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53
54<H2><A NAME="SECTION005380000000000000000"></A><A NAME="coercion-rules"></A>
55<BR>
563.3.8 Coercion rules
57</H2>
58
59<P>
60This section used to document the rules for coercion. As the language
61has evolved, the coercion rules have become hard to document
62precisely; documenting what one version of one particular
63implementation does is undesirable. Instead, here are some informal
64guidelines regarding coercion. In Python 3.0, coercion will not be
65supported.
66
67<P>
68
69<UL>
70<LI><P>
71If the left operand of a % operator is a string or Unicode object, no
72coercion takes place and the string formatting operation is invoked
73instead.
74
75<P>
76</LI>
77<LI><P>
78It is no longer recommended to define a coercion operation.
79Mixed-mode operations on types that don't define coercion pass the
80original arguments to the operation.
81
82<P>
83</LI>
84<LI><P>
85New-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
87time <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>
93For most intents and purposes, an operator that returns
94<code>NotImplemented</code> is treated the same as one that is not
95implemented at all.
96
97<P>
98</LI>
99<LI><P>
100Below, <tt class="method">__op__()</tt> and <tt class="method">__rop__()</tt> are used to signify
101the generic method names corresponding to an operator;
102<tt class="method">__iop__()</tt> is used for the corresponding in-place operator. For
103example, 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
105binary operator, and <tt class="method">__iadd__()</tt> for the in-place variant.
106
107<P>
108</LI>
109<LI><P>
110For objects <var>x</var> and <var>y</var>, first <code><var>x</var>.__op__(<var>y</var>)</code>
111is 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
113implemented or returns <code>NotImplemented</code>, a <tt class="exception">TypeError</tt>
114exception is raised. But see the following exception:
115
116<P>
117</LI>
118<LI><P>
119Exception to the previous item: if the left operand is an instance of
120a built-in type or a new-style class, and the right operand is an
121instance of a proper subclass of that type or class, the right
122operand's <tt class="method">__rop__()</tt> method is tried <em>before</em> the left
123operand's <tt class="method">__op__()</tt> method. This is done so that a subclass can
124completely override binary operators. Otherwise, the left operand's
125__op__ method would always accept the right operand: when an instance
126of a given class is expected, an instance of a subclass of that class
127is always acceptable.
128
129<P>
130</LI>
131<LI><P>
132When either operand type defines a coercion, this coercion is called
133before that type's <tt class="method">__op__()</tt> or <tt class="method">__rop__()</tt> method is
134called, but no sooner. If the coercion returns an object of a
135different type for the operand whose coercion is invoked, part of the
136process is redone using the new object.
137
138<P>
139</LI>
140<LI><P>
141When an in-place operator (like `<code>+=</code>') is used, if the left
142operand implements <tt class="method">__iop__()</tt>, it is invoked without any
143coercion. 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>
149In <var>x</var><code>+</code><var>y</var>, if <var>x</var> is a sequence that implements
150sequence concatenation, sequence concatenation is invoked.
151
152<P>
153</LI>
154<LI><P>
155In <var>x</var><code>*</code><var>y</var>, if one operator is a sequence that
156implements 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>
162Rich comparisons (implemented by methods <tt class="method">__eq__()</tt> and so on)
163never use coercion. Three-way comparison (implemented by
164<tt class="method">__cmp__()</tt>) does use coercion under the same conditions as
165other binary operations use it.
166
167<P>
168</LI>
169<LI><P>
170In 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
173apparent when subclassing these types. Over time, the type
174<tt class="class">complex</tt> may be fixed to avoid coercion. All these types
175implement 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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