05-operators

05 · Operators, Expressions & Statements

Part 1 — Fundamentals · Estimated time: 20–30 min · Prerequisite: 04 · Booleans & Comparison

You've already used +, =, and ==. This lesson names the building blocks properly: an expression produces a value, a statement performs an action, and operators are the symbols that do the work. Understanding operator precedence (the order Python applies them) saves you from a whole class of silent bugs.

What you'll learn

• The difference between an expression and a statement
• The arithmetic operators and augmented assignment (+=, -=, …)
• Operator precedence and how parentheses control it
• Using + and * with strings

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1. Expressions vs. statements

An expression is anything that evaluates to a value:

3 + 4            # evaluates to 7
"Hi " + "there"  # evaluates to "Hi there"
age >= 18        # evaluates to True or False

A statement does something — like assigning a variable or printing:

x = 3 + 4        # assignment statement (it contains the expression 3 + 4)
print(x)         # a statement that performs an action

Rule of thumb: if you could put it on the right of an =, it's an expression.

▶️ Run it: python examples/01_expressions_vs_statements.py

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2. Arithmetic & augmented assignment

print(10 + 3, 10 - 3, 10 * 3)   # 13 7 30
print(10 / 3)    # 3.33...  (always a float)
print(10 // 3)   # 3        (floor division)
print(10 % 3)    # 1        (remainder)
print(2 ** 4)    # 16       (power)

Augmented assignment updates a variable using its own value:

total = 0
total += 5     # same as total = total + 5
total *= 2     # same as total = total * 2
print(total)   # 10

▶️ Run it: python examples/02_arithmetic_operators.py

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3. Operator precedence (order of operations)

Python follows maths rules (think PEMDAS): ** first, then * / // %, then + -. Parentheses override everything — when in doubt, add them.

print(2 + 3 * 4)     # 14  (multiplication happens first)
print((2 + 3) * 4)   # 20  (parentheses force the addition first)
print(10 - 2 - 3)    # 5   (left to right)

▶️ Run it: python examples/03_operator_precedence.py

💡 Try it yourself: what does 8 / 2 + 2 give? What about 8 / (2 + 2)? Predict, then check.

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4. Operators on strings

+ joins strings; * repeats them.

print("ab" + "cd")   # abcd
print("ha" * 3)      # hahaha
print("-" * 10)      # ----------  (great for separator lines)

▶️ Run it: python examples/04_string_operators.py

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Common mistakes

Mistake	What happens	Fix
2 + 3 * 4 expecting 20	gives 14 — * binds tighter than +	Add parentheses: (2 + 3) * 4.
"5" * "3"	TypeError	* repeats a string by an int, e.g. "5" * 3.
Writing x =+ 1	sets x to +1, not "add 1"	The operator is +=, not =+.
Relying on remembered precedence	subtle bugs	When unsure, just use parentheses.

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Recap / cheat-sheet

# expression -> a value;  statement -> an action
+  -  *  /  //  %  **      # arithmetic
+=  -=  *=  /=            # augmented assignment
# precedence: ()  >  **  >  * / // %  >  + -
"a" + "b"                 # join strings
"ab" * 3                  # repeat strings

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_fix_precedence.py — add parentheses so an average comes out right.
2. exercises/02_receipt_bar.py — use string repetition to draw a separator.
3. exercises/03_celsius_to_fahrenheit.py — apply a formula with correct precedence.

Try each yourself before opening the solution.

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Next → 06 · Conditionals (coming soon)