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