Chapter 9. Classes 66 Python Tutorial, Release 3.9.5 9.2 Python Scopes and Namespaces Before introducing classes, I first have to tell you something about Python’s scope rules. Class definitions play some neat tricks with namespaces, and you need to know how scopes and namespaces work to fully understand what’s going on. Incidentally, knowledge about this subject is useful for any advanced Python programmer. Let’s begin with some definitions. 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 (containing functions such as$_{abs()}$ , 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. 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! 1 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$_{del}$ statement. For example,$_{del modname.the_answer}$ will remove the attribute$_{the_answer}$from the object named by$_{modname}$. 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$_{__main__}$ , so they have their own global namespace. (The built-in names actually also live in a module; this is called$_{builtins}$.) 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. 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. Although scopes are determined statically, they are used dynamically. At any time during execution, there are 3 or 4 nested scopes whose namespaces are directly accessible: If a name is declared global, then all references and assignments go directly to the middle scope containing the module’s global names. To rebind variables found outside of the innermost scope, the$_{nonlocal}$ statement can be used; if not declared nonlocal, those variables are read-only (an attempt to write to such a variable will simply create a new local variable in the innermost scope, leaving the identically named outer variable unchanged). Usually, the local scope references the local names of the (textually) current function. Outside 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. 1 Except for one thing. Module objects have a secret read-only attribute called __dict__ which returns the dictionary used to implement the module’s namespace; the name __dict__ is an attribute but not a global name. Obviously, using this violates the abstraction of namespace implementation, and should be restricted to things like post-mortem debuggers. • the outermost scope (searched last) is the namespace containing built-in names • the next-to-last scope contains the current module’s global names • the scopes of any enclosing functions, which are searched starting with the nearest enclosing scope, contains non-local, but also non-global names • the innermost scope, which is searched first, contains the local names