python.org — list.sort
docs.python.org — sorting HOWTO
Sorting a list temporarily with sorted()
cars = ['bmw', 'audi', 'toyota', 'subaru']
print("Here is the original list:")
print(cars)
print("\nHere is the sorted list:")
print(sorted(cars))
print("\nHere is the original list again:")
print(cars)Here is the original list:
['bmw', 'audi', 'toyota', 'subaru']

Here is the sorted list:
['audi', 'bmw', 'subaru', 'toyota']

Here is the original list again:
['bmw', 'audi', 'toyota', 'subaru']
Comparison
Source: docs.python.org/3/library/functions.html#sorted
Uppercase letters sort before lowercase — use key=
cars = ['bmw', 'Audi', 'toyota', 'Subaru']
print(sorted(cars))
print(sorted(cars, key=str.lower))['Audi', 'Subaru', 'bmw', 'toyota']
['Audi', 'bmw', 'Subaru', 'toyota']Strings compare by Unicode code point, so 'S' (83) comes before 'b' (98). The key function converts each item before comparison without changing the stored values.
Security example — sorting IP addresses correctly
import ipaddress
ips = ['10.0.0.2', '10.0.0.10', '9.1.1.1']
print(sorted(ips))
print(sorted(ips, key=ipaddress.ip_address))['10.0.0.10', '10.0.0.2', '9.1.1.1']
['9.1.1.1', '10.0.0.2', '10.0.0.10']Key message: text sorting puts "10.0.0.10" before "10.0.0.2"; analysts should sort IPs as addresses.
Sources: docs.python.org — key functions
docs.python.org — ipaddress
Printing in reverse order · Finding the length
cars = ['bmw', 'audi', 'toyota', 'subaru']
cars.reverse()
print(cars)
print(len(cars))['subaru', 'toyota', 'audi', 'bmw']
4
Notes
reverse() flips the current order in place (not alphabetical); calling it again restores the original order.
len() counts items starting from 1; the last valid index is len(lst) - 1.
Related built-in: reversed(lst) returns an iterator without changing the list.
Sources: docs.python.org — mutable sequence types
docs.python.org — len
docs.python.org — reversed
Avoiding index errors when working with lists
motorcycles = ['honda', 'yamaha', 'suzuki']
print(motorcycles[3])Traceback (most recent call last):
  File "motorcycles.py", line 2, in <module>
    print(motorcycles[3])
IndexError: list index out of rangeA list of 3 items has indexes 0, 1, 2; index 3 asks for a fourth item.
Other cases
motorcycles = [] then motorcycles[-1] → IndexError: list index out of range
[].pop() → IndexError: pop from empty list
Debugging tip (coursebook): when an index error appears, print the list or print len(list) to see what it really contains.
Source: docs.python.org — IndexError
Why index errors matter in security
Key message: Python checks every index at runtime, which removes a whole class of memory-corruption bugs; programs must still validate indexes that come from user input.
Sources: cwe.mitre.org — CWE-125 · cwe.mitre.org — CWE-129
docs.python.org — IndexError
Looping through an entire list
magicians = ['alice', 'david', 'carolina']
for magician in magicians:
    print(magician)alice
david
carolina
Flow
The for statement iterates over the items of any sequence in order; no index variable is needed.
Sources: docs.python.org — for statements
docs.python.org — the for statement
A closer look at looping
Loop variable naming
The loop variable name is chosen by the programmer; singular/plural pairs make code readable:
for cat in cats:
for dog in dogs:
for item in list_of_items:
for port in open_ports:The loop runs once per item, whether the list has 3 items or 1,000,000.
C++ classic loop vs Python
// C++
for (int i = 0; i < n; i++) { cout << arr[i]; }

# Python
for item in items:
    print(item)When the index is also needed, use enumerate():
for i, magician in enumerate(magicians):
    print(i, magician)0 alice
1 david
2 carolinaSource: docs.python.org — enumerate
More work within a loop · Doing something after a loop
magicians = ['alice', 'david', 'carolina']
for magician in magicians:
    print(f"{magician.title()}, that was a great trick!")
    print(f"I can't wait to see your next trick, {magician.title()}.\n")
print("Thank you, everyone. That was a great magic show!")Alice, that was a great trick!
I can't wait to see your next trick, Alice.

David, that was a great trick!
I can't wait to see your next trick, David.

Carolina, that was a great trick!
I can't wait to see your next trick, Carolina.

