Python Quick Reference

This page is a small companion page for Python practice. It does not replace the course materials, but it is useful when you want to quickly recall the shape of a construct.

Read the Structure First

Before focusing on syntax, ask three small questions:

1. What is the program supposed to do?
2. What input does it need, and what output does it produce?
3. What is the shape — linear, branching, or looping?

That habit becomes especially useful once functions, loops, and files appear.

Printing and Asking for Input

print("Hello, world!")

name = input("What is your name? ")
print(f"Hello, {name}!")

Common notes:

• print displays its argument.
• input always returns text, even when the user types digits.
• f-strings (f"...") are the preferred way to include a variable’s value in a piece of text.

Variables and Assignment

minutes = 45
subject = "Math"

= is assignment, not equality. A variable is created the first time you assign to it, and you can reassign it later.

Type Conversion

minutes_text = input("Minutes: ")
minutes = int(minutes_text)
total = minutes + 15

Use int(...), float(...), or str(...) to convert between types. int("forty-five") raises ValueError.

f-strings

subject = "Math"
minutes = 45
print(f"Logged {minutes} minutes of {subject}.")

Inside { }, any expression works:

print(f"That is {minutes / 60} hours.")

Joining Strings with +

first_name = "Ada"
last_name = "Lovelace"
full_name = first_name + " " + last_name

+ joins two strings end-to-end. Use it when you have two existing string values you want to join, and prefer f-strings when you are inserting values into a fixed sentence. + between a string and a number raises TypeError.

Common String Methods

name = "  Ada Lovelace  "
clean = name.strip()             # "Ada Lovelace"
upper = name.upper()             # "  ADA LOVELACE  "
swapped = name.replace("Ada", "Grace")

Each method returns a new string. The original is unchanged.

if / elif / else

if minutes < 15:
    print("Short session.")
elif minutes <= 90:
    print("Solid session.")
else:
    print("Long session.")

Common notes:

• < is strict, <= includes the boundary.
• elif chains continue checking only if previous conditions failed.
• else runs when all of the above were false.

Boolean Operators

minutes = 45
answer = "quit"

if minutes >= 15 and minutes <= 90:
    print("Solid session.")

if answer == "done" or answer == "quit":
    print("Finished.")

and, or, and not combine boolean expressions.

for Loop Over a List

subjects = ["Math", "Physics", "History"]
for subject in subjects:
    print(subject)

The loop variable (subject) is set to each item in turn.

for Loop with range

for i in range(5):
    print(i)

This prints 0, 1, 2, 3, 4. range(n) produces numbers from 0 up to but not including n. range(start, end) and range(start, end, step) are also available.

while Loop

while True:
    answer = input("Subject (or 'done' to finish): ").strip().lower()
    if answer == "done":
        break
    print(f"Logged: {answer}")

while True: runs until something inside the loop ends it. break ends the loop immediately.

Common List Operations

minutes_list = [45, 30, 60]

# Length
n = len(minutes_list)              # 3

# Access by index
first = minutes_list[0]            # 45
last = minutes_list[-1]            # 60

# Add an item to the end
minutes_list.append(15)

# Check membership
"Math" in subjects                 # True or False

# Sum, max, min
total = sum(minutes_list)
longest = max(minutes_list)
shortest = min(minutes_list)

max and min raise ValueError on an empty list.

Defining a Function

def classify_session(minutes):
    """Return a short label for a session length."""
    if minutes < 15:
        return "short"
    elif minutes <= 90:
        return "solid"
    else:
        return "long"

Common notes:

• def introduces a function definition.
• The parameter minutes is local to the function.
• The function ends as soon as a return runs.
• The triple-quoted string just below def is a docstring: a short description of what the function does.

Calling a Function

label = classify_session(45)
print(label)                       # "solid"

Returning More Than One Value

def get_session():
    subject = input("Subject: ").strip().lower()
    minutes = int(input("Minutes: ").strip())
    return subject, minutes

subject, minutes = get_session()

The function returns a tuple; the caller unpacks it into two variables.

Testing a Function with assert

def total_minutes(minutes_list):
    total = 0
    for m in minutes_list:
        total += m
    return total

assert total_minutes([]) == 0
assert total_minutes([45]) == 45
assert total_minutes([45, 30, 60]) == 135
print("All tests passed.")

Each assert checks a specific case. The empty case and the single-element case are often the most informative.

Raising an Exception

def average_minutes(minutes_list):
    if len(minutes_list) == 0:
        raise ValueError("cannot average an empty list")
    return sum(minutes_list) / len(minutes_list)

Raise an exception when a function receives input it cannot handle.

Catching an Exception with try / except

while True:
    minutes_text = input("Minutes: ").strip()
    try:
        minutes = int(minutes_text)
        break
    except ValueError:
        print("Please enter a whole number.")

Catch specifically (except ValueError:), not broadly. A bare except: is almost always a bug.

Reading an Error

When Python prints a traceback:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    minutes = int("forty-five")
ValueError: invalid literal for int() with base 10: 'forty-five'

Read the last line first: the exception type (ValueError) and the message tell you what went wrong. The lines above show where in the program it happened.

Clause Patterns at a Glance

A program with input, branching, looping, and a function call looks roughly like this:

def classify_session(minutes):
    if minutes < 15:
        return "short"
    elif minutes <= 90:
        return "solid"
    else:
        return "long"

while True:
    subject = input("Subject (or 'done'): ").strip().lower()
    if subject == "done":
        break
    minutes = int(input("Minutes: ").strip())
    label = classify_session(minutes)
    print(f"  {label.capitalize()} session on {subject}.")

Three patterns are visible at once: a function definition, a while True: loop with a break, and an if/elif/else chain inside the function.

Safe Working Habits

• Read each line carefully before running it.
• Predict the output before pressing run.
• For loops, predict what happens on the empty input.
• For conditions, predict what happens at the exact boundary.
• Use try/except around input that may be malformed; raise exceptions for inputs you do not want to handle.
• Run tests after every change that could break something.
• When a piece of code surprises you, simplify it until the surprise goes away.