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#5144: document that PySys_SetArgv prepends the script directory (or the empty string) to sys.path.
author Georg Brandl <georg@python.org>
date Thu, 05 Feb 2009 10:37:07 +0000
parents 92d00f5f3c0d
children f3b26a729e82
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2 .. _execmodel:
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4 ***************
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5 Execution model
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6 ***************
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8 .. index:: single: execution model
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11 .. _naming:
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13 Naming and binding
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14 ==================
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16 .. index::
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17 pair: code; block
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18 single: namespace
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19 single: scope
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21 .. index::
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22 single: name
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23 pair: binding; name
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24
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25 :dfn:`Names` refer to objects. Names are introduced by name binding operations.
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26 Each occurrence of a name in the program text refers to the :dfn:`binding` of
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27 that name established in the innermost function block containing the use.
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29 .. index:: single: block
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30
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31 A :dfn:`block` is a piece of Python program text that is executed as a unit.
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32 The following are blocks: a module, a function body, and a class definition.
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33 Each command typed interactively is a block. A script file (a file given as
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34 standard input to the interpreter or specified on the interpreter command line
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35 the first argument) is a code block. A script command (a command specified on
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36 the interpreter command line with the '**-c**' option) is a code block. The
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37 file read by the built-in function :func:`execfile` is a code block. The string
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38 argument passed to the built-in function :func:`eval` and to the :keyword:`exec`
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39 statement is a code block. The expression read and evaluated by the built-in
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40 function :func:`input` is a code block.
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41
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42 .. index:: pair: execution; frame
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43
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44 A code block is executed in an :dfn:`execution frame`. A frame contains some
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45 administrative information (used for debugging) and determines where and how
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46 execution continues after the code block's execution has completed.
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48 .. index:: single: scope
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50 A :dfn:`scope` defines the visibility of a name within a block. If a local
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51 variable is defined in a block, its scope includes that block. If the
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52 definition occurs in a function block, the scope extends to any blocks contained
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53 within the defining one, unless a contained block introduces a different binding
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54 for the name. The scope of names defined in a class block is limited to the
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55 class block; it does not extend to the code blocks of methods -- this includes
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56 generator expressions since they are implemented using a function scope. This
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57 means that the following will fail::
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58
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59 class A:
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60 a = 42
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61 b = list(a + i for i in range(10))
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63 .. index:: single: environment
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64
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65 When a name is used in a code block, it is resolved using the nearest enclosing
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66 scope. The set of all such scopes visible to a code block is called the block's
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67 :dfn:`environment`.
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68
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69 .. index:: pair: free; variable
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71 If a name is bound in a block, it is a local variable of that block. If a name
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72 is bound at the module level, it is a global variable. (The variables of the
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73 module code block are local and global.) If a variable is used in a code block
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74 but not defined there, it is a :dfn:`free variable`.
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76 .. index::
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77 single: NameError (built-in exception)
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78 single: UnboundLocalError
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79
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80 When a name is not found at all, a :exc:`NameError` exception is raised. If the
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81 name refers to a local variable that has not been bound, a
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82 :exc:`UnboundLocalError` exception is raised. :exc:`UnboundLocalError` is a
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83 subclass of :exc:`NameError`.
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84
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85 .. index:: statement: from
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86
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87 The following constructs bind names: formal parameters to functions,
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88 :keyword:`import` statements, class and function definitions (these bind the
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89 class or function name in the defining block), and targets that are identifiers
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90 if occurring in an assignment, :keyword:`for` loop header, or in the second
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91 position of an :keyword:`except` clause header. The :keyword:`import` statement
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92 of the form "``from ...import *``" binds all names defined in the imported
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93 module, except those beginning with an underscore. This form may only be used
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94 at the module level.
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95
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96 A target occurring in a :keyword:`del` statement is also considered bound for
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97 this purpose (though the actual semantics are to unbind the name). It is
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98 illegal to unbind a name that is referenced by an enclosing scope; the compiler
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99 will report a :exc:`SyntaxError`.
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100
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101 Each assignment or import statement occurs within a block defined by a class or
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102 function definition or at the module level (the top-level code block).
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103
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104 If a name binding operation occurs anywhere within a code block, all uses of the
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105 name within the block are treated as references to the current block. This can
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106 lead to errors when a name is used within a block before it is bound. This rule
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107 is subtle. Python lacks declarations and allows name binding operations to
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108 occur anywhere within a code block. The local variables of a code block can be
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109 determined by scanning the entire text of the block for name binding operations.
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110
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111 If the global statement occurs within a block, all uses of the name specified in
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112 the statement refer to the binding of that name in the top-level namespace.
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113 Names are resolved in the top-level namespace by searching the global namespace,
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114 i.e. the namespace of the module containing the code block, and the builtin
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115 namespace, the namespace of the module :mod:`__builtin__`. The global namespace
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116 is searched first. If the name is not found there, the builtin namespace is
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117 searched. The global statement must precede all uses of the name.
