What a Program Is: Values, Variables, and Your First Script

A program is text that tells a computer what to do, one instruction at a time. The computer does not guess what you meant, skip the boring parts, or fix your typos. It runs each line in order, from the top of the file to the bottom, and does exactly what each line says.

print("Hello")
print("Goodbye")

Run that and you get Hello on one line, then Goodbye. Swap the two lines and the output swaps too. That sounds trivial, but it is the rule everything else in this course builds on: order is meaning. Later lessons add ways to skip lines (decisions) and repeat lines (loops), but even those follow precise rules about which line runs next.

Definition: A program is a sequence of instructions a computer executes in order. A programming language is the set of rules for writing those instructions so the computer can read them without ambiguity.

Any language can teach the fundamentals, because variables, decisions, loops and functions exist in nearly all of them. Python is a common first choice for practical reasons: the syntax is short, blocks are marked by indentation rather than braces, and you can type a line and see its result immediately.

It is also what most learners already reach for. In the Stack Overflow Developer Survey 2025, Python was the most used language among respondents who said they were learning to code:

Among all respondents the same survey put Python at 57.9%. The ideas transfer: once you understand a loop in Python, a loop in JavaScript is new spelling, not a new idea.

Python comes with an interpreter: a program that reads your Python and carries it out. You can talk to it in two ways.

REPL stands for read, evaluate, print, loop, which is exactly what it does. On some systems the command is python3 rather than python; the official Python tutorial covers both.

Gotcha: In the REPL, typing 2 + 3 shows 5. In a script, the same line computes 5 and throws it away silently. If a script prints nothing, check whether you forgot print.

Every piece of data in a program is a value, and every value has a type that decides what you can do with it. Four types cover most beginner programs:

The built-in type() function tells you what you are holding:

print(type(42))     # <class 'int'>
print(type(3.5))    # <class 'float'>
print(type("42"))   # <class 'str'>
print(type(True))   # <class 'bool'>

Notice "42" in quotes is text, not a number. It looks the same on screen, but Python treats it completely differently, which is the source of one of the most common beginner bugs later in this lesson.

An expression is anything Python can reduce to a single value. Arithmetic is the first kind you meet:

Precedence follows school maths: ** first, then * / // %, then + -. Use parentheses whenever you are not sure; (2 + 3) * 4 is 20, while 2 + 3 * 4 is 14.

Gotcha: 0.1 + 0.2 prints 0.30000000000000004. Floats are stored in binary, and most decimal fractions cannot be represented exactly. The Python tutorial's appendix on floating point explains why. For display, format the number: round(0.1 + 0.2, 2) gives 0.3.

A variable is a name that refers to a value. You create one with =, the assignment operator:

score = 0
score = score + 1
score += 1          # shorthand for score = score + 1
print(score)        # 2

Read = as "now refers to", never as "equals". score = score + 1 is nonsense as algebra but perfectly clear as an instruction: compute the right side using the current value, then point the name at the result.

a = 5
b = a
a = 10
print(b)

Names can contain letters, digits and underscores, cannot start with a digit, and are case sensitive, so Total and total are different variables.

Text values are strings, and Python gives you a lot to do with them:

word = "Python"
print(len(word))        # 6, the number of characters
print(word[0])          # P, positions start at 0
print(word[-1])         # n, negative counts from the end
print(word[0:3])        # Pyt, from position 0 up to (not including) 3
print(word + "!" * 3)   # Python!!!
print(f"{word} has {len(word)} letters")   # Python has 6 letters

Two ideas here recur everywhere in programming. First, counting starts at 0: the first character is at position 0, so the last one of a six-letter word is at position 5. Second, ranges exclude their end: word[0:3] gives positions 0, 1 and 2.

The last line is an f-string. Put f before the opening quote and anything inside curly braces is evaluated and inserted. It is the clearest way to build a message out of values, and you will use it constantly.

input() shows a prompt, waits for the user to type something and press Enter, and gives back what they typed. It always gives back a string, even if the user typed digits.

print("3" + "4")             # 34, joining two strings
print(int("3") + int("4"))   # 7, adding two numbers
print(str(7) + " days")      # 7 days

So a program that asks for a number must convert it: int(...) for whole numbers, float(...) for decimals. Going the other way, str(...) turns a number into text.

Gotcha: age = input("Age? ") followed by age + 1 crashes with TypeError: can only concatenate str (not "int") to str. Python refuses to guess whether you meant text or arithmetic. The fix is age = int(input("Age? ")), converting at the moment the value enters the program.

If the user types three where you call int(), Python stops with a ValueError. Handling bad input gracefully is a later skill; for now, know where the error comes from.

Input, conversion, computation, output: the tip calculator in the next step follows this path exactly.

Save this as tip.py and run it with python tip.py:

# tip.py: split a restaurant bill
bill = float(input("Bill total: "))
tip_percent = float(input("Tip percent: "))
people = int(input("Number of people: "))

tip = bill * tip_percent / 100
total = bill + tip
each = total / people

print(f"Tip: {tip:.2f}")
print(f"Total: {total:.2f}")
print(f"Each person pays: {each:.2f}")

Typing 84, 15 and 3 gives:

Tip: 12.60
Total: 96.60
Each person pays: 32.20

Every idea from this lesson is here: instructions in order, input converted to numbers, arithmetic expressions, variables naming intermediate results, and f-strings for output. The :.2f inside the braces formats a float with two decimal places. The line starting with # is a comment, ignored by Python and written for humans.

Try it: Add a fourth question asking for a rounding amount, or print how much the tip adds per person. Change one thing, run it, check the output by hand.