4. Execution model
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 – this includes generator expressions since they are implemented using a function scope. This means that the following will fail:
class A: a = 42 b = list(a + i for i in range(10))
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, in the second position of an except clause header or after as in a with statement. 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 builtins namespace, the namespace of the module builtin. The global namespace is searched first. If the name is not found there, the builtins namespace is searched. The global statement must precede all uses of the name.
The builtins 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). By default, when in the main module,
__builtins__ is the built-in module builtin (note: no ‘s’); when in any other module,
__builtins__ is an alias for the dictionary of the builtin module itself.
__builtins__ can be set to a user-created dictionary to create a weak form of restricted execution.
CPython implementation detail: Users should not touch
__builtins__ ; it is strictly an implementation detail. Users wanting to override values in the builtins namespace should import the builtin (no ‘s’) module and modify its attributes appropriately.
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 define 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.
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 —
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.
Exceptions are a means of breaking out of the normal flow of control of a code block in order to handle errors or other exceptional conditions. An exception is raised at the point where the error is detected; it may be handled by the surrounding code block or by any code block that directly or indirectly invoked the code block where the error occurred.
The Python interpreter raises an exception when it detects a run-time error (such as division by zero). A Python program can also explicitly raise an exception with the raise statement. Exception handlers are specified with the try … except statement. The finally clause of such a statement can be used to specify cleanup code which does not handle the exception, but is executed whether an exception occurred or not in the preceding code.
Python uses the “termination” model of error handling: an exception handler can find out what happened and continue execution at an outer level, but it cannot repair the cause of the error and retry the failing operation (except by re-entering the offending piece of code from the top).
When an exception is not handled at all, the interpreter terminates execution of the program, or returns to its interactive main loop. In either case, it prints a stack backtrace, except when the exception is SystemExit.
Exceptions are identified by class instances. The except clause is selected depending on the class of the instance: it must reference the class of the instance or a base class thereof. The instance can be received by the handler and can carry additional information about the exceptional condition.
Exceptions can also be identified by strings, in which case the except clause is selected by object identity. An arbitrary value can be raised along with the identifying string which can be passed to the handler.
Messages to exceptions are not part of the Python API. Their contents may change from one version of Python to the next without warning and should not be relied on by code which will run under multiple versions of the interpreter.
- This limitation occurs because the code that is executed by these operations is not available at the time the module is compiled.