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118
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119 .. index:: pair: restricted; execution
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120
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121 The built-in namespace associated with the execution of a code block is actually
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122 found by looking up the name ``__builtins__`` in its global namespace; this
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123 should be a dictionary or a module (in the latter case the module's dictionary
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124 is used). By default, when in the :mod:`__main__` module, ``__builtins__`` is
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125 the built-in module :mod:`__builtin__` (note: no 's'); when in any other module,
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126 ``__builtins__`` is an alias for the dictionary of the :mod:`__builtin__` module
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127 itself. ``__builtins__`` can be set to a user-created dictionary to create a
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128 weak form of restricted execution.
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129
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130 .. note::
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131
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132 Users should not touch ``__builtins__``; it is strictly an implementation
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133 detail. Users wanting to override values in the built-in namespace should
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134 :keyword:`import` the :mod:`__builtin__` (no 's') module and modify its
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135 attributes appropriately.
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136
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137 .. index:: module: __main__
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138
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139 The namespace for a module is automatically created the first time a module is
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140 imported. The main module for a script is always called :mod:`__main__`.
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141
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142 The global statement has the same scope as a name binding operation in the same
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143 block. If the nearest enclosing scope for a free variable contains a global
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144 statement, the free variable is treated as a global.
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145
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146 A class definition is an executable statement that may use and define names.
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147 These references follow the normal rules for name resolution. The namespace of
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148 the class definition becomes the attribute dictionary of the class. Names
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149 defined at the class scope are not visible in methods.
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150
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151
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152 .. _dynamic-features:
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153
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154 Interaction with dynamic features
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155 ---------------------------------
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156
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157 There are several cases where Python statements are illegal when used in
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158 conjunction with nested scopes that contain free variables.
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159
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160 If a variable is referenced in an enclosing scope, it is illegal to delete the
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161 name. An error will be reported at compile time.
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162
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163 If the wild card form of import --- ``import *`` --- is used in a function and
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164 the function contains or is a nested block with free variables, the compiler
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165 will raise a :exc:`SyntaxError`.
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166
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167 If :keyword:`exec` is used in a function and the function contains or is a
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168 nested block with free variables, the compiler will raise a :exc:`SyntaxError`
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169 unless the exec explicitly specifies the local namespace for the
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170 :keyword:`exec`. (In other words, ``exec obj`` would be illegal, but ``exec obj
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171 in ns`` would be legal.)
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172
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173 The :func:`eval`, :func:`execfile`, and :func:`input` functions and the
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174 :keyword:`exec` statement do not have access to the full environment for
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175 resolving names. Names may be resolved in the local and global namespaces of
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176 the caller. Free variables are not resolved in the nearest enclosing namespace,
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177 but in the global namespace. [#]_ The :keyword:`exec` statement and the
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178 :func:`eval` and :func:`execfile` functions have optional arguments to override
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179 the global and local namespace. If only one namespace is specified, it is used
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180 for both.
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181
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182
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183 .. _exceptions:
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184
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185 Exceptions
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186 ==========
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187
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188 .. index:: single: exception
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189
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190 .. index::
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191 single: raise an exception
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192 single: handle an exception
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193 single: exception handler
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194 single: errors
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195 single: error handling
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196
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197 Exceptions are a means of breaking out of the normal flow of control of a code
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198 block in order to handle errors or other exceptional conditions. An exception
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199 is *raised* at the point where the error is detected; it may be *handled* by the
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200 surrounding code block or by any code block that directly or indirectly invoked
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201 the code block where the error occurred.
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202
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203 The Python interpreter raises an exception when it detects a run-time error
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204 (such as division by zero). A Python program can also explicitly raise an
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205 exception with the :keyword:`raise` statement. Exception handlers are specified
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206 with the :keyword:`try` ... :keyword:`except` statement. The :keyword:`finally`
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207 clause of such a statement can be used to specify cleanup code which does not
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208 handle the exception, but is executed whether an exception occurred or not in
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209 the preceding code.
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210
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211 .. index:: single: termination model
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212
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213 Python uses the "termination" model of error handling: an exception handler can
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214 find out what happened and continue execution at an outer level, but it cannot
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215 repair the cause of the error and retry the failing operation (except by
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216 re-entering the offending piece of code from the top).
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217
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218 .. index:: single: SystemExit (built-in exception)
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219
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220 When an exception is not handled at all, the interpreter terminates execution of
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221 the program, or returns to its interactive main loop. In either case, it prints
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222 a stack backtrace, except when the exception is :exc:`SystemExit`.
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223
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224 Exceptions are identified by class instances. The :keyword:`except` clause is
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225 selected depending on the class of the instance: it must reference the class of
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226 the instance or a base class thereof. The instance can be received by the
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227 handler and can carry additional information about the exceptional condition.
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228
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229 Exceptions can also be identified by strings, in which case the
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230 :keyword:`except` clause is selected by object identity. An arbitrary value can
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231 be raised along with the identifying string which can be passed to the handler.
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232
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233 .. warning::
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234
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235 Messages to exceptions are not part of the Python API. Their contents may
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236 change from one version of Python to the next without warning and should not be
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237 relied on by code which will run under multiple versions of the interpreter.
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238
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239 See also the description of the :keyword:`try` statement in section :ref:`try`
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240 and :keyword:`raise` statement in section :ref:`raise`.
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241
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242 .. rubric:: Footnotes
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243
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244 .. [#] This limitation occurs because the code that is executed by these operations is
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245 not available at the time the module is compiled.
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246