Variables, expressions and operators

A variable is a name given to a value in the program’s memory. In Python it is created with an assignment statement using =: x = 5. Here = does not mean equality; it is a command: “bind the value on the right to the name on the left”. Later the variable can be given another value: the statement x = x + 1 increases its value by one.

Every value has a type: integer (int), floating-point number (float), string (str), Boolean (bool); the type can be found with type(x). In Python the type is determined by the value (dynamic typing), so after x = 5 you may write x = "abc". A name starts with a letter or an underscore and consists only of letters, digits and _; case matters (count and Count are different names). The input() function always returns a string, so a number must be converted with int() or float().

An expression is a combination of values, variables and operators that evaluates to a single value: 2 * (x + 3). The arithmetic operators are +, -, *, / (division; the result is always a float), // (the integer part) and % (the remainder). For example, 7 / 2 equals 3.5, 7 // 2 equals 3 and 7 % 2 equals 1. The remainder operator is used very often: the condition n % 2 == 0 checks whether a number is even.

Comparison operators (==, !=, <, >, <=, >=) return a bool value, and the logical operators and, or, not combine them. Operations are performed by precedence: first parentheses, then *, /, //, %, then +, -, so 2 + 3 * 4 equals 14 and (2 + 3) * 4 equals 20. Floating-point numbers are stored approximately in binary, so the result of 0.1 + 0.2 is not exactly 0.3 but 0.30000000000000004; avoid comparing floating-point numbers with ==.

Key takeaways

• A variable is a name given to a value; in Python it is created with = and can later be bound to another value.
• The main types are int, float, str and bool; input() returns a string, so convert it with int() or float().
• An expression evaluates to one value using operators; / always gives a float, // the integer part and % the remainder.
• Comparison operators return bool, and and, or, not combine them.
• Precedence affects the result, so use parentheses for clarity; floating-point numbers are stored approximately.