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Local and Global Scope

The scope of a variable is which part of the code it can be read/modified. Think of a scope as a container for variables. When a scope is destroyed, all the values stored in the scope’s variables are forgotten.

The global scope is the scope that is applicable to the entire program. There is only one global scope, and it is created when your program begins. When your program terminates, the global scope is destroyed, and all its variables are forgotten. Otherwise, the next time you ran your program, the variables would remember their values from the last time you ran it. A variable that exists in the global scope is called a global variable.

A local scope is a scope that applies only during the execution of a function. A local scope is created whenever a function is called. Parameters and variables that are assigned in the called function are said to exist in that function’s local scope. A variable that exists in a local scope is called a local variable. When the function returns, the local scope is destroyed, and these variables are forgotten. The next time you call this function, the local variables will not remember the values stored in them from the last time the function was called.

A variable must be in the global scope or the local scope; it cannot be in both.

_{%%[Some parts of the above explanation were adapted from Automate the Boring Stuff]%%}

Consider the code given below %%(apparently, from a program related to a farm)%%. The global scope and the local scope of each function indicated by shaded areas.

Note the following rules about scope:

• Rule 1: Local scopes can read global variables e.g., raise_chicken function can access the global variable total_chickens (see line 6).
• Rule 2: Global scope cannot read/write local variables e.g., line 11 will be rejected because it reads the local variable eggs.
• Rule 3: Local scopes cannot read/write variables of other local scopes e.g., line 15 will be rejected because it reads the local variable eggs.
• Rule 4: If a variable is ==assigned== in a local scope, it becomes a local variable even if a global variable has the same name. That is, there can be separate global and local variables with the same name. e.g., line 16 (total_cows = 10) creates a local variable total_cows although there is also a global variable total_cows; the line 19 prints 5 because the total_cows global variable remains 5 and the local variable total_cows which has the value 10 is destroyed after the function milk_cows is executed.

To modify a global variable within a local scope, use the global statement on that variable.

{{ icon_example }} The breed_cows function below can increase the global variable total_cows to 10 from the local scope because it has a global total_cows statement. If you remove that statement, the last print statement will print 5 instead of 10

total_cows = 5
print('total cows before breeding:', total_cows)

def breed_cows():
    global total_cows
    print('breeding cows')
    total_cows = 10
    print('total cows at the end of breeding:', total_cows)

breed_cows()
print('total cows after breeding:', total_cows) # prints 10

{{ icon_output }} total cows before breeding: 5 breeding cows total cows at the end of breeding: 10 total cows after breeding: 10

{{ icon_output }} without the global total_cows statement total cows before breeding: 5 breeding cows total cows at the end of breeding: 10 total cows after breeding: 5

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