Concepts of Programming Languages

226 Chapter 5 Names, Bindings, and Scopes

if it has been declared to be global in the function. Consider the following

examples:

day = "Monday"

def tester():

print "The global day is:", day

tester()

The output of this script, because globals can be referenced directly in func-

tions, is as follows:

The global day is: Monday

The following script attempts to assign a new value to the global day:

day = "Monday"

def tester():

print "The global day is:", day

day = "Tuesday"

print "The new value of day is:", day

tester()

This script creates an UnboundLocalError error message, because the

assignment to day in the second line of the body of the function makes day a

local variable, which makes the reference to day in the first line of the body of

the function an illegal forward reference to the local.

The assignment to day can be to the global variable if day is declared to

be global at the beginning of the function. This prevents the assignment to day

from creating a local variable. This is shown in the following script:

day = "Monday"

def tester():

global day

print "The global day is:", day

day = "Tuesday"

print "The new value of day is:", day

tester()

The output of this script is as follows:

The global day is: Monday

The new value of day is: Tuesday