Variables, Types & Your First Programs

▶Today's videos

Watch each video, then work the matching sections below. Watching alone will not do it.

1What a program really is

Before syntax, one idea: the computer has no judgment. Precision is not pedantry - it is the whole job.

Imagine you write instructions for someone who follows them exactly, has read every dictionary ever printed, and has never once used common sense. You write "spread the peanut butter on the bread." They pick up the sealed jar and press it against the loaf. Technically correct. Completely wrong.

That is your computer. It is astonishingly fast and astonishingly literal. Every bug you will ever write is a gap between what you meant and what you actually said.

From your text to actual behaviour

You write source code - plain text in a .py file. The Python interpreter reads it line by line, translates each line into operations the machine understands, and performs them immediately. This is why you can run a Python file the moment you save it, with no extra step.

Your first program

# hello.py - the traditional first program
print("Hello, world!")

print() is a function: a named piece of behaviour you can trigger. The parentheses mean "do it now," and what goes inside them is what you are handing over. A line like this is a statement - one complete instruction. Python runs statements top to bottom, one per line.

Comments

Anything after # on a line is ignored by Python entirely. Comments are for humans.

# This whole line is a comment.
print("Visible")   # This part is a comment too

# Good comment - explains WHY:
# Prices are stored in cents to avoid floating-point rounding errors.

# Useless comment - just restates the code:
# print the total
print(1250)

2Variables: names bound to values

Almost every beginner is taught the box metaphor. It is wrong, and it breaks the moment you meet lists. Learn the right model now.

A variable is a name that refers to a value. When you write:

score = 42

you are not putting 42 inside a container called score. You are creating the value 42 and attaching the label score to it. The name points at the value.

The difference matters immediately:

a = 5
b = a      # b now points at the SAME value a points at
a = 10     # a is re-pointed at a new value; b is untouched

print(a)   # 10
print(b)   # 5

If a were a box and b = a poured its contents into another box, you would get the same answer here - so the metaphor survives this example. It will not survive Day 8, when b = a on a list means both names point at one list and changing either changes both. Get the model right now and Day 8 costs you nothing.

Naming rules and naming taste

3The five types you need today

Python tracks what kind of thing every value is, and that determines what you are allowed to do with it.

print(type(42))        # <class 'int'>
print(type(3.14))      # <class 'float'>
print(type("hello"))   # <class 'str'>
print(type(True))      # <class 'bool'>
print(type(None))      # <class 'NoneType'>

# Note: 2 and 2.0 are DIFFERENT types
print(type(2), type(2.0))

Type decides what operations make sense:

print("ha" * 3)     # 'hahaha'  - repetition
print(3 * 3)        # 9         - multiplication
print("ha" + "ha")  # 'haha'    - joining
print("ha" - "ha")  # TypeError - subtraction is meaningless for text

Why 0.1 + 0.2 is not 0.3

print(0.1 + 0.2)          # 0.30000000000000004
print(0.1 + 0.2 == 0.3)   # False

This is not a Python bug. Computers store floats in binary, and 0.1 in binary is an infinitely repeating fraction - exactly like 1/3 is 0.333... in decimal. The stored value is very slightly off, and the errors accumulate. Every mainstream language does this.

4Strings and f-strings

Most real data arrives as text. And unformatted output is the fastest way to make a working program look broken.

Strings can use single or double quotes - pick one and be consistent. Triple quotes span multiple lines.

single = 'Python'
double = "Python"
multi = """Line one
Line two"""

# Use the OTHER quote when your text contains one:
quote = "It's fine."
other = 'She said "hello".'

# Escape sequences
print("Tab\there")
print("Line one\nLine two")
print("A backslash: \\")

f-strings: the only formatting you need

Put f before the opening quote, then write any expression inside { }.

name = "Amin"
items = 3
price = 12.5

# The painful old way:
print("Hi " + name + ", you have " + str(items) + " items.")

# The f-string way:
print(f"Hi {name}, you have {items} items.")

# Expressions work inside the braces:
print(f"Total: {items * price}")

# Format specifiers control the display:
print(f"Total: ${items * price:.2f}")     # 2 decimal places
print(f"|{name:>10}|")                    # right-aligned in 10 columns
print(f"|{name:<10}|")                    # left-aligned
print(f"|{name:^10}|")                    # centred
print(f"{1234567:,}")                     # thousands separators

5Type conversion - and where it bites

Python will not silently guess what you meant. That is a feature. It does mean you have to ask explicitly.

int("42")       # 42        - text that looks like a whole number
int(3.9)        # 3         - TRUNCATES, does not round
int("3.9")      # ValueError - not a whole number in text form
float("3.14")   # 3.14
str(99)         # '99'
bool(0)         # False
bool("")        # False
bool("False")   # True  <- it is a non-empty string!

The bug that catches everyone

input() reads what the user typed and hands it back as a string. Always. Even if they typed digits.

a = input("First number: ")   # user types 5
b = input("Second number: ")  # user types 3
print(a + b)                  # '53'  <- string joining, not addition!

The fix is to convert on the way in:

a = int(input("First number: "))
b = int(input("Second number: "))
print(a + b)                  # 8

6Putting it together

A complete small program, built the way you should build every program: decide the parts first, then type.

Before writing a line, name the three parts: what comes in, what processing happens, what goes out.

# unit_converter.py - Celsius to Fahrenheit

FREEZING_C = 0          # named constant, not a magic number

celsius_text = input("Temperature in Celsius: ")
celsius = float(celsius_text)          # convert immediately

fahrenheit = celsius * 9 / 5 + 32

print(f"{celsius:.1f} C is {fahrenheit:.1f} F")
if_below = celsius < FREEZING_C
print(f"Below freezing: {if_below}")

Notice four habits worth stealing:

• Convert at the boundary. The moment text enters your program, turn it into the type you actually need.
• Name your constants. FREEZING_C beats a bare 0 six months from now.
• Format your output. :.1f is the difference between 70.7 and 70.70000000000001.
• One idea per line. You can cram it all into one statement. Do not.

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