1 2.. _execmodel: 3 4*************** 5Execution model 6*************** 7 8.. index:: single: execution model 9 10 11.. _naming: 12 13Naming and binding 14================== 15 16.. index:: 17 pair: code; block 18 single: namespace 19 single: scope 20 21.. index:: 22 single: name 23 pair: binding; name 24 25:dfn:`Names` refer to objects. Names are introduced by name binding operations. 26Each occurrence of a name in the program text refers to the :dfn:`binding` of 27that name established in the innermost function block containing the use. 28 29.. index:: single: block 30 31A :dfn:`block` is a piece of Python program text that is executed as a unit. 32The following are blocks: a module, a function body, and a class definition. 33Each command typed interactively is a block. A script file (a file given as 34standard input to the interpreter or specified on the interpreter command line 35the first argument) is a code block. A script command (a command specified on 36the interpreter command line with the '**-c**' option) is a code block. The 37file read by the built-in function :func:`execfile` is a code block. The string 38argument passed to the built-in function :func:`eval` and to the :keyword:`exec` 39statement is a code block. The expression read and evaluated by the built-in 40function :func:`input` is a code block. 41 42.. index:: pair: execution; frame 43 44A code block is executed in an :dfn:`execution frame`. A frame contains some 45administrative information (used for debugging) and determines where and how 46execution continues after the code block's execution has completed. 47 48.. index:: single: scope 49 50A :dfn:`scope` defines the visibility of a name within a block. If a local 51variable is defined in a block, its scope includes that block. If the 52definition occurs in a function block, the scope extends to any blocks contained 53within the defining one, unless a contained block introduces a different binding 54for the name. The scope of names defined in a class block is limited to the 55class block; it does not extend to the code blocks of methods -- this includes 56generator expressions since they are implemented using a function scope. This 57means that the following will fail:: 58 59 class A: 60 a = 42 61 b = list(a + i for i in range(10)) 62 63.. index:: single: environment 64 65When a name is used in a code block, it is resolved using the nearest enclosing 66scope. The set of all such scopes visible to a code block is called the block's 67:dfn:`environment`. 68 69.. index:: pair: free; variable 70 71If a name is bound in a block, it is a local variable of that block. If a name 72is bound at the module level, it is a global variable. (The variables of the 73module code block are local and global.) If a variable is used in a code block 74but not defined there, it is a :dfn:`free variable`. 75 76.. index:: 77 single: NameError (built-in exception) 78 single: UnboundLocalError 79 80When a name is not found at all, a :exc:`NameError` exception is raised. If the 81name refers to a local variable that has not been bound, a 82:exc:`UnboundLocalError` exception is raised. :exc:`UnboundLocalError` is a 83subclass of :exc:`NameError`. 84 85.. index:: statement: from 86 87The following constructs bind names: formal parameters to functions, 88:keyword:`import` statements, class and function definitions (these bind the 89class or function name in the defining block), and targets that are identifiers 90if occurring in an assignment, :keyword:`for` loop header, in the second 91position of an :keyword:`except` clause header or after :keyword:`as` in a 92:keyword:`with` statement. The :keyword:`import` statement 93of the form ``from ... import *`` binds all names defined in the imported 94module, except those beginning with an underscore. This form may only be used 95at the module level. 96 97A target occurring in a :keyword:`del` statement is also considered bound for 98this purpose (though the actual semantics are to unbind the name). It is 99illegal to unbind a name that is referenced by an enclosing scope; the compiler 100will report a :exc:`SyntaxError`. 101 102Each assignment or import statement occurs within a block defined by a class or 103function definition or at the module level (the top-level code block). 104 105If a name binding operation occurs anywhere within a code block, all uses of the 106name within the block are treated as references to the current block. This can 107lead to errors when a name is used within a block before it is bound. This rule 108is subtle. Python lacks declarations and allows name binding operations to 109occur anywhere within a code block. The local variables of a code block can be 110determined by scanning the entire text of the block for name binding operations. 111 112If the global statement occurs within a block, all uses of the name specified in 113the statement refer to the binding of that name in the top-level namespace. 114Names are resolved in the top-level namespace by searching the global namespace, 115i.e. the namespace of the module containing the code block, and the builtins 116namespace, the namespace of the module :mod:`__builtin__`. The global namespace 117is searched first. If the name is not found there, the builtins namespace is 118searched. The global statement must precede all uses of the name. 119 120.. index:: pair: restricted; execution 121 122The builtins namespace associated with the execution of a code block is actually 123found by looking up the name ``__builtins__`` in its global namespace; this 124should be a dictionary or a module (in the latter case the module's dictionary 125is used). By default, when in the :mod:`__main__` module, ``__builtins__`` is 126the built-in module :mod:`__builtin__` (note: no 's'); when in any other module, 127``__builtins__`` is an alias for the dictionary of the :mod:`__builtin__` module 128itself. ``__builtins__`` can be set to a user-created dictionary to create a 129weak form of restricted execution. 