Function Nesting: How to Define Another Function Inside a Function in Python?

Python function nesting refers to defining an inner function within an outer function, which can hide functionality or implement complex logic. It has two calling methods: one is to directly call the inner function within the outer function; the other is to have the outer function return the inner function object for external invocation. The scope of the inner function is limited to the outer function. It can access the parameters or local variables of the outer function, but the outer function cannot access the local variables of the inner function, which is a core feature of nesting. Common uses of function nesting include implementing closures (where the inner function retains the state of the outer function) and decorators (which add additional functionality to functions, such as timing and logging). It enables code modular encapsulation and temporary state preservation, serving as a foundation for advanced Python features like closures and decorators. Beginners can start with nested calls and scope rules to gradually master its application in development.