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| 54 | <H2><A NAME="SECTION005380000000000000000"></A><A NAME="coercion-rules"></A> |
| 55 | <BR> |
| 56 | 3.3.8 Coercion rules |
| 57 | </H2> |
| 58 | |
| 59 | <P> |
| 60 | This section used to document the rules for coercion. As the language |
| 61 | has evolved, the coercion rules have become hard to document |
| 62 | precisely; documenting what one version of one particular |
| 63 | implementation does is undesirable. Instead, here are some informal |
| 64 | guidelines regarding coercion. In Python 3.0, coercion will not be |
| 65 | supported. |
| 66 | |
| 67 | <P> |
| 68 | |
| 69 | <UL> |
| 70 | <LI><P> |
| 71 | If the left operand of a % operator is a string or Unicode object, no |
| 72 | coercion takes place and the string formatting operation is invoked |
| 73 | instead. |
| 74 | |
| 75 | <P> |
| 76 | </LI> |
| 77 | <LI><P> |
| 78 | It is no longer recommended to define a coercion operation. |
| 79 | Mixed-mode operations on types that don't define coercion pass the |
| 80 | original arguments to the operation. |
| 81 | |
| 82 | <P> |
| 83 | </LI> |
| 84 | <LI><P> |
| 85 | New-style classes (those derived from <tt class="class">object</tt>) never invoke the |
| 86 | <tt class="method">__coerce__()</tt> method in response to a binary operator; the only |
| 87 | time <tt class="method">__coerce__()</tt> is invoked is when the built-in function |
| 88 | <tt class="function">coerce()</tt> is called. |
| 89 | |
| 90 | <P> |
| 91 | </LI> |
| 92 | <LI><P> |
| 93 | For most intents and purposes, an operator that returns |
| 94 | <code>NotImplemented</code> is treated the same as one that is not |
| 95 | implemented at all. |
| 96 | |
| 97 | <P> |
| 98 | </LI> |
| 99 | <LI><P> |
| 100 | Below, <tt class="method">__op__()</tt> and <tt class="method">__rop__()</tt> are used to signify |
| 101 | the generic method names corresponding to an operator; |
| 102 | <tt class="method">__iop__()</tt> is used for the corresponding in-place operator. For |
| 103 | example, for the operator `<code>+</code>', <tt class="method">__add__()</tt> and |
| 104 | <tt class="method">__radd__()</tt> are used for the left and right variant of the |
| 105 | binary operator, and <tt class="method">__iadd__()</tt> for the in-place variant. |
| 106 | |
| 107 | <P> |
| 108 | </LI> |
| 109 | <LI><P> |
| 110 | For objects <var>x</var> and <var>y</var>, first <code><var>x</var>.__op__(<var>y</var>)</code> |
| 111 | is tried. If this is not implemented or returns <code>NotImplemented</code>, |
| 112 | <code><var>y</var>.__rop__(<var>x</var>)</code> is tried. If this is also not |
| 113 | implemented or returns <code>NotImplemented</code>, a <tt class="exception">TypeError</tt> |
| 114 | exception is raised. But see the following exception: |
| 115 | |
| 116 | <P> |
| 117 | </LI> |
| 118 | <LI><P> |
| 119 | Exception to the previous item: if the left operand is an instance of |
| 120 | a built-in type or a new-style class, and the right operand is an |
| 121 | instance of a proper subclass of that type or class, the right |
| 122 | operand's <tt class="method">__rop__()</tt> method is tried <em>before</em> the left |
| 123 | operand's <tt class="method">__op__()</tt> method. This is done so that a subclass can |
| 124 | completely override binary operators. Otherwise, the left operand's |
| 125 | __op__ method would always accept the right operand: when an instance |
| 126 | of a given class is expected, an instance of a subclass of that class |
| 127 | is always acceptable. |
| 128 | |
| 129 | <P> |
| 130 | </LI> |
| 131 | <LI><P> |
| 132 | When either operand type defines a coercion, this coercion is called |
| 133 | before that type's <tt class="method">__op__()</tt> or <tt class="method">__rop__()</tt> method is |
| 134 | called, but no sooner. If the coercion returns an object of a |
| 135 | different type for the operand whose coercion is invoked, part of the |
| 136 | process is redone using the new object. |
| 137 | |
| 138 | <P> |
| 139 | </LI> |
| 140 | <LI><P> |
| 141 | When an in-place operator (like `<code>+=</code>') is used, if the left |
| 142 | operand implements <tt class="method">__iop__()</tt>, it is invoked without any |
| 143 | coercion. When the operation falls back to <tt class="method">__op__()</tt> and/or |
| 144 | <tt class="method">__rop__()</tt>, the normal coercion rules apply. |
| 145 | |
| 146 | <P> |
| 147 | </LI> |
| 148 | <LI><P> |
| 149 | In <var>x</var><code>+</code><var>y</var>, if <var>x</var> is a sequence that implements |
| 150 | sequence concatenation, sequence concatenation is invoked. |
| 151 | |
| 152 | <P> |
| 153 | </LI> |
| 154 | <LI><P> |
| 155 | In <var>x</var><code>*</code><var>y</var>, if one operator is a sequence that |
| 156 | implements sequence repetition, and the other is an integer |
| 157 | (<tt class="class">int</tt> or <tt class="class">long</tt>), sequence repetition is invoked. |
| 158 | |
| 159 | <P> |
| 160 | </LI> |
| 161 | <LI><P> |
| 162 | Rich comparisons (implemented by methods <tt class="method">__eq__()</tt> and so on) |
| 163 | never use coercion. Three-way comparison (implemented by |
| 164 | <tt class="method">__cmp__()</tt>) does use coercion under the same conditions as |
| 165 | other binary operations use it. |
| 166 | |
| 167 | <P> |
| 168 | </LI> |
| 169 | <LI><P> |
| 170 | In the current implementation, the built-in numeric types <tt class="class">int</tt>, |
| 171 | <tt class="class">long</tt> and <tt class="class">float</tt> do not use coercion; the type |
| 172 | <tt class="class">complex</tt> however does use it. The difference can become |
| 173 | apparent when subclassing these types. Over time, the type |
| 174 | <tt class="class">complex</tt> may be fixed to avoid coercion. All these types |
| 175 | implement a <tt class="method">__coerce__()</tt> method, for use by the built-in |
| 176 | <tt class="function">coerce()</tt> function. |
| 177 | |
| 178 | <P> |
| 179 | </LI> |
| 180 | </UL> |
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