Naming and binding
Names refer to objects. Names are introduced by name binding operations. Each occurrence of a name in the program text refers to the binding of that name established in the innermost function block containing the use.
A block is a piece of Python program text that is executed as a unit. The following are blocks: a module, a function body, and a class definition. Each command typed interactively is a block. A script file (a file given as standard input to the interpreter or specified on the interpreter command line the first argument) is a code block. A script command (a command specified on the interpreter command line with the -c option) is a code block. The file read by the built-in function execfile is a code block. The string argument passed to the built-in function eval and to the exec statement is a code block. The expression read and evaluated by the built-in function input is a code block.
A code block is executed in an execution frame. A frame contains some administrative information (used for debugging) and determines where and how execution continues after the code block’s execution has completed.
A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block. If the definition occurs in a function block, the scope extends to any blocks contained within the defining one, unless a contained block introduces a different binding for the name. The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods.
When a name is used in a code block, it is resolved using the nearest enclosing scope. The set of all such scopes visible to a code block is called the block’s environment.
If a name is bound in a block, it is a local variable of that block. If a name is bound at the module level, it is a global variable. (The variables of the module code block are local and global.) If a variable is used in a code block but not defined there, it is a free variable.
When a name is not found at all, a NameError exception is raised. If the name refers to a local variable that has not been bound, a UnboundLocalError exception is raised. UnboundLocalError is a subclass of NameError.
The following constructs bind names: formal parameters to functions, import statements, class and function definitions (these bind the class or function name in the defining block), and targets that are identifiers if occurring in an assignment, for loop header, or in the second position of an except clause header. The import statement of the form “from …import *” binds all names defined in the imported module, except those beginning with an underscore. This form may only be used at the module level.
A target occurring in a del statement is also considered bound for this purpose (though the actual semantics are to unbind the name). It is illegal to unbind a name that is referenced by an enclosing scope; the compiler will report a SyntaxError.
Each assignment or import statement occurs within a block defined by a class or function definition or at the module level (the top-level code block).
If a name binding operation occurs anywhere within a code block, all uses of the name within the block are treated as references to the current block. This can lead to errors when a name is used within a block before it is bound. This rule is subtle. Python lacks declarations and allows name binding operations to occur anywhere within a code block. The local variables of a code block can be determined by scanning the entire text of the block for name binding operations.
If the global statement occurs within a block, all uses of the name specified in the statement refer to the binding of that name in the top-level namespace. Names are resolved in the top-level namespace by searching the global namespace, i.e. the namespace of the module containing the code block, and the builtin namespace, the namespace of the module __builtin__. The global namespace is searched first. If the name is not found there, the builtin namespace is searched. The global statement must precede all uses of the name.
The built-in namespace associated with the execution of a code block
is actually found by looking up the name __builtins__ in its
global namespace; this should be a dictionary or a module (in the
latter case the module’s dictionary is used). Normally, the
__builtins__ namespace is the dictionary of the built-in module
__builtin__ (note: no
s). If it isn’t, restricted execution mode
is in effect.
The namespace for a module is automatically created the first time a module is imported. The main module for a script is always called __main__.
The global statement has the same scope as a name binding operation in the same block. If the nearest enclosing scope for a free variable contains a global statement, the free variable is treated as a global.
A class definition is an executable statement that may use and defin names. These references follow the normal rules for name resolution. The namespace of the class definition becomes the attribute dictionary of the class. Names defined at the class scope are not visible in methods.
Interaction with dynamic features
There are several cases where Python statements are illegal when used in conjunction with nested scopes that contain free variables.
If a variable is referenced in an enclosing scope, it is illegal to delete the name. An error will be reported at compile time.
If the wild card form of import — “from … import *” — is used in a function and the function contains or is a nested block with free variables, the compiler will raise a SyntaxError.
If exec is used in a function and the function contains or is a nested block with free variables, the compiler will raise a SyntaxError unless the exec explicitly specifies the local namespace for the exec. (In other words, “exec obj” would be illegal, but “exec obj in ns” would be legal.)
The eval, execfile, and input functions and the exec statement do not have access to the full environment for resolving names. Names may be resolved in the local and global namespaces of the caller. Free variables are not resolved in the nearest enclosing namespace, but in the global namespace. The exec statement and the eval and execfile functions have optional arguments to override the global and local namespace. If only one namespace is specified, it is used for both.