When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence. For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:
- Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first,
2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even if it doesn't change the result.
- Exponentiation has the next highest precedence, so
2**1+1 is 3, not 4, and 3*1**3 is 3, not 27.
- Multiplication and Division have the same precedence, which is higher than Addition and Subtraction, which also have the same precedence. So
2*3-1 is 5, not 4, and 6+4/2 is 8.0, not 5.
- Operators with the same precedence are evaluated from left to right. So the expression
5-3-1 is 1, not 3, because the 5-3 happens first and then 1 is subtracted from 2.
When in doubt, always put parentheses in your expressions to make sure the computations are performed in the order you intend.