Thank you, everyone. That was a great magic show!
Key point
Indented lines run once per item; the unindented final line runs once after the loop.
Security parallel
A scanner reports each host inside the loop, then prints one summary line after the loop.
Avoiding indentation errors
Key message: Python uses indentation to define blocks; PEP 8 recommends 4 spaces per level and no mixing of tabs and spaces.
Sources: docs.python.org — indentation · PEP 8 — indentation · docs.python.org — 3.10 error messages
Using the range() function
for value in range(1, 5):
    print(value)1
2
3
4
Explanation
range(start, stop) stops BEFORE stop ("off-by-one" behavior); range(1, 6) is needed to reach 5.
range(6) starts from 0 → 0, 1, 2, 3, 4, 5.
A range object stores only start, stop and step, so it uses the same small amount of memory regardless of its length.
Source: docs.python.org — ranges
Using range() to make a list of numbers
numbers = list(range(1, 6))
print(numbers)
even_numbers = list(range(2, 11, 2))
print(even_numbers)
squares = []
for value in range(1, 11):
    squares.append(value ** 2)
print(squares)[1, 2, 3, 4, 5]
[2, 4, 6, 8, 10]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
Security example: well-known port range
well_known_ports = range(0, 1024)
print(len(well_known_ports), well_known_ports[0], well_known_ports[-1])1024 0 1023Sources: docs.python.org — range function
RFC 6335 (port number ranges: System Ports 0–1023)
Simple statistics with a list of numbers
digits = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
print(min(digits))
print(max(digits))
print(sum(digits))0
9
45
Security example — failed logins per hour
failed_logins = [3, 0, 1, 42, 2]
print(max(failed_logins), sum(failed_logins) / len(failed_logins))42 9.6The peak (42) stands out against the mean (9.6) — the basis of simple threshold alerts.
Optional: the statistics module provides mean(), median(), stdev().
Sources: docs.python.org — min
docs.python.org — statistics
List comprehensions
squares = [value ** 2 for value in range(1, 11)]
print(squares)[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]Anatomy: [ expression for variable in iterable ] → [ value**2 | for value in range(1, 11) ]
Comparison: 4-line loop with append() (previous slide) vs 1-line comprehension — same result.
Extension (with a condition)
ports = [21, 22, 80, 443, 3389]
risky = [p for p in ports if p in (21, 23, 3389)]
print(risky)[21, 3389]Guideline: use a comprehension when it stays readable on one line; otherwise use a normal loop.
Sources: docs.python.org — list comprehensions
PEP 202
Slicing a list
players = ['charles', 'martina', 'michael', 'florence', 'eli']
print(players[0:3])
print(players[1:4])
print(players[:4])
print(players[2:])
print(players[-3:])
print(players[::2])['charles', 'martina', 'michael']
['martina', 'michael', 'florence']
['charles', 'martina', 'michael', 'florence']
['michael', 'florence', 'eli']
['michael', 'florence', 'eli']
['charles', 'michael', 'eli']
Notes
Index ruler: start (included) and stop (excluded).
Slicing never raises IndexError — players[10:20] returns [].
Source: docs.python.org — common sequence operations
Looping through a slice
players = ['charles', 'martina', 'michael', 'florence', 'eli']
print("Here are the first three players on my team:")
for player in players[:3]:
    print(player.title())Here are the first three players on my team:
Charles
Martina
Michael
Security parallel
Show the top 5 offending IP addresses after sorting by number of alerts:
for ip in sorted_ips[:5]:
    ...Note: "last N lines of a log" is lines[-N:], similar to the Unix tail command.
Copying a list — a copy is not the same as a second name
Correct copy with a slice
my_foods = ['pizza', 'falafel', 'carrot cake']
friend_foods = my_foods[:]
my_foods.append('cannoli')
friend_foods.append('ice cream')
print(my_foods)
print(friend_foods)['pizza', 'falafel', 'carrot cake', 'cannoli']
['pizza', 'falafel', 'carrot cake', 'ice cream']
This does NOT copy
my_foods = ['pizza', 'falafel', 'carrot cake']
friend_foods = my_foods
my_foods.append('cannoli')
friend_foods.append('ice cream')
print(my_foods)['pizza', 'falafel', 'carrot cake', 'cannoli', 'ice cream']Memory diagram: two names → one list object (alias) vs two names → two list objects (copy). Link back to Chapter 2 "variables are labels".
Advanced note — shallow vs deep copy
import copy
rules = [['22', 'deny'], ['80', 'allow']]
shallow = rules[:]
shallow[0][1] = 'allow'
print(rules)[['22', 'allow'], ['80', 'allow']]A slice copies only the outer list; nested lists are shared. copy.deepcopy(rules) copies them too — important when testing changes to a firewall rule set without altering the original.
Sources: docs.python.org — copy · docs.python.org — copying FAQ
Defining a tuple — an immutable list
dimensions = (200, 50)
print(dimensions[0])
print(dimensions[1])
dimensions[0] = 250200
50
Traceback (most recent call last):
  File "dimensions.py", line 4, in <module>
    dimensions[0] = 250
TypeError: 'tuple' object does not support item assignment
Note: a tuple is defined by the comma, not the parentheses
my_t = (3,)
print(type(my_t), type((3)))<class 'tuple'> <class 'int'>Source: docs.python.org — tuples and sequences
Looping through a tuple · Writing over a tuple
dimensions = (200, 50)
print("Original dimensions:")
for dimension in dimensions:
    print(dimension)
dimensions = (400, 100)
print("\nModified dimensions:")
for dimension in dimensions:
    print(dimension)Original dimensions:
200
50

