1 2.. _execmodel: 3 4*************** 5Execution model 6*************** 7 8.. index:: 9 single: execution model 10 pair: code; block 11 12.. _prog_structure: 13 14Structure of a program 15====================== 16 17.. index:: block 18 19A Python program is constructed from code blocks. 20A :dfn:`block` is a piece of Python program text that is executed as a unit. 21The following are blocks: a module, a function body, and a class definition. 22Each command typed interactively is a block. A script file (a file given as 23standard input to the interpreter or specified as a command line argument to the 24interpreter) is a code block. A script command (a command specified on the 25interpreter command line with the :option:`-c` option) is a code block. 26A module run as a top level script (as module ``__main__``) from the command 27line using a :option:`-m` argument is also a code block. The string 28argument passed to the built-in functions :func:`eval` and :func:`exec` is a 29code block. 30 31.. index:: pair: execution; frame 32 33A code block is executed in an :dfn:`execution frame`. A frame contains some 34administrative information (used for debugging) and determines where and how 35execution continues after the code block's execution has completed. 36 37.. _naming: 38 39Naming and binding 40================== 41 42.. index:: 43 single: namespace 44 single: scope 45 46.. _bind_names: 47 48Binding of names 49---------------- 50 51.. index:: 52 single: name 53 pair: binding; name 54 55:dfn:`Names` refer to objects. Names are introduced by name binding operations. 56 57.. index:: single: from; import statement 58 59The following constructs bind names: formal parameters to functions, 60:keyword:`import` statements, class and function definitions (these bind the 61class or function name in the defining block), and targets that are identifiers 62if occurring in an assignment, :keyword:`for` loop header, or after 63:keyword:`!as` in a :keyword:`with` statement or :keyword:`except` clause. 64The :keyword:`!import` statement 65of the form ``from ... import *`` binds all names defined in the imported 66module, except those beginning with an underscore. This form may only be used 67at the module level. 68 69A target occurring in a :keyword:`del` statement is also considered bound for 70this purpose (though the actual semantics are to unbind the name). 71 72Each assignment or import statement occurs within a block defined by a class or 73function definition or at the module level (the top-level code block). 74 75.. index:: pair: free; variable 76 77If a name is bound in a block, it is a local variable of that block, unless 78declared as :keyword:`nonlocal` or :keyword:`global`. If a name is bound at 79the module level, it is a global variable. (The variables of the module code 80block are local and global.) If a variable is used in a code block but not 81defined there, it is a :dfn:`free variable`. 82 83Each occurrence of a name in the program text refers to the :dfn:`binding` of 84that name established by the following name resolution rules. 85 86.. _resolve_names: 87 88Resolution of names 89------------------- 90 91.. index:: scope 92 93A :dfn:`scope` defines the visibility of a name within a block. If a local 94variable is defined in a block, its scope includes that block. If the 95definition occurs in a function block, the scope extends to any blocks contained 96within the defining one, unless a contained block introduces a different binding 97for the name. 98 99.. index:: single: environment 100 101When a name is used in a code block, it is resolved using the nearest enclosing 102scope. The set of all such scopes visible to a code block is called the block's 103:dfn:`environment`. 104 105.. index:: 106 single: NameError (built-in exception) 107 single: UnboundLocalError 108 109When a name is not found at all, a :exc:`NameError` exception is raised. 110If the current scope is a function scope, and the name refers to a local 111variable that has not yet been bound to a value at the point where the name is 112used, an :exc:`UnboundLocalError` exception is raised. 113:exc:`UnboundLocalError` is a subclass of :exc:`NameError`. 114 115If a name binding operation occurs anywhere within a code block, all uses of the 116name within the block are treated as references to the current block. This can 117lead to errors when a name is used within a block before it is bound. This rule 118is subtle. Python lacks declarations and allows name binding operations to 119occur anywhere within a code block. The local variables of a code block can be 120determined by scanning the entire text of the block for name binding operations. 121 122If the :keyword:`global` statement occurs within a block, all uses of the name 123specified in the statement refer to the binding of that name in the top-level 124namespace. Names are resolved in the top-level namespace by searching the 125global namespace, i.e. the namespace of the module containing the code block, 126and the builtins namespace, the namespace of the module :mod:`builtins`. The 127global namespace is searched first. If the name is not found there, the 128builtins namespace is searched. The :keyword:`!global` statement must precede 129all uses of the name. 130 131The :keyword:`global` statement has the same scope as a name binding operation 132in the same block. If the nearest enclosing scope for a free variable contains 133a global statement, the free variable is treated as a global. 134 135.. XXX say more about "nonlocal" semantics here 136 137The :keyword:`nonlocal` statement causes corresponding names to refer 138to previously bound variables in the nearest enclosing function scope. 