python.pptx
- 1. NADAR SARASWATHI COLLEGE OF ARTS AND SCIENCE Department of CS &IT SCOPE OF OBJECTS AND NAMES Presented by: S.SABTHAMI I.MSC(IT)
- 2. Python Scopes and Namespaces • 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. • Examples of namespaces are: – the set of built-in names (functions such as abs(), and built-in exception names) – the global names in a module; – and the local names in a function invocation.
- 3. Python Scopes and Namespaces – 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.
- 4. Python Scopes and Namespaces • 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!
- 5. Python Scopes and Namespaces • 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.
- 6. Python Scopes and Namespaces • 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 __main__, – so they have their own global namespace. • The built-in names actually also live in a module; – this is called __builtin__.
- 7. Python Scopes and Namespaces • 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. – Of course, recursive invocations each have their own local namespace.
- 8. Python Scopes and Namespaces • 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.
- 9. Python Scopes and Namespaces • 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: – the innermost scope, which is searched first, contains the local names; the namespaces of any enclosing functions,
- 10. Python Scopes and Namespaces – which are searched starting with the nearest enclosing scope; the middle scope, searched next, contains the current module’s global names; – and the outermost scope (searched last) is the namespace containing built-in names.
- 11. Python Scopes and Namespaces • If a name is declared global, then all references and assignments go directly to the middle scope containing the module’s global names. • Otherwise, all variables found outside of the innermost scope are read-only.
- 12. Python Scopes and Namespaces • Usually, the local scope references the local names of the current function. • Outside of functions, the local scope references the same namespace as the global scope: – the module’s namespace. • Class definitions place yet another namespace in the local scope.
- 13. Python Scopes and Namespaces • A special quirk of Python is that assignments always go into the innermost scope. • Assignments do not copy data— – they just bind names to objects. • The same is true for deletions: – the statement ‘del x’ removes the binding of x from the namespace referenced by the local scope.
- 14. Python Scopes and Namespaces • In fact, all operations that introduce new names use the local scope: – in particular, import statements and function definitions bind the module or function name in the local scope. (The global statement can be used to indicate that particular variables live in the global scope.)