In Python, understanding the difference between expressions and statements is like understanding the difference between a phrase and a complete sentence in English.
Expressions (The "Phrases")
An expression is a piece of code that produces a value. Whenever Python "looks" at an expression, it calculates it down to a single result.
- Think of it as: A question that Python answers.
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Examples:
- 5 + 5 (evaluates to 10)
- print(round(30/4)) (evaluates to 8)
- user_score > 10 (evaluates to True or False)
Statements (The "Sentences")
A statement is a complete action. It tells Python to do something. While an expression produces a value, a statement executes a command.
- Think of it as: An instruction or a move the program makes.
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Examples:
- Assignment Statement: x = 10 (Tells Python to store 10 in x).
- Print Statement: print("Hi") (Tells Python to show text on the screen).
- If Statement: if x > 5: (Tells Python to make a decision).
The Key Difference: The "Print Test"
A simple trick to tell them apart is the Print Test: If you can put the code inside print() and it outputs a value without crashing, it is likely an expression.
print(5 + 5) ✅ Works. (Expression)
print(x = 10) ❌ Fails. (Statement—you can't "print" an assignment).
Important Note: Nested Logic
Expressions can be part of a statement. In the code score = 10 + 5:
- 10 + 5 is the expression (it evaluates to 15).
- The entire line score = 15 is the statement (the act of assignment).
Test Your Knowledge
Select the correct answer below to see if you've mastered Expressions and Statements.
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Time to experiment!
Coding Exercises (VS Code) Instructions:
- Create a file named expressions_vs_statements.py in your part8 folder.
- Complete the tasks and use # comments to explain your answers.
Exercise 1: The Print Test
- In your editor, try to run print(10 + 5).
- On the next line, try to run print(y = 20).
The Task: One of these will crash.
In a comment, identify which one failed and explain why based on the "Print Test" rule.
Exercise 2: Building Statements
Write a script that calculates the area of a rectangle (length multiplied by width).
The Task: Create three statements.
- Statement 1 sets the length
- Statement 2 sets the width
- Statement 3 calculates the area
Label the expression part of your third statement with a comment.
Exercise 3: Expression Hunting
Copy these lines:
result = (10 + 2) * 3
print(result)
The Task: Below this code, add a comment listing every expression you see. (Hint: even result by itself is an expression when used inside print())
Exercise 4: Constants and Conventions
Create a variable for Gravity (9.8) using the Constant naming convention.
The Task: Write a statement that calculates weight by multiplying a mass variable by your gravity constant. Ensure you use snake_case for mass and ALL_CAPS for gravity.
Exercise 5: The Dunder Mystery
Type: print(__name__)
The Task: Run the code. In a comment, explain if __name__ is acting as an expression or a statement here. Also, note the "Dunder" naming convention used.
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