{"id":955,"date":"2020-11-05T03:07:39","date_gmt":"2020-11-05T03:07:39","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=955"},"modified":"2025-03-27T13:42:22","modified_gmt":"2025-03-27T13:42:22","slug":"python-nonlocal","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/advanced-python\/python-nonlocal\/","title":{"rendered":"Python nonlocal"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python nonlocal scopes and how to use the nonlocal<\/code> keyword to change the variables of the nonlocal scopes.<\/p>\n\n\n\n

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

In Python, you can define a function<\/a> inside another function. For example:<\/p>\n\n\n

def<\/span> outer<\/span>()<\/span>:<\/span>\n    print('outer function'<\/span>)\n\n    def<\/span> inner<\/span>()<\/span>:<\/span>\n        print('inner function'<\/span>)\n\n    inner()\n\n\nouter()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
outer function\ninner function<\/code><\/span>Code language:<\/span> plaintext<\/span> (<\/span>plaintext<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we defined a function called outer<\/code>.<\/p>\n\n\n\n
Inside the outer<\/code> function, we defined another function called inner<\/code>. And we called the inner<\/code> function from the inside of the outer<\/code> function.<\/p>\n\n\n\n
Often, we say that the inner<\/code> function is nested in the outer<\/code> function. In practice, you define nested functions when you don’t want these functions to be global.<\/p>\n\n\n\n
And the inner<\/code> function also has access to its enclosing scope, which is the scope of the outer<\/code> function.<\/p>\n\n\n\n
From the inner()<\/code> function perspective, its enclosing scope is neither local nor global. And Python calls this a nonlocal scope.<\/p>\n\n\n\n
Let’s modify the outer<\/code> and inner<\/code> functions:<\/p>\n\n\n
def<\/span> outer<\/span>()<\/span>:<\/span>\n    message = 'outer function'<\/span>\n    print(message)\n\n    def<\/span> inner<\/span>()<\/span>:<\/span>\n        print(message)\n\n    inner()\n\n\nouter()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
outer function<\/span>\nouter<\/span> function<\/span><\/span><\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
When we call the outer<\/code> function, Python creates the inner<\/code> function and executes it.<\/p>\n\n\n\n
When the inner<\/code> function executes, Python doesn’t find the message<\/code> variable in the local scope. So Python looks for it in the enclosing scope, which is the scope of the outer<\/code> function:<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
See the following example:<\/p>\n\n\n
message = 'global scope'<\/span>\n\n\ndef<\/span> outer<\/span>()<\/span>:<\/span>\n\n    def<\/span> inner<\/span>()<\/span>:<\/span>\n        print(message)\n\n    inner()\n\n\nouter()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
global<\/span> scope<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, Python searches for the message<\/code> variable in the local scope of the inner<\/code> function.<\/p>\n\n\n\n
Since Python doesn’t find the variable, it searches for the variable in its enclosing scope, which is the scope of the outer<\/code> function.<\/p>\n\n\n\n
And in this case, Python goes up to the global scope to find the variable:<\/p>\n\n\n\n
\"Python<\/figure>\n\n\n\n
Python nonlocal keyword #<\/a><\/h2>\n\n\n\n
To modify variables from a nonlocal scope in a local scope, you use the nonlocal<\/code> keyword. For example:<\/p>\n\n\n
def<\/span> outer<\/span>()<\/span>:<\/span>\n    message = 'outer scope'<\/span>\n    print(message)\n\n    def<\/span> inner<\/span>()<\/span>:<\/span>\n        nonlocal<\/span> message\n        message = 'inner scope'<\/span>\n        print(message)\n\n    inner()\n\n    print(message)\n\n\nouter()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
outer scope\ninner scope\ninner scope    <\/code><\/span><\/pre>\n\n\n
In this example, we use nonlocal<\/code> keyword to explicitly instruct Python that we’re modifying a nonlocal variable.<\/p>\n\n\n\n
When you use the nonlocal<\/code> keyword for a variable, Python will look for the variable in the enclosing local scopes chain until it first encounters the variable name.<\/p>\n\n\n\n
More importantly, Python won’t look for the variable in the global scope.<\/p>\n\n\n\n
Consider the following example:<\/p>\n\n\n
message = 'outer scope'<\/span>\n\n\ndef<\/span> outer<\/span>()<\/span>:<\/span>\n    print(message)\n\n    def<\/span> inner<\/span>()<\/span>:<\/span>\n        nonlocal<\/span> message\n        message = 'inner scope'<\/span>\n        print(message)\n\n    inner()\n\n    print(message)\n\n\nouter()<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you run this code, you’ll get the following error:<\/p>\n\n\n
SyntaxError<\/span>: no binding for<\/span> nonlocal 'message'<\/span> found<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
From inside of the inner<\/code> function, we use the nonlocal<\/code> keyword for the message<\/code> variable.<\/p>\n\n\n\n
Therefore, Python searches for the message<\/code> variable in the enclosing scope, which is the scope of the outer<\/code> function.<\/p>\n\n\n\n
Since the scope of the outer<\/code> function doesn’t have message<\/code> variable and Python doesn’t look further in the global scope, it issues an error:<\/p>\n\n\n\n
\"Python<\/figure>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • The enclosing scopes of inner functions are called nonlocal scopes.<\/li>\n\n\n\n
  • Use the nonlocal<\/code> keyword to modify the variable from the nonlocal scopes.<\/li>\n\n\n\n
  • And Python will look up the nonlocal variables in the enclosing local scopes chain. It won’t search for the variable in the global scope.<\/li>\n<\/ul>\n
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    Was this tutorial helpful ?<\/div>\n\t\t
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Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
Both outer<\/code> and inner<\/code> have access to the global and built-in scopes<\/a> as well as their local scopes.<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n