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4: Scope, Lifetime, and Closure
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Summary
============

- A variable that a function assigns to is created as a local variable
- To reference a global variable that a a function assigns to, it has to be declared as global
- Functions can be defined within other functions and the inner functions have access to the containing functions local variables, but not vice versa
- if an inner functions assigns to a variable then it is created as a local variable
- To refer to a local variable of a containing function in an assignment you have to declare the variable nonlocal
- Closure is a natural consequence of function objects outliving their local variables
    - If an inner function exists and can be invoked after its containing function has ended, then it still has access to the same local variables via the closure
- The closure consists of all of the variables in scope when a  function is declared
- All functions declared within the same execution context share that context as their closure
- The values of variables in the closure are the last values they had before the outer function terminated
- Closures has many uses, but the main ones are to provide private state variables, provide a self-reference, and to provide context to callback functions
- You can work with the "__closure__" magic attribute to access, and even change, the closure, but how this works is platform-dependent.

Program
============

.. literalinclude:: programs/chapterFour.py
   :language: python

Program Output
=================

.. code-block:: console

   (docs-env) root@BMitchellLTOP:~/git/sphinx_students/source/programming_lang/book/programs# python3 chapterFour.py
    Inside outer_function:
    Local outer_var: Outer Variable
    Accessing global counter: 100

    Inside inner_function:
    Local inner_var: Inner Variable
    Modified outer_var: Outer Variable Modified by Inner
    Accessing global counter: 100
    Message from outer_function: Hello from closure!

    Back in global scope:
    Global counter: 100