Data Has Types

Core path: 25 minutes

Why does Python treat 10 differently from "10"?

Remember our mystery from Week 01?

print("10 + 5")    # 10 + 5
print(10 + 5)      # 15
print("10" + "5") # 105

Those expressions look similar to a human. Python sees different data types and therefore different operations.

Four types we'll start with

Diagrams open at a readable shape-aware scale. Zoom or expand when you need more detail.

• Strings (str) represent text: name = "Nova".
• Integers (int) represent whole numbers: score = 100.
• Floats (float) represent decimal values: temperature = 72.5.
• Booleans (bool) represent True or False: game_over = False.

Quotation marks matter. "42" and 42 contain similar characters on screen, but Python does not treat them as the same kind of value.

Ask Python

print(type("Robotnix"))
print(type(42))
print(type(3.14))
print(type(True))

Use type() as an investigation tool when the program's behavior does not match your assumption.

Now inspect several values directly and change one literal to see whether the type changes with it:

Run the code to see output.

Guided example: trace the types before the operation

Predict the type and result of each expression before running it:

a = 100
b = "100"
c = 100.0

print(a + 5)
print(b + "5")
print(c + 5)

The useful step is not memorizing the answers. Trace the operands first:

a + 5     → int + int     → 105
b + "5"   → str + str     → "1005"
c + 5     → float + int   → 105.0

Now make this fail:

age = 15
print("I am " + age + " years old.")

Failure analysis: Python is not refusing to print an age. The operation is wrong for the operand types: string concatenation cannot directly combine a str with an int. Fix the representation with an f-string or an explicit conversion.

Expressions and formatted output

An expression is code that produces a value. For readable output, prefer an f-string:

name = "Nova"
score = 850
print(f"{name} has {score} points.")

Success check

Given any short expression in this lesson, you should be able to identify the type of each value, predict the operation Python will attempt, and explain a type-related failure without guessing.