Modified dimensions:
400
100
Explanation
Items of a tuple cannot be changed, but the variable can be bound to a new tuple. Python raises no error because the name is reassigned; the old tuple is not modified.
Source: docs.python.org — objects, values and types
When to use a list and when to use a tuple
Security example — socket addresses are tuples
server_address = ("127.0.0.1", 8080)
host, port = server_address
print(host, port)127.0.0.1 8080Sources: docs.python.org — list vs tuple FAQ
docs.python.org — socket families
Practice exercises (Chapter 3)
Names · Greetings · Your Own List — create lists, access items, use f-strings.
Guest List · Changing Guest List · More Guests · Shrinking Guest List — append, insert, pop, del.
Seeing the World · Locations · Every Function — sort(), sorted(), reverse(), len().
Pizzas · Animals — for loops with a final line after the loop.
Counting to Twenty · One Million · Summing a Million · Odd Numbers · Threes · Cubes · Cube Comprehension — range(), min/max/sum, comprehensions.
Slices · My Pizzas, Your Pizzas · More Loops — slicing and copying.
Buffet — tuples; confirm Python rejects item assignment.
Optional security twist: store 10 failed-login counts in a list; print the max, the mean, and the top 3 values using sorted(..., reverse=True)[:3].
Chapter 3 — key takeaways
Lists are ordered, mutable, zero-indexed; negative indexes count from the end.
Add with append()/insert(); remove with del, pop(), remove().
sort() changes the list; sorted() returns a new one; key= controls comparison.
for loops + indentation define what repeats; range() and comprehensions build numeric lists.
Slices select parts of a list and make shallow copies; assignment only creates an alias.
Tuples are immutable sequences for fixed groups of values.
Chapter 4 — If Statements
A simple example
cars = ['audi', 'bmw', 'subaru', 'toyota']
for car in cars:
    if car == 'bmw':
        print(car.upper())
    else:
        print(car.title())Audi
BMW
Subaru
Toyota
Explanation
Inside the loop, each car is tested; 'bmw' is an acronym and is printed in uppercase, all others in title case.
Source: docs.python.org — if statements
Checking for equality · Checking for inequality
>>> car = 'bmw'
>>> car == 'bmw'
True
>>> car = 'audi'
>>> car == 'bmw'
Falserequested_topping = 'mushrooms'
if requested_topping != 'anchovies':
    print("Hold the anchovies!")Hold the anchovies!
Key point table
Note: every conditional test evaluates to True or False (type bool).
Source: docs.python.org — comparisons
Ignoring case when checking for equality
>>> car = 'Audi'
>>> car == 'audi'
False
>>> car.lower() == 'audi'
True
>>> car
'Audi'lower() returns a new string for the comparison; the stored value keeps its original case.
Security box — normalise before checking
blocked_users = ['admin', 'root']
username = ' Admin '
print(username in blocked_users)
print(username.strip().lower() in blocked_users)False
Truecasefold() is the recommended method for caseless matching of international text.
When comparing secrets (tokens, password hashes), use hmac.compare_digest() instead of == to reduce timing side channels.
Sources: docs.python.org — str.casefold
docs.python.org — hmac.compare_digest
Numerical comparisons
>>> age = 19
>>> age < 21
True
>>> age <= 21
True
>>> age > 21
False
>>> age >= 21
Falseanswer = 17
if answer != 42:
    print("That is not the correct answer. Please try again!")That is not the correct answer. Please try again!
Extras
Chained comparison: 18 <= 21 < 65 → True (same as 18 <= 21 and 21 < 65).
Floats: 0.1 + 0.2 == 0.3 → False; use math.isclose() (link to Chapter 2).
Operator table
Sources: docs.python.org — comparisons
docs.python.org — math.isclose
Checking multiple conditions — and, or
>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 and age_1 >= 21
False
>>> age_1 = 22
>>> age_0 >= 21 and age_1 >= 21
True
>>> age_0 = 22
>>> age_1 = 18
>>> age_0 >= 21 or age_1 >= 21
True
>>> age_0 = 18
>>> age_0 >= 21 or age_1 >= 21
False
Truth table
