{"id":941,"date":"2020-11-05T00:33:11","date_gmt":"2020-11-05T00:33:11","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=941"},"modified":"2025-03-27T13:39:04","modified_gmt":"2025-03-27T13:39:04","slug":"python-variable-scopes","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-variable-scopes\/","title":{"rendered":"Python Variable Scopes"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how Python variable scopes work. After the tutorial, you’ll have a good understanding of built-in, local, and global scopes.<\/p>\n\n\n\n

Introduction to Python variable scopes #<\/a><\/h2>\n\n\n\n

When you assign an object to a variable<\/a>, the variable will reference<\/a> that object in the memory. It’s saying that the variable is bound to the object.<\/p>\n\n\n\n

After the assignment, you can access the object using the variable name in various parts of your code. However, you cannot access the variable everywhere in the code.<\/p>\n\n\n\n

The variable name and its binding (name and object) only exist in specific parts of your code.<\/p>\n\n\n\n

The part of the code where you define the name\/binding is called the lexical scope<\/strong> of the variables.<\/p>\n\n\n\n

Python stores these bindings in something called namespaces. Every scope has its own namespace.<\/p>\n\n\n\n

You can think that a namespace is a table which contains the label and the reference that the label is bound to.<\/p>\n\n\n\n

Global scopes #<\/a><\/h2>\n\n\n\n

The global scope is the module<\/a> scope. The global scope spans a single Python source code file only.<\/p>\n\n\n\n

Python doesn’t have a truly global scope that spans across all modules except for the built-in scope.<\/p>\n\n\n\n

The built-in scope is a special scope that provides globally available objects such as print<\/code>, len<\/code>, None<\/code>, True<\/code>, and False<\/code>.<\/p>\n\n\n\n

Basically, the built-in and global variables exist everywhere inside a module.<\/p>\n\n\n\n

Internally, global scopes are nested inside the built-in scope:<\/p>\n\n\n

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If you access a variable from a scope and Python doesn’t find it in the namespace of that scope, it’ll search in the enclosing scope’s namespace.<\/p>\n\n\n\n

Suppose that you have the following statement in a module called app.py<\/code>:<\/p>\n\n\n

print('Hello'<\/span>)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this app.py<\/code> module, Python looks for the print<\/code> function in the module scope (app.py<\/code>).<\/p>\n\n\n\n
Since Python doesn’t find the definition of the print<\/code> function in the app.py<\/code> module scope, Python goes up to the enclosing scope, which is the built-in scope, and looks for the print<\/code> function there. In this case, it can find the print<\/code> function in the built-in scope.<\/p>\n\n\n
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If you change the statement to the following, you’ll get an runtime error:<\/p>\n\n\n
print(counter)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, Python doesn’t find the counter<\/code> in the current global scope. Therefore, Python looks for it in the enclosing scope, which is the built-in scope.<\/p>\n\n\n\n
However, the variable counter<\/code> doesn’t exist in the built-in scope. Therefore, Python issues a NameError<\/code> exception:<\/p>\n\n\n
NameError: name 'counter' is not defined<\/code><\/span>Code language:<\/span> Shell Session<\/span> (<\/span>shell<\/span>)<\/span><\/small><\/pre>\n\n\n
Local scopes #<\/a><\/h2>\n\n\n\n
When creating a function<\/a>, you can define parameters and variables for the function. For example:<\/p>\n\n\n
def<\/span> increment<\/span>(counter, by=1<\/span>)<\/span>:<\/span>\n    result = counter + by\n    return<\/span> result<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
When you execute the code, Python carries two phases: <\/p>\n\n\n\n