130 131.. impl-detail:: 132 133 Users should not touch ``__builtins__``; it is strictly an implementation 134 detail. Users wanting to override values in the builtins namespace should 135 :keyword:`import` the :mod:`__builtin__` (no 's') module and modify its 136 attributes appropriately. 137 138.. index:: module: __main__ 139 140The namespace for a module is automatically created the first time a module is 141imported. The main module for a script is always called :mod:`__main__`. 142 143The :keyword:`global` statement has the same scope as a name binding operation 144in the same block. If the nearest enclosing scope for a free variable contains 145a global statement, the free variable is treated as a global. 146 147A class definition is an executable statement that may use and define names. 148These references follow the normal rules for name resolution. The namespace of 149the class definition becomes the attribute dictionary of the class. Names 150defined at the class scope are not visible in methods. 151 152 153.. _dynamic-features: 154 155Interaction with dynamic features 156--------------------------------- 157 158There are several cases where Python statements are illegal when used in 159conjunction with nested scopes that contain free variables. 160 161If a variable is referenced in an enclosing scope, it is illegal to delete the 162name. An error will be reported at compile time. 163 164If the wild card form of import --- ``import *`` --- is used in a function and 165the function contains or is a nested block with free variables, the compiler 166will raise a :exc:`SyntaxError`. 167 168If :keyword:`exec` is used in a function and the function contains or is a 169nested block with free variables, the compiler will raise a :exc:`SyntaxError` 170unless the exec explicitly specifies the local namespace for the 171:keyword:`exec`. (In other words, ``exec obj`` would be illegal, but ``exec obj 172in ns`` would be legal.) 173 174The :func:`eval`, :func:`execfile`, and :func:`input` functions and the 175:keyword:`exec` statement do not have access to the full environment for 176resolving names. Names may be resolved in the local and global namespaces of 177the caller. Free variables are not resolved in the nearest enclosing namespace, 178but in the global namespace. [#]_ The :keyword:`exec` statement and the 179:func:`eval` and :func:`execfile` functions have optional arguments to override 180the global and local namespace. If only one namespace is specified, it is used 181for both. 182 183 184.. _exceptions: 185 186Exceptions 187========== 188 189.. index:: single: exception 190 191.. index:: 192 single: raise an exception 193 single: handle an exception 194 single: exception handler 195 single: errors 196 single: error handling 197 198Exceptions are a means of breaking out of the normal flow of control of a code 199block in order to handle errors or other exceptional conditions. An exception 200is *raised* at the point where the error is detected; it may be *handled* by the 201surrounding code block or by any code block that directly or indirectly invoked 202the code block where the error occurred. 203 204The Python interpreter raises an exception when it detects a run-time error 205(such as division by zero). A Python program can also explicitly raise an 206exception with the :keyword:`raise` statement. Exception handlers are specified 207with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally` 208clause of such a statement can be used to specify cleanup code which does not 209handle the exception, but is executed whether an exception occurred or not in 210the preceding code. 211 212.. index:: single: termination model 213 214Python uses the "termination" model of error handling: an exception handler can 215find out what happened and continue execution at an outer level, but it cannot 216repair the cause of the error and retry the failing operation (except by 217re-entering the offending piece of code from the top). 218 219.. index:: single: SystemExit (built-in exception) 220 221When an exception is not handled at all, the interpreter terminates execution of 222the program, or returns to its interactive main loop. In either case, it prints 223a stack backtrace, except when the exception is :exc:`SystemExit`. 224 225Exceptions are identified by class instances. The :keyword:`except` clause is 226selected depending on the class of the instance: it must reference the class of 227the instance or a base class thereof. The instance can be received by the 228handler and can carry additional information about the exceptional condition. 229 230Exceptions can also be identified by strings, in which case the 231:keyword:`except` clause is selected by object identity. An arbitrary value can 232be raised along with the identifying string which can be passed to the handler. 233 234.. note:: 235 236 Messages to exceptions are not part of the Python API. Their contents may 237 change from one version of Python to the next without warning and should not be 238 relied on by code which will run under multiple versions of the interpreter. 239 240See also the description of the :keyword:`try` statement in section :ref:`try` 241and :keyword:`raise` statement in section :ref:`raise`. 242 243.. rubric:: Footnotes 244 245.. [#] This limitation occurs because the code that is executed by these operations is 246 not available at the time the module is compiled. 247 248