Parentheses improve readability: (age_0 >= 21) and (age_1 >= 21).
Short-circuit evaluation: in A and B, B is not evaluated when A is False — useful for guards such as if users and users[0] == 'admin':
Source: docs.python.org — boolean operations
Checking whether a value is (or is not) in a list
>>> requested_toppings = ['mushrooms', 'onions', 'pineapple']
>>> 'mushrooms' in requested_toppings
True
>>> 'pepperoni' in requested_toppings
Falsebanned_users = ['andrew', 'carolina', 'david']
user = 'marie'
if user not in banned_users:
    print(f"{user.title()}, you can post a response if you wish.")Marie, you can post a response if you wish.
Security box — allow-list vs block-list
Block-list: reject known-bad values (banned_users) — easy to bypass with unknown values.
Allow-list: accept only known-good values (allowed_ports = [80, 443]) — recommended by OWASP for input validation.
Performance note: x in list scans items one by one (O(n)); sets (Chapter 6) give fast lookups for large lists.
Sources: docs.python.org — membership test
OWASP Input Validation Cheat Sheet
wiki.python.org — TimeComplexity
Boolean expressions and truth values (supplement)
game_active = True
can_edit = False
print(bool([]), bool([0]), bool(''), bool('0'))False True False TrueValues that are False in an if test
False, None, 0, 0.0, '' (empty string), [] (empty list), () (empty tuple), {} (empty dict), set(), range(0)
Everything else counts as True, including '0' and [0].
PEP 8 recommendations
For sequences, use if not seq: / if seq: rather than if len(seq) == 0:.
Don't compare booleans to True/False with ==.
Sources: docs.python.org — truth value testing
PEP 8 — programming recommendations
if statements · if-else statements
Syntax: if conditional_test:
    do something
age = 19
if age >= 18:
    print("You are old enough to vote!")
    print("Have you registered to vote yet?")You are old enough to vote!
Have you registered to vote yet?
age = 17
if age >= 18:
    print("You are old enough to vote!")
else:
    print("Sorry, you are too young to vote.")
    print("Please register to vote as soon as you turn 18!")Sorry, you are too young to vote.
Please register to vote as soon as you turn 18!
Source: docs.python.org — the if statement
The if-elif-else chain
Version 1
age = 12
if age < 4:
    print("Your admission cost is 0 USD.")
elif age < 18:
    print("Your admission cost is 25 USD.")
else:
    print("Your admission cost is 40 USD.")Your admission cost is 25 USD.
Version 2 (preferred — one print, easier to change)
age = 12
if age < 4:
    price = 0
elif age < 18:
    price = 25
else:
    price = 40
print(f"Your admission cost is {price} USD.")Your admission cost is 25 USD.
Key point: Python tests conditions top-down and runs ONLY the first block whose test is True.
Using multiple elif blocks · Omitting the else block
age = 70
if age < 4:
    price = 0
elif age < 18:
    price = 25
elif age < 65:
    price = 40
elif age >= 65:
    price = 20
print(f"Your admission cost is {price} USD.")Your admission cost is 20 USD.
Explanation
Any number of elif blocks is allowed.
Replacing the final else with an explicit elif age >= 65 makes the code reject unexpected cases: else catches EVERYTHING, including invalid or malicious input.
Security parallel: "default deny" — a final else that grants access is risky; prefer explicit conditions for allow and a default branch that denies.
Optional note: Python 3.10 adds the match statement (structural pattern matching) for multi-way branching on values and shapes.
Sources: docs.python.org — if statements
PEP 636
Testing multiple conditions — independent if statements
Correct — independent ifs
requested_toppings = ['mushrooms', 'extra cheese']
if 'mushrooms' in requested_toppings:
    print("Adding mushrooms.")
if 'pepperoni' in requested_toppings:
    print("Adding pepperoni.")
if 'extra cheese' in requested_toppings:
    print("Adding extra cheese.")
print("\nFinished making your pizza!")Adding mushrooms.
Adding extra cheese.

