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: 60 61* formal parameters to functions, 62* class definitions, 63* function definitions, 64* assignment expressions, 65* :ref:`targets <assignment>` that are identifiers if occurring in 66 an assignment: 67 68 + :keyword:`for` loop header, 69 + after :keyword:`!as` in a :keyword:`with` statement, :keyword:`except` 70 clause or in the as-pattern in structural pattern matching, 71 + in a capture pattern in structural pattern matching 72 73* :keyword:`import` statements. 74 75The :keyword:`!import` statement of the form ``from ... import *`` binds all 76names defined in the imported module, except those beginning with an underscore. 77This form may only be used at the module level. 78 79A target occurring in a :keyword:`del` statement is also considered bound for 80this purpose (though the actual semantics are to unbind the name). 81 82Each assignment or import statement occurs within a block defined by a class or 83function definition or at the module level (the top-level code block). 84 85.. index:: pair: free; variable 86 87If a name is bound in a block, it is a local variable of that block, unless 88declared as :keyword:`nonlocal` or :keyword:`global`. If a name is bound at 89the module level, it is a global variable. (The variables of the module code 90block are local and global.) If a variable is used in a code block but not 91defined there, it is a :dfn:`free variable`. 92 93Each occurrence of a name in the program text refers to the :dfn:`binding` of 94that name established by the following name resolution rules. 95 96.. _resolve_names: 97 98Resolution of names 99------------------- 100 101.. index:: scope 102 103A :dfn:`scope` defines the visibility of a name within a block. If a local 104variable is defined in a block, its scope includes that block. If the 105definition occurs in a function block, the scope extends to any blocks contained 106within the defining one, unless a contained block introduces a different binding 107for the name. 108 109.. index:: single: environment 110 111When a name is used in a code block, it is resolved using the nearest enclosing 112scope. The set of all such scopes visible to a code block is called the block's 113:dfn:`environment`. 114 115.. index:: 116 single: NameError (built-in exception) 117 single: UnboundLocalError 118 119When a name is not found at all, a :exc:`NameError` exception is raised. 120If the current scope is a function scope, and the name refers to a local 121variable that has not yet been bound to a value at the point where the name is 122used, an :exc:`UnboundLocalError` exception is raised. 123:exc:`UnboundLocalError` is a subclass of :exc:`NameError`. 124 125If a name binding operation occurs anywhere within a code block, all uses of the 126name within the block are treated as references to the current block. This can 127lead to errors when a name is used within a block before it is bound. This rule 128is subtle. Python lacks declarations and allows name binding operations to 129occur anywhere within a code block. The local variables of a code block can be 130determined by scanning the entire text of the block for name binding operations. 131 132If the :keyword:`global` statement occurs within a block, all uses of the names 133specified in the statement refer to the bindings of those names in the top-level 134namespace. Names are resolved in the top-level namespace by searching the 135global namespace, i.e. the namespace of the module containing the code block, 136and the builtins namespace, the namespace of the module :mod:`builtins`. The 137global namespace is searched first. If the names are not found there, the 138builtins namespace is searched. The :keyword:`!global` statement must precede 139all uses of the listed names. 140 141The :keyword:`global` statement has the same scope as a name binding operation 142in the same block. If the nearest enclosing scope for a free variable contains 143a global statement, the free variable is treated as a global. 144 145.. XXX say more about "nonlocal" semantics here 146 147The :keyword:`nonlocal` statement causes corresponding names to refer 148to previously bound variables in the nearest enclosing function scope. 149:exc:`SyntaxError` is raised at compile time if the given name does not 150exist in any enclosing function scope. 151 152.. index:: module: __main__ 153 154The namespace for a module is automatically created the first time a module is 155imported. The main module for a script is always called :mod:`__main__`. 156 157Class definition blocks and arguments to :func:`exec` and :func:`eval` are 158special in the context of name resolution. 159A class definition is an executable statement that may use and define names. 160These references follow the normal rules for name resolution with an exception 161that unbound local variables are looked up in the global namespace. 