139:exc:`SyntaxError` is raised at compile time if the given name does not 140exist in any enclosing function scope. 141 142.. index:: module: __main__ 143 144The namespace for a module is automatically created the first time a module is 145imported. The main module for a script is always called :mod:`__main__`. 146 147Class definition blocks and arguments to :func:`exec` and :func:`eval` are 148special in the context of name resolution. 149A class definition is an executable statement that may use and define names. 150These references follow the normal rules for name resolution with an exception 151that unbound local variables are looked up in the global namespace. 152The namespace of the class definition becomes the attribute dictionary of 153the class. The scope of names defined in a class block is limited to the 154class block; it does not extend to the code blocks of methods -- this includes 155comprehensions and generator expressions since they are implemented using a 156function scope. This means that the following will fail:: 157 158 class A: 159 a = 42 160 b = list(a + i for i in range(10)) 161 162.. _restrict_exec: 163 164Builtins and restricted execution 165--------------------------------- 166 167.. index:: pair: restricted; execution 168 169.. impl-detail:: 170 171 Users should not touch ``__builtins__``; it is strictly an implementation 172 detail. Users wanting to override values in the builtins namespace should 173 :keyword:`import` the :mod:`builtins` module and modify its 174 attributes appropriately. 175 176The builtins namespace associated with the execution of a code block 177is actually found by looking up the name ``__builtins__`` in its 178global namespace; this should be a dictionary or a module (in the 179latter case the module's dictionary is used). By default, when in the 180:mod:`__main__` module, ``__builtins__`` is the built-in module 181:mod:`builtins`; when in any other module, ``__builtins__`` is an 182alias for the dictionary of the :mod:`builtins` module itself. 183 184 185.. _dynamic-features: 186 187Interaction with dynamic features 188--------------------------------- 189 190Name resolution of free variables occurs at runtime, not at compile time. 191This means that the following code will print 42:: 192 193 i = 10 194 def f(): 195 print(i) 196 i = 42 197 f() 198 199.. XXX from * also invalid with relative imports (at least currently) 200 201The :func:`eval` and :func:`exec` functions do not have access to the full 202environment for resolving names. Names may be resolved in the local and global 203namespaces of the caller. Free variables are not resolved in the nearest 204enclosing namespace, but in the global namespace. [#]_ The :func:`exec` and 205:func:`eval` functions have optional arguments to override the global and local 206namespace. If only one namespace is specified, it is used for both. 207 208 209.. _exceptions: 210 211Exceptions 212========== 213 214.. index:: single: exception 215 216.. index:: 217 single: raise an exception 218 single: handle an exception 219 single: exception handler 220 single: errors 221 single: error handling 222 223Exceptions are a means of breaking out of the normal flow of control of a code 224block in order to handle errors or other exceptional conditions. An exception 225is *raised* at the point where the error is detected; it may be *handled* by the 226surrounding code block or by any code block that directly or indirectly invoked 227the code block where the error occurred. 228 229The Python interpreter raises an exception when it detects a run-time error 230(such as division by zero). A Python program can also explicitly raise an 231exception with the :keyword:`raise` statement. Exception handlers are specified 232with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally` 233clause of such a statement can be used to specify cleanup code which does not 234handle the exception, but is executed whether an exception occurred or not in 235the preceding code. 236 237.. index:: single: termination model 238 239Python uses the "termination" model of error handling: an exception handler can 240find out what happened and continue execution at an outer level, but it cannot 241repair the cause of the error and retry the failing operation (except by 242re-entering the offending piece of code from the top). 243 244.. index:: single: SystemExit (built-in exception) 245 246When an exception is not handled at all, the interpreter terminates execution of 247the program, or returns to its interactive main loop. In either case, it prints 248a stack traceback, except when the exception is :exc:`SystemExit`. 249 250Exceptions are identified by class instances. The :keyword:`except` clause is 251selected depending on the class of the instance: it must reference the class of 252the instance or a base class thereof. The instance can be received by the 253handler and can carry additional information about the exceptional condition. 254 255.. note:: 256 257 Exception messages are not part of the Python API. Their contents may change 258 from one version of Python to the next without warning and should not be 259 relied on by code which will run under multiple versions of the interpreter. 260 261See also the description of the :keyword:`try` statement in section :ref:`try` 262and :keyword:`raise` statement in section :ref:`raise`. 263 264 265.. rubric:: Footnotes 266 267.. [#] This limitation occurs because the code that is executed by these operations 268 is not available at the time the module is compiled. 269