Original Translation
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A *namespace* is a mapping from names to objects. Most namespaces are currently implemented as Python dictionaries, but that's normally not noticeable in any way (except for performance), and it may change in the future. Examples of namespaces are: the set of built-in names (functions such as :func:`abs`, and built-in exception names); the global names in a module; and the local names in a function invocation. In a sense the set of attributes of an object also form a namespace. The important thing to know about namespaces is that there is absolutely no relation between names in different namespaces; for instance, two different modules may both define a function ``maximize`` without confusion --- users of the modules must prefix it with the module name.
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By the way, I use the word *attribute* for any name following a dot --- for example, in the expression ``z.real``, ``real`` is an attribute of the object ``z``. Strictly speaking, references to names in modules are attribute references: in the expression ``modname.funcname``, ``modname`` is a module object and ``funcname`` is an attribute of it. In this case there happens to be a straightforward mapping between the module's attributes and the global names defined in the module: they share the same namespace! [#]_
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Attributes may be read-only or writable. In the latter case, assignment to attributes is possible. Module attributes are writable: you can write ``modname.the_answer = 42``. Writable attributes may also be deleted with the :keyword:`del` statement. For example, ``del modname.the_answer`` will remove the attribute :attr:`the_answer` from the object named by ``modname``.
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Namespaces are created at different moments and have different lifetimes. The namespace containing the built-in names is created when the Python interpreter starts up, and is never deleted. The global namespace for a module is created when the module definition is read in; normally, module namespaces also last until the interpreter quits. The statements executed by the top-level invocation of the interpreter, either read from a script file or interactively, are considered part of a module called :mod:`__main__`, so they have their own global namespace. (The built-in names actually also live in a module; this is called :mod:`builtins`.)
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The local namespace for a function is created when the function is called, and deleted when the function returns or raises an exception that is not handled within the function. (Actually, forgetting would be a better way to describe what actually happens.) Of course, recursive invocations each have their own local namespace.
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A *scope* is a textual region of a Python program where a namespace is directly accessible. "Directly accessible" here means that an unqualified reference to a name attempts to find the name in the namespace.
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Although scopes are determined statically, they are used dynamically. At any time during execution, there are at least three nested scopes whose namespaces are directly accessible:
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the innermost scope, which is searched first, contains the local names
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the scopes of any enclosing functions, which are searched starting with the nearest enclosing scope, contains non-local, but also non-global names
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the next-to-last scope contains the current module's global names