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copied!<p>It's faster <em>and</em> more readable to construct the object and then attach it to <code>self</code>.</p> <pre><code>class Test1(object): def __init__(self): d = {} d['a'] = 1 d['b'] = 2 d['c'] = 3 self.d = d class Test2(object): def __init__(self): self.d = {} self.d['a'] = 1 self.d['b'] = 2 self.d['c'] = 3 import dis print "Test1.__init__" dis.dis(Test1.__init__) print "Test2.__init__" dis.dis(Test2.__init__) </code></pre> <p>disassemles to:</p> <pre><code>Test1.__init__ 4 0 BUILD_MAP 0 3 STORE_FAST 1 (d) 5 6 LOAD_CONST 1 (1) 9 LOAD_FAST 1 (d) 12 LOAD_CONST 2 ('a') 15 STORE_SUBSCR 6 16 LOAD_CONST 3 (2) 19 LOAD_FAST 1 (d) 22 LOAD_CONST 4 ('b') 25 STORE_SUBSCR 7 26 LOAD_CONST 5 (3) 29 LOAD_FAST 1 (d) 32 LOAD_CONST 6 ('c') 35 STORE_SUBSCR 8 36 LOAD_FAST 1 (d) 39 LOAD_FAST 0 (self) 42 STORE_ATTR 0 (d) 45 LOAD_CONST 0 (None) 48 RETURN_VALUE Test2.__init__ 12 0 BUILD_MAP 0 3 LOAD_FAST 0 (self) 6 STORE_ATTR 0 (d) 13 9 LOAD_CONST 1 (1) 12 LOAD_FAST 0 (self) 15 LOAD_ATTR 0 (d) 18 LOAD_CONST 2 ('a') 21 STORE_SUBSCR 14 22 LOAD_CONST 3 (2) 25 LOAD_FAST 0 (self) 28 LOAD_ATTR 0 (d) 31 LOAD_CONST 4 ('b') 34 STORE_SUBSCR 15 35 LOAD_CONST 5 (3) 38 LOAD_FAST 0 (self) 41 LOAD_ATTR 0 (d) 44 LOAD_CONST 6 ('c') 47 STORE_SUBSCR 48 LOAD_CONST 0 (None) 51 RETURN_VALUE </code></pre> <p>You can see that <code>STORE_ATTR</code> only gets called once doing it the first way at the end. Doing it the other way, <code>STORE_ATTR</code> still gets called right at the beginning but now <code>LOAD_ATTR</code> gets called for every access to the dictionary. The more assignments there are, the higher the cost. Every other instruction is the same. It's still a ridiculously small cost.</p> <p>This trick can be exploited to make loops with many iterations run faster. It's not uncommon to see things like</p> <pre><code>foo = self.foo factorial = math.factorial for x in really_big_iterator: foo(factorial(x)) </code></pre> <p>another trick is to pass global functions as default arguments to a function that has a loop like that or gets called a whole bunch to save some attribute lookups: it's in the local scope which is the first one looked in.</p> <pre><code>def fast(iterators, sum=sum): for i in iterator: yield sum(i) </code></pre> <p>now sum is right in the local scope.</p>

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