Procedure calls

When you compute an expression of the form

(,  ..., )

the Python interpreter treats  this as a procedure call. It will be simpler to talk about  a speci?c case of invoking a function, so we will explain with the example

>>> square(a + 3)

computed in environment E1. Here are the steps:

1.  The expression that determines the procedure () is computed. In this case, we compute

square in E1 and get Procedure1.

2.  The expressions that  determine the arguments (, ..., )  are computed. In this case, we compute a + 3 in E1 and get 5.

3.  A new environment (in this case E2) is prepared, which:

-  binds  the formal  parameters of the function (in this case x) to the values  of its arguments(in this case, 5); and

-  has as its base the environment in which  the procedure was de?ned we ?nd  a pointer .

The dotted line between E1 and  E2 is used to show that  E1 is the parent environmentof E2.

4.  The statements of the  procedure body  are computed in the  new  environment until  either  a return statement or the end of the list of statements is got. If a result expression is calculated, the expression after the return is computed and its value is returned as the value of the function call expression. Otherwise, the function has no return value,  and  the expression has the unique Python value  None.

In our  example, the  only  statement in the  body  is a return statement.  So, we  compute the expression x * x in E2, obtaining a value  of 25.  That  value  is returned as the value  of the entire  procedure-call expression square(a + 3).

This basic mechanism can generate behavior of arbitrary hardness, when grouped with  recursion or other control  structures.