2.3. Expressions

Expressions, like we just saw in the assignment statement syntax pattern, are values, variables, operators, and function calls (which we’ll learn more about in a bit) either by themselves or in combination with one another.

A value all by itself is considered an expression (called a literal), and so is a variable, so the following are all legal expressions (assuming that the variable x has been assigned a value):

17
x
x + 17
len("Hi there!")

Note

len() is a built-in Python function that returns the length of a string (the number of characters in it). This is the third built-in function we’ve seen so far, including print() and type().

Expressions seem so simple that beginning programmers often make incorrect assumptions about them and misunderstand how they work. What do you think the following code outputs?

x = 6 x x + 1 len("Hi there!")

Are you surprised it outputs nothing?

You might have guessed the code would output 6, 7, and 9, because those are the values of the expressions on each line. But an expression by itself doesn’t output anything. This is a common source of confusion when you first start to code.

To see the value of an expression, we can use the print() function:

x = 6 print(x) print(x + 1) print(len("Hi there!"))

So expressions don’t automatically create output. Then what does Python do with an expression? It evaluates it.

Syntax Rule

In Python, any expression, anywhere in the program, will be evaluated to produce a value. That value is then used in place of the expression as the statement it is in executes.

Evaluation Rules:

• A variable in an expression is replaced with the value that it refers to.

• Operations (like 1 + 1 and x * 5) are replaced with the result of the operation.

• Function calls (like len("Hi there!")) are replaced with the return value of the function (which we’ll learn more about later).

Examples:

• An expression on a line by itself is evaluated, but then the result is discarded.

  5
  x + 1
  

  In both of those statements, the expression is evaluated and a value is obtained (5 and whatever x+1 equals, respectively), but it isn’t used or saved anywere.

• An expression on the right hand side of an assignment statement is evaluated, and then the resulting value is saved in the variable on the left hand side.

  x = 5 + 10
  years = 12
  days = years * 365
  

  In each of these statements, first the expression on the right hand side is evaluated, then that value is saved in the variable on the left.

• An expression inside the parentheses of a function call is evaluated, and then the resulting value is given to the function to use.

  print(5 + 10)
  print(years)
  

  Here, the print() function is given the values 15 and 12 (assuming years was assigned 12 as above), so that is what it prints. Note that it does not print the string "years".