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53 | <H1><A NAME="SECTION000600000000000000000"> | |
54 | 5 PEP 318: Decorators for Functions and Methods</A> | |
55 | </H1> | |
56 | ||
57 | <P> | |
58 | Python 2.2 extended Python's object model by adding static methods and | |
59 | class methods, but it didn't extend Python's syntax to provide any new | |
60 | way of defining static or class methods. Instead, you had to write a | |
61 | <tt class="keyword">def</tt> statement in the usual way, and pass the resulting | |
62 | method to a <tt class="function">staticmethod()</tt> or <tt class="function">classmethod()</tt> | |
63 | function that would wrap up the function as a method of the new type. | |
64 | Your code would look like this: | |
65 | ||
66 | <P> | |
67 | <div class="verbatim"><pre> | |
68 | class C: | |
69 | def meth (cls): | |
70 | ... | |
71 | ||
72 | meth = classmethod(meth) # Rebind name to wrapped-up class method | |
73 | </pre></div> | |
74 | ||
75 | <P> | |
76 | If the method was very long, it would be easy to miss or forget the | |
77 | <tt class="function">classmethod()</tt> invocation after the function body. | |
78 | ||
79 | <P> | |
80 | The intention was always to add some syntax to make such definitions | |
81 | more readable, but at the time of 2.2's release a good syntax was not | |
82 | obvious. Today a good syntax <em>still</em> isn't obvious but users are | |
83 | asking for easier access to the feature; a new syntactic feature has | |
84 | been added to meet this need. | |
85 | ||
86 | <P> | |
87 | The new feature is called ``function decorators''. The name comes | |
88 | from the idea that <tt class="function">classmethod</tt>, <tt class="function">staticmethod</tt>, | |
89 | and friends are storing additional information on a function object; | |
90 | they're <em>decorating</em> functions with more details. | |
91 | ||
92 | <P> | |
93 | The notation borrows from Java and uses the "<tt class="character">@</tt>" character as an | |
94 | indicator. Using the new syntax, the example above would be written: | |
95 | ||
96 | <P> | |
97 | <div class="verbatim"><pre> | |
98 | class C: | |
99 | ||
100 | @classmethod | |
101 | def meth (cls): | |
102 | ... | |
103 | </pre></div> | |
104 | ||
105 | <P> | |
106 | The <code>@classmethod</code> is shorthand for the | |
107 | <code>meth=classmethod(meth)</code> assignment. More generally, if you have | |
108 | the following: | |
109 | ||
110 | <P> | |
111 | <div class="verbatim"><pre> | |
112 | @A @B @C | |
113 | def f (): | |
114 | ... | |
115 | </pre></div> | |
116 | ||
117 | <P> | |
118 | It's equivalent to the following pre-decorator code: | |
119 | ||
120 | <P> | |
121 | <div class="verbatim"><pre> | |
122 | def f(): ... | |
123 | f = A(B(C(f))) | |
124 | </pre></div> | |
125 | ||
126 | <P> | |
127 | Decorators must come on the line before a function definition, and | |
128 | can't be on the same line, meaning that <code>@A def f(): ...</code> is | |
129 | illegal. You can only decorate function definitions, either at the | |
130 | module level or inside a class; you can't decorate class definitions. | |
131 | ||
132 | <P> | |
133 | A decorator is just a function that takes the function to be decorated | |
134 | as an argument and returns either the same function or some new | |
135 | callable thing. It's easy to write your own decorators. The | |
136 | following simple example just sets an attribute on the function | |
137 | object: | |
138 | ||
139 | <P> | |
140 | <div class="verbatim"><pre> | |
141 | >>> def deco(func): | |
142 | ... func.attr = 'decorated' | |
143 | ... return func | |
144 | ... | |
145 | >>> @deco | |
146 | ... def f(): pass | |
147 | ... | |
148 | >>> f | |
149 | <function f at 0x402ef0d4> | |
150 | >>> f.attr | |
151 | 'decorated' | |
152 | >>> | |
153 | </pre></div> | |
154 | ||
155 | <P> | |
156 | As a slightly more realistic example, the following decorator checks | |
157 | that the supplied argument is an integer: | |
158 | ||
159 | <P> | |
160 | <div class="verbatim"><pre> | |
161 | def require_int (func): | |
162 | def wrapper (arg): | |
163 | assert isinstance(arg, int) | |
164 | return func(arg) | |
165 | ||
166 | return wrapper | |
167 | ||
168 | @require_int | |
169 | def p1 (arg): | |
170 | print arg | |
171 | ||
172 | @require_int | |
173 | def p2(arg): | |
174 | print arg*2 | |
175 | </pre></div> | |
176 | ||
177 | <P> | |
178 | An example in <a class="rfc" id='rfcref-1445' xml:id='rfcref-1445' | |
179 | href="http://www.python.org/peps/pep-0318.html">PEP 318</a> contains a fancier version of this idea that | |
180 | lets you both specify the required type and check the returned type. | |
181 | ||
182 | <P> | |
183 | Decorator functions can take arguments. If arguments are supplied, | |
184 | your decorator function is called with only those arguments and must | |
185 | return a new decorator function; this function must take a single | |
186 | function and return a function, as previously described. In other | |
187 | words, <code>@A @B @C(args)</code> becomes: | |
188 | ||
189 | <P> | |
190 | <div class="verbatim"><pre> | |
191 | def f(): ... | |
192 | _deco = C(args) | |
193 | f = A(B(_deco(f))) | |
194 | </pre></div> | |
195 | ||
196 | <P> | |
197 | Getting this right can be slightly brain-bending, but it's not too | |
198 | difficult. | |
199 | ||
200 | <P> | |
201 | A small related change makes the <tt class="member">func_name</tt> attribute of | |
202 | functions writable. This attribute is used to display function names | |
203 | in tracebacks, so decorators should change the name of any new | |
204 | function that's constructed and returned. | |
205 | ||
206 | <P> | |
207 | <div class="seealso"> | |
208 | <p class="heading">See Also:</p> | |
209 | ||
210 | <dl compact="compact" class="seerfc"> | |
211 | <dt><a href="http://www.python.org/peps/pep-0318.html" | |
212 | title="Decorators for Functions, Methods and Classes" | |
213 | >PEP 318, <em>Decorators for Functions, Methods and Classes</em></a> | |
214 | <dd>Written | |
215 | by Kevin D. Smith, Jim Jewett, and Skip Montanaro. Several people | |
216 | wrote patches implementing function decorators, but the one that was | |
217 | actually checked in was patch #979728, written by Mark Russell. | |
218 | </dl> | |
219 | </div> | |
220 | ||
221 | <P> | |
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