2.6. Statements and Expressions¶
A statement is an instruction that the Python interpreter can execute. We
have only seen the assignment statement so far. Some other kinds of statements
that we’ll see shortly are while statements, for statements, if
statements, and import statements. (There are other kinds too!)
An expression is a combination of values, variables, operators, and calls
to functions. Expressions need to be evaluated. If you ask Python to print an expression, the interpreter
evaluates the expression and displays the result.
In this example len is a built-in Python function that returns the number
of characters in a string. We’ve previously seen the print and the
type functions, so this is our third example of a function!
The evaluation of an expression produces a value, which is why expressions can appear on the right hand side of assignment statements. A value all by itself is a simple expression, and so is a variable. Evaluating a variable gives the value that the variable refers to.
If we take a look at this same example in the Python shell, we will see one of the distinct differences between statements and expressions.
>>> y = 3.14
>>> x = len("hello")
>>> print(x)
5
>>> print(y)
3.14
>>> y
3.14
>>>
Note that when we enter the assignment statement, y = 3.14, only the prompt is returned. There is no value. This
is due to the fact that statements, such as the assignment statement, do not return a value. They are simply executed.
On the other hand, the result of executing the assignment statement is the creation of a reference from a variable, y, to a value, 3.14. When we execute the print function working on y, we see the value that y is referring to. In fact, evaluating y by itself results in the same response.