Finished making your pizza!
Incorrect for this task — elif stops after the first match
Output would be:
Adding mushrooms.

Finished making your pizza!Rule table
Security parallel: a log line may match several detection rules (brute force AND unusual country) — use independent ifs so every alert fires.
Applying if-elif-else: alert severity
failed_logins = 42
if failed_logins >= 50:
    severity = "critical"
elif failed_logins >= 20:
    severity = "high"
elif failed_logins >= 5:
    severity = "medium"
else:
    severity = "low"
print(f"{failed_logins} failed logins -> severity: {severity}")42 failed logins -> severity: high
Notes
Order matters: the largest threshold must be tested first; reversing the order would label 42 as "medium".
Thresholds are illustrative; real values come from the organisation's monitoring policy.
Checking for special items · Checking that a list is not empty
requested_toppings = ['mushrooms', 'green peppers', 'extra cheese']
for requested_topping in requested_toppings:
    if requested_topping == 'green peppers':
        print("Sorry, we are out of green peppers right now.")
    else:
        print(f"Adding {requested_topping}.")
print("\nFinished making your pizza!")Adding mushrooms.
Sorry, we are out of green peppers right now.
Adding extra cheese.

Finished making your pizza!
requested_toppings = []
if requested_toppings:
    for requested_topping in requested_toppings:
        print(f"Adding {requested_topping}.")
    print("\nFinished making your pizza!")
else:
    print("Are you sure you want a plain pizza?")Are you sure you want a plain pizza?Note: an empty list evaluates to False in an if test (see the truthiness slide).
Source: docs.python.org — truth value testing
Using multiple lists
available_toppings = ['mushrooms', 'olives', 'green peppers',
                      'pepperoni', 'pineapple', 'extra cheese']
requested_toppings = ['mushrooms', 'french fries', 'extra cheese']
for requested_topping in requested_toppings:
    if requested_topping in available_toppings:
        print(f"Adding {requested_topping}.")
    else:
        print(f"Sorry, we don't have {requested_topping}.")
print("\nFinished making your pizza!")Adding mushrooms.
Sorry, we don't have french fries.
Adding extra cheese.

Finished making your pizza!
Security mapping (same structure)
allowed_ports = [80, 443]
requested_ports = [443, 23, 80]
for port in requested_ports:
    if port in allowed_ports:
        print(f"Port {port}: allow")
    else:
        print(f"Port {port}: deny")Port 443: allow
Port 23: deny
Port 80: allowKey message: available_toppings is an allow-list; the same pattern underlies firewall rules and input validation.
Sources: OWASP Input Validation Cheat Sheet
IANA port registry
Practice exercises (Chapter 4)
Conditional Tests — at least 10 tests (5 True, 5 False) with printed predictions.
More Conditional Tests — string ==/!=, lower(), numeric comparisons, and/or, in, not in.
Alien Colors #1 — simple if (one passing version, one failing version).
Alien Colors #2 — if-else (5 or 10 points).
Alien Colors #3 — if-elif-else (green 5, yellow 10, red 15).
Stages of Life — baby, toddler, kid, teenager, adult, elder.
Favorite Fruit — five independent if statements.
Hello Admin — special greeting for 'admin'.
No Users — handle an empty list.
Checking Usernames — case-insensitive uniqueness check (lowercase copy of current_users).
Ordinal Numbers — 1st 2nd 3rd 4th ... 9th with if-elif-else inside a loop.
Security link: Checking Usernames is the same normalise-then-compare pattern used in account registration; Hello Admin and No Users mirror role checks and empty-input handling.
Session 2 summary
Chapter 3
Lists: indexing (0-based, negative), modifying, append/insert, del/pop/remove
Organizing: sort(), sorted(), key=, reverse(), len(); IndexError
for loops, indentation, range(), min/max/sum, comprehensions
Slicing, copy vs alias, tuples
Chapter 4
Conditional tests: ==, !=, <, <=, >, >=, and, or, in, not in, truthiness
if, if-else, if-elif-else; independent ifs when several actions may apply
if + lists: special items, empty-list checks, allow-list pattern
Next session — Chapter 5: Dictionaries
Eric Matthes, Python Crash Course, 2nd ed., No Starch Press, 2019 · Official docs: https://docs.python.org/3/