How Can a Python Function Access a Global Variable Without the `global` Keyword?

Access Global Variables Without global Keyword

In Python, global variables are stored in a separate namespace from local variables. To access a global variable within a function, the global keyword is typically used. However, there are situations where a global variable can be accessed without explicitly declaring it as such.

Consider the following code:

sub = ['0', '0', '0', '0']

def getJoin():
    return '.'.join(sub)

getJoin()

In this code, the getJoin function returns the joined elements of the sub list, which is a global variable. Surprisingly, the function is able to access sub without using the global keyword.

Why does this occur?

Dynamic Scope

Python uses dynamic scope, which means that variables are searched for in the current scope and then in enclosing scopes until they are found. In the getJoin function, the variable sub is not found locally. Therefore, it searches the enclosing scope, which is the global scope, and finds the sub variable there.

Variable Binding

When a variable is accessed, it is bound to a memory location. In the case of sub, it is bound to the memory location containing the list of '0's. This binding process occurs when the interpreter encounters the assignment statement sub = ['0', '0', '0', '0'].

Function Access

When the getJoin function is called, it executes and the interpreter searches for the variables it needs. Since sub is not found locally, it looks in the enclosing scope and finds the already bound sub variable. The function then accesses the data stored at this memory location, which contains the list of '0's.

Global Keyword

The global keyword is useful when you want to modify or create global variables within a local context. However, it is not strictly required to access global variables if they are already bound to a memory location within the enclosing scope.

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