230 Chapter 5 Names, Bindings, and Scopes
Scope and lifetime are also unrelated when subprogram calls are involved.
Consider the following C++ functions:void printheader() {} /* end of printheader */
void compute() {
int sum;printheader();
} /* end of compute */The scope of the variable sum is completely contained within the compute
function. It does not extend to the body of the function printheader, although
printheader executes in the midst of the execution of compute. However,
the lifetime of sum extends over the time during which printheader executes.
Whatever storage location sum is bound to before the call to printheader,
that binding will continue during and after the execution of printheader.5.7 Referencing Environments
The referencing environment of a statement is the collection of all variables
that are visible in the statement. The referencing environment of a statement in
a static-scoped language is the variables declared in its local scope plus the col-
lection of all variables of its ancestor scopes that are visible. In such a language,
the referencing environment of a statement is needed while that statement is
being compiled, so code and data structures can be created to allow references
to variables from other scopes during run time. Techniques for implementing
references to nonlocal variables in both static- and dynamic-scoped languages
are discussed in Chapter 10.
In Python, scopes can be created by function definitions. The referencing
environment of a statement includes the local variables, plus all of the variables
declared in the functions in which the statement is nested (excluding variables
in nonlocal scopes that are hidden by declarations in nearer functions). Each
function definition creates a new scope and thus a new environment. Consider
the following Python skeletal program:g = 3; # A globaldef sub1():
a = 5; # Creates a local
b = 7; # Creates another local... 1
def sub2():
global g; # Global g is now assignable here