Python Casting

Python Casting: Converting Between Strings, Numbers, and Booleans

Python casting converts a value from one data type to another using constructor functions like int(), float(), str(), and bool(). Because input() and environment variables always arrive as strings, explicit casting is an everyday task in Python scripts.

Python performs very few implicit conversions automatically. Writing explicit casts makes your intent clear and avoids unexpected type mismatches.

Using int(), float(), and str()

int() parses whole number strings or truncates floats toward zero. float() accepts integer values or decimal strings, and str() turns almost any object into text.

raw_qty = "12"
raw_discount = "4.5"

quantity = int(raw_qty)         # 12
discount = float(raw_discount)  # 4.5
truncated = int(9.87)           # 9 (drops decimal, does not round)
summary = "Qty: " + str(quantity)

print(quantity, discount, truncated, summary)

Truthy and Falsy Casting with bool()

Passing any object to bool() evaluates whether Python considers it truthy or falsy. Empty containers ("", [], {}), 0, 0.0, and None become False; almost everything else becomes True.

print(bool("0"))   # True (non-empty string!)
print(bool(""))    # False
print(bool(0))     # False

Common Mistakes to Avoid

• Passing decimal strings to int(): int("3.5") raises ValueError; parse with int(float("3.5")) first.
• Assuming bool("False") is False: Any non-empty string evaluates to True, including "False" and "0".
• Skipping error handling on external input: Wrap int(user_text) inside try ... except ValueError when parsing user input or CSV columns.

Frequently Asked Questions

Does int(3.9) round up to 4?

No. int() truncates toward zero, returning 3. Use round(3.9) when you want standard rounding.

How do I convert a string "true" to a boolean?

Compare the normalized text explicitly: value.strip().lower() in {"true", "1", "yes"}.

Can I cast a list into a tuple or set?

Yes. Call tuple(my_list) to freeze sequence order or set(my_list) to remove duplicate items.