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, :keyword:`except* <except_star>` 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. 131See :ref:`the FAQ entry on UnboundLocalError <faq-unboundlocalerror>` 132for examples. 133 134If the :keyword:`global` statement occurs within a block, all uses of the names 135specified in the statement refer to the bindings of those names in the top-level 136namespace. Names are resolved in the top-level namespace by searching the 137global namespace, i.e. the namespace of the module containing the code block, 138and the builtins namespace, the namespace of the module :mod:`builtins`. The 139global namespace is searched first. If the names are not found there, the 140builtins namespace is searched. The :keyword:`!global` statement must precede 141all uses of the listed names. 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. 146 147.. XXX say more about "nonlocal" semantics here 148 149The :keyword:`nonlocal` statement causes corresponding names to refer 150to previously bound variables in the nearest enclosing function scope. 151:exc:`SyntaxError` is raised at compile time if the given name does not 152exist in any enclosing function scope. 153 154.. index:: pair: module; __main__ 155 156The namespace for a module is automatically created the first time a module is 157imported. The main module for a script is always called :mod:`__main__`. 158 159Class definition blocks and arguments to :func:`exec` and :func:`eval` are 160special in the context of name resolution. 161A class definition is an executable statement that may use and define names. 162These references follow the normal rules for name resolution with an exception 163that unbound local variables are looked up in the global namespace. 164The namespace of the class definition becomes the attribute dictionary of 165the class. The scope of names defined in a class block is limited to the 166class block; it does not extend to the code blocks of methods -- this includes 167comprehensions and generator expressions since they are implemented using a 168function scope. This means that the following will fail:: 169 170 class A: 171 a = 42 172 b = list(a + i for i in range(10)) 173 174.. _restrict_exec: 175 176Builtins and restricted execution 177--------------------------------- 178 179.. index:: pair: restricted; execution 180 181.. impl-detail:: 182 183 Users should not touch ``__builtins__``; it is strictly an implementation 184 detail. Users wanting to override values in the builtins namespace should 185 :keyword:`import` the :mod:`builtins` module and modify its 186 attributes appropriately. 187 188The builtins namespace associated with the execution of a code block 189is actually found by looking up the name ``__builtins__`` in its 190global namespace; this should be a dictionary or a module (in the 191latter case the module's dictionary is used). By default, when in the 192:mod:`__main__` module, ``__builtins__`` is the built-in module 193:mod:`builtins`; when in any other module, ``__builtins__`` is an 194alias for the dictionary of the :mod:`builtins` module itself. 195 196 197.. _dynamic-features: 198 199Interaction with dynamic features 200--------------------------------- 201 202Name resolution of free variables occurs at runtime, not at compile time. 203This means that the following code will print 42:: 204 205 i = 10 206 def f(): 207 print(i) 208 i = 42 209 f() 210 211.. XXX from * also invalid with relative imports (at least currently) 212 213The :func:`eval` and :func:`exec` functions do not have access to the full 214environment for resolving names. Names may be resolved in the local and global 215namespaces of the caller. Free variables are not resolved in the nearest 216enclosing namespace, but in the global namespace. [#]_ The :func:`exec` and 217:func:`eval` functions have optional arguments to override the global and local 218namespace. If only one namespace is specified, it is used for both. 219 220 221.. _exceptions: 222 223Exceptions 224========== 225 226.. index:: single: exception 227 228.. index:: 229 single: raise an exception 230 single: handle an exception 231 single: exception handler 232 single: errors 233 single: error handling 234 235Exceptions are a means of breaking out of the normal flow of control of a code 236block in order to handle errors or other exceptional conditions. An exception 237is *raised* at the point where the error is detected; it may be *handled* by the 238surrounding code block or by any code block that directly or indirectly invoked 239the code block where the error occurred. 240 241The Python interpreter raises an exception when it detects a run-time error 242(such as division by zero). A Python program can also explicitly raise an 243exception with the :keyword:`raise` statement. Exception handlers are specified 244with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally` 245clause of such a statement can be used to specify cleanup code which does not 246handle the exception, but is executed whether an exception occurred or not in 247the preceding code. 248 249.. index:: single: termination model 250 251Python uses the "termination" model of error handling: an exception handler can 252find out what happened and continue execution at an outer level, but it cannot 253repair the cause of the error and retry the failing operation (except by 254re-entering the offending piece of code from the top). 255 256.. index:: single: SystemExit (built-in exception) 257 258When an exception is not handled at all, the interpreter terminates execution of 259the program, or returns to its interactive main loop. In either case, it prints 260a stack traceback, except when the exception is :exc:`SystemExit`. 261 262Exceptions are identified by class instances. The :keyword:`except` clause is 263selected depending on the class of the instance: it must reference the class of 264the instance or a :term:`non-virtual base class <abstract base class>` thereof. 265The instance can be received by the handler and can carry additional information 266about the exceptional condition. 267 268.. note:: 269 270 Exception messages are not part of the Python API. Their contents may change 271 from one version of Python to the next without warning and should not be 272 relied on by code which will run under multiple versions of the interpreter. 273 274See also the description of the :keyword:`try` statement in section :ref:`try` 275and :keyword:`raise` statement in section :ref:`raise`. 276 277 278.. rubric:: Footnotes 279 280.. [#] This limitation occurs because the code that is executed by these operations 281 is not available at the time the module is compiled. 282