Programming Fundamentals › Programming Basics

Type Conversion

Turning a value of one type into another, explicitly or implicitly.

Also known as: type casting, casting, type cast

Type conversion means turning a value of one type into another, such as the string "42" into the number 42. When you ask for it yourself, it’s explicit (also called casting). When the language does it for you, it’s coercion.

int("42")        # 42
float("3.5")     # 3.5
str(42)          # "42"
bool("")         # False
list("abc")      # ["a", "b", "c"]
int("abc")       # ValueError

Number("42");    // 42
Number("abc");   // NaN
String(42);      // "42"
parseInt("42px", 10);   // 42  (stops at the first non-digit)

When you need it

Almost all outside data arrives as text: form fields, query parameters, CSV cells, environment variables, JSON values you didn’t control. Convert it at the edge of your program, and validate that it worked.

What can go wrong

• Invalid input. int("abc") raises an error, Number("abc") quietly gives NaN. Decide how to handle bad data (runtime validation).
• Losing information. int(3.9) is 3 (truncated, not rounded). A big number may not fit the target type.
• Surprising truthiness. bool("false") is True, since the string isn’t empty.
• Floats and money. Converting 0.1 + 0.2 to a decimal doesn’t undo the error (floating point).
• Dates and times need parsers, not casts (ISO 8601).
• Locale formats. "1,234.50" and "1.234,50" need different handling.

Habits

• Convert once, near the input, then keep the right types inside.
• Prefer functions that fail loudly over ones that guess.
• Catch the specific error you expect, not every exception.