162The namespace of the class definition becomes the attribute dictionary of 163the class. The scope of names defined in a class block is limited to the 164class block; it does not extend to the code blocks of methods -- this includes 165comprehensions and generator expressions since they are implemented using a 166function scope. This means that the following will fail:: 167 168 class A: 169 a = 42 170 b = list(a + i for i in range(10)) 171 172.. _restrict_exec: 173 174Builtins and restricted execution 175--------------------------------- 176 177.. index:: pair: restricted; execution 178 179.. impl-detail:: 180 181 Users should not touch ``__builtins__``; it is strictly an implementation 182 detail. Users wanting to override values in the builtins namespace should 183 :keyword:`import` the :mod:`builtins` module and modify its 184 attributes appropriately. 185 186The builtins namespace associated with the execution of a code block 187is actually found by looking up the name ``__builtins__`` in its 188global namespace; this should be a dictionary or a module (in the 189latter case the module's dictionary is used). By default, when in the 190:mod:`__main__` module, ``__builtins__`` is the built-in module 191:mod:`builtins`; when in any other module, ``__builtins__`` is an 192alias for the dictionary of the :mod:`builtins` module itself. 193 194 195.. _dynamic-features: 196 197Interaction with dynamic features 198--------------------------------- 199 200Name resolution of free variables occurs at runtime, not at compile time. 201This means that the following code will print 42:: 202 203 i = 10 204 def f(): 205 print(i) 206 i = 42 207 f() 208 209.. XXX from * also invalid with relative imports (at least currently) 210 211The :func:`eval` and :func:`exec` functions do not have access to the full 212environment for resolving names. Names may be resolved in the local and global 213namespaces of the caller. Free variables are not resolved in the nearest 214enclosing namespace, but in the global namespace. [#]_ The :func:`exec` and 215:func:`eval` functions have optional arguments to override the global and local 216namespace. If only one namespace is specified, it is used for both. 217 218 219.. _exceptions: 220 221Exceptions 222========== 223 224.. index:: single: exception 225 226.. index:: 227 single: raise an exception 228 single: handle an exception 229 single: exception handler 230 single: errors 231 single: error handling 232 233Exceptions are a means of breaking out of the normal flow of control of a code 234block in order to handle errors or other exceptional conditions. An exception 235is *raised* at the point where the error is detected; it may be *handled* by the 236surrounding code block or by any code block that directly or indirectly invoked 237the code block where the error occurred. 238 239The Python interpreter raises an exception when it detects a run-time error 240(such as division by zero). A Python program can also explicitly raise an 241exception with the :keyword:`raise` statement. Exception handlers are specified 242with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally` 243clause of such a statement can be used to specify cleanup code which does not 244handle the exception, but is executed whether an exception occurred or not in 245the preceding code. 246 247.. index:: single: termination model 248 249Python uses the "termination" model of error handling: an exception handler can 250find out what happened and continue execution at an outer level, but it cannot 251repair the cause of the error and retry the failing operation (except by 252re-entering the offending piece of code from the top). 253 254.. index:: single: SystemExit (built-in exception) 255 256When an exception is not handled at all, the interpreter terminates execution of 257the program, or returns to its interactive main loop. In either case, it prints 258a stack traceback, except when the exception is :exc:`SystemExit`. 259 260Exceptions are identified by class instances. The :keyword:`except` clause is 261selected depending on the class of the instance: it must reference the class of 262the instance or a base class thereof. The instance can be received by the 263handler and can carry additional information about the exceptional condition. 264 265.. note:: 266 267 Exception messages are not part of the Python API. Their contents may change 268 from one version of Python to the next without warning and should not be 269 relied on by code which will run under multiple versions of the interpreter. 270 271See also the description of the :keyword:`try` statement in section :ref:`try` 272and :keyword:`raise` statement in section :ref:`raise`. 273 274 275.. rubric:: Footnotes 276 277.. [#] This limitation occurs because the code that is executed by these operations 278 is not available at the time the module is compiled. 279