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Every value in Python has a type. Sometimes you need to change a value from one type to another — most often when reading user input (which comes in as a string) and using it as a number.

The classic beginner bug

input() always returns a string, even if the user typed digits. "25" + 1 makes no sense to Python — strings concatenate, ints add. Python raises a TypeError rather than guess what you meant.

The fix is explicit conversion:

The four conversion functions you need

What can be converted to what

• str → int works only if the string is a clean integer: int("42") ✓, int("3.14") ✗ ValueError, int("hi") ✗ ValueError.
• str → float is more forgiving: float("3.14") ✓, float("42") ✓ (returns 42.0).
• float → int truncates toward zero: int(3.7) is 3, int(-3.7) is -3 (not -4!). To round properly, use round() first: int(round(3.7)).
• anything → str always works: str(True) → "True", str(None) → "None".

Truthy and falsy

Python treats some values as "falsy" — they evaluate to False in a boolean context. Everything else is "truthy."

Falsy values:

• False, None, 0, 0.0, "" (empty string), [] (empty list), {} (empty dict)

Truthy: everything else.

This lets you write idiomatic Python:

Common conversion mistakes

• Forgetting to convert input: n = input(); print(n + 1) → TypeError.
• Converting unparseable text: int("3.14") → ValueError. Use float() first if you might have a decimal.
• Assuming int() rounds: int(2.9) is 2, not 3. Use round(2.9) for proper rounding.
• Comparing types directly: "5" == 5 is False — they're different types! Convert first.