4.Execution model

4.1.Structure of a program

A Python program is constructed from code blocks.Ablock 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 asstandard input to the interpreter or specified as a command line argument to theinterpreter) is a code block. A script command (a command specified on theinterpreter command line with the-c option) is a code block.A module run as a top level script (as module__main__) from the commandline using a-m argument is also a code block. The stringargument passed to the built-in functionseval() andexec() is acode block.

A code block is executed in anexecution frame. A frame contains someadministrative information (used for debugging) and determines where and howexecution continues after the code block’s execution has completed.

4.2.Naming and binding

4.2.1.Binding of names

Names refer to objects. Names are introduced by name binding operations.

The following constructs bind names: formal parameters to functions,import statements, class and function definitions (these bind theclass or function name in the defining block), and targets that are identifiersif occurring in an assignment,for loop header, or afteras in awith statement orexcept clause.Theimport statementof the formfrom...import* binds all names defined in the importedmodule, except those beginning with an underscore. This form may only be usedat the module level.

A target occurring in adel statement is also considered bound forthis purpose (though the actual semantics are to unbind the name).

Each assignment or import statement occurs within a block defined by a class orfunction definition or at the module level (the top-level code block).

If a name is bound in a block, it is a local variable of that block, unlessdeclared asnonlocal orglobal. If a name is bound atthe module level, it is a global variable. (The variables of the module codeblock are local and global.) If a variable is used in a code block but notdefined there, it is afree variable.

Each occurrence of a name in the program text refers to thebinding ofthat name established by the following name resolution rules.

4.2.2.Resolution of names

Ascope defines the visibility of a name within a block. If a localvariable is defined in a block, its scope includes that block. If thedefinition occurs in a function block, the scope extends to any blocks containedwithin the defining one, unless a contained block introduces a different bindingfor the name.

When a name is used in a code block, it is resolved using the nearest enclosingscope. The set of all such scopes visible to a code block is called the block’senvironment.

When a name is not found at all, aNameError exception is raised.If the current scope is a function scope, and the name refers to a localvariable that has not yet been bound to a value at the point where the name isused, anUnboundLocalError exception is raised.UnboundLocalError is a subclass ofNameError.

If a name binding operation occurs anywhere within a code block, all uses of thename within the block are treated as references to the current block. This canlead to errors when a name is used within a block before it is bound. This ruleis subtle. Python lacks declarations and allows name binding operations tooccur anywhere within a code block. The local variables of a code block can bedetermined by scanning the entire text of the block for name binding operations.

If theglobal statement occurs within a block, all uses of the namesspecified in the statement refer to the bindings of those names in the top-levelnamespace. Names are resolved in the top-level namespace by searching theglobal namespace, i.e. the namespace of the module containing the code block,and the builtins namespace, the namespace of the modulebuiltins. Theglobal namespace is searched first. If the names are not found there, thebuiltins namespace is searched. Theglobal statement must precedeall uses of the listed names.

Theglobal statement has the same scope as a name binding operationin the same block. If the nearest enclosing scope for a free variable containsa global statement, the free variable is treated as a global.

Thenonlocal statement causes corresponding names to referto previously bound variables in the nearest enclosing function scope.SyntaxError is raised at compile time if the given name does notexist in any enclosing function scope.

The namespace for a module is automatically created the first time a module isimported. The main module for a script is always called__main__.

Class definition blocks and arguments toexec() andeval() arespecial in the context of name resolution.A class definition is an executable statement that may use and define names.These references follow the normal rules for name resolution with an exceptionthat unbound local variables are looked up in the global namespace.The namespace of the class definition becomes the attribute dictionary ofthe class. The scope of names defined in a class block is limited to theclass block; it does not extend to the code blocks of methods – this includescomprehensions and generator expressions since they are implemented using afunction scope. This means that the following will fail:

classA:a=42b=list(a+iforiinrange(10))

4.2.3.Builtins and restricted execution

CPython implementation detail: Users should not touch__builtins__; it is strictly an implementationdetail. Users wanting to override values in the builtins namespace shouldimport thebuiltins module and modify itsattributes appropriately.

The builtins namespace associated with the execution of a code blockis actually found by looking up the name__builtins__ in itsglobal namespace; this should be a dictionary or a module (in thelatter case the module’s dictionary is used). By default, when in the__main__ module,__builtins__ is the built-in modulebuiltins; when in any other module,__builtins__ is analias for the dictionary of thebuiltins module itself.

4.2.4.Interaction with dynamic features

Name resolution of free variables occurs at runtime, not at compile time.This means that the following code will print 42:

i=10deff():print(i)i=42f()

Theeval() andexec() functions do not have access to the fullenvironment for resolving names. Names may be resolved in the local and globalnamespaces of the caller. Free variables are not resolved in the nearestenclosing namespace, but in the global namespace.1 Theexec() andeval() functions have optional arguments to override the global and localnamespace. If only one namespace is specified, it is used for both.

4.3.Exceptions

Exceptions are a means of breaking out of the normal flow of control of a codeblock in order to handle errors or other exceptional conditions. An exceptionisraised at the point where the error is detected; it may behandled by thesurrounding code block or by any code block that directly or indirectly invokedthe 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 anexception with theraise statement. Exception handlers are specifiedwith thetry …except statement. Thefinallyclause of such a statement can be used to specify cleanup code which does nothandle the exception, but is executed whether an exception occurred or not inthe preceding code.

Python uses the “termination” model of error handling: an exception handler canfind out what happened and continue execution at an outer level, but it cannotrepair the cause of the error and retry the failing operation (except byre-entering the offending piece of code from the top).

When an exception is not handled at all, the interpreter terminates execution ofthe program, or returns to its interactive main loop. In either case, it printsa stack traceback, except when the exception isSystemExit.

Exceptions are identified by class instances. Theexcept clause isselected depending on the class of the instance: it must reference the class ofthe instance or anon-virtual base class thereof.The instance can be received by the handler and can carry additional informationabout the exceptional condition.

Note

Exception messages are not part of the Python API. Their contents may changefrom one version of Python to the next without warning and should not berelied on by code which will run under multiple versions of the interpreter.

See also the description of thetry statement in sectionThe try statementandraise statement in sectionThe raise statement.

Footnotes

1

This limitation occurs because the code that is executed by these operationsis not available at the time the module is compiled.