2) Logical Operators
There are three logical operators: and, or, and not. The semantics (meaning) of these operators is similar to their meaning in English. For example, x > 0 and x < 10 is true only if x is greater than 0 and at the same time, x is less than 10. How would you describe this in words? You would say that x is between 0 and 10, not including the endpoints.
n % 2 == 0 or n % 3 == 0 is true if either of the conditions is true, that is, if the number is divisible by 2 or divisible by 3. In this case, one, or the other, or both of the parts has to be true for the result to be true.
Finally, the not operator negates a boolean expression, so not x > y is true if x > y is false, that is, if x is less than or equal to y.
Common Mistake!
There is a very common mistake that occurs when programmers try to write boolean expressions. For example, what if we have a variable number and we want to check to see if its value is 5,6, or 7. In words we might say: “number equal to 5 or 6 or 7”. However, if we translate this into Python, number == 5 or 6 or 7, it will not be correct. The or operator must join the results of three equality checks. The correct way to write this isnumber == 5 or number == 6 or number == 7. This may seem like a lot of typing but it is absolutely necessary. You cannot take a shortcut.
Precedence of Operators
We have now added a number of additional operators to those we learned in the previous chapters. It is important to understand how these operators relate to the others with respect to operator precedence. Python will always evaluate the arithmetic operators first (** is highest, then multiplication/division, then addition/subtraction). Next comes the relational operators. Finally, the logical operators are done last. This means that the expression x*5 >= 10 and y-6 <= 20 will be evaluated so as to first perform the arithmetic and then check the relationships. The and will be done last. Although many programmers might place parenthesis around the two relational expressions, it is not necessary.
The following table summarizes the operator precedence from highest to lowest. A complete table for the entire language can be found in the Python Documentation.