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POwhile (1) Vs. for while(True) -- Why is there a difference?
 

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  1. POwhile (1) Vs. for while(True) -- Why is there a difference?
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    <p>Intrigued by this question about infinite loops in perl: <a href="https://stackoverflow.com/questions/885908/while-1-vs-for-is-there-a-speed-difference">while (1) Vs. for (;;) Is there a speed difference?</a>, I decided to run a similar comparison in python. I expected that the compiler would generate the same byte code for <code>while(True): pass</code> and <code>while(1): pass</code>, but this is actually not the case in python2.7.</p> <p>The following script:</p> <pre><code>import dis def while_one(): while 1: pass def while_true(): while True: pass print("while 1") print("----------------------------") dis.dis(while_one) print("while True") print("----------------------------") dis.dis(while_true) </code></pre> <p>produces the following results:</p> <pre><code>while 1 ---------------------------- 4 0 SETUP_LOOP 3 (to 6) 5 &gt;&gt; 3 JUMP_ABSOLUTE 3 &gt;&gt; 6 LOAD_CONST 0 (None) 9 RETURN_VALUE while True ---------------------------- 8 0 SETUP_LOOP 12 (to 15) &gt;&gt; 3 LOAD_GLOBAL 0 (True) 6 JUMP_IF_FALSE 4 (to 13) 9 POP_TOP 9 10 JUMP_ABSOLUTE 3 &gt;&gt; 13 POP_TOP 14 POP_BLOCK &gt;&gt; 15 LOAD_CONST 0 (None) 18 RETURN_VALUE </code></pre> <p>Using <code>while True</code> is noticeably more complicated. Why is this?</p> <p>In other contexts, python acts as though <code>True</code> equals 1:</p> <pre><code>&gt;&gt;&gt; True == 1 True &gt;&gt;&gt; True + True 2 </code></pre> <p>Why does <code>while</code> distinguish the two?</p> <p>I noticed that python3 does evaluate the statements using identical operations:</p> <pre><code>while 1 ---------------------------- 4 0 SETUP_LOOP 3 (to 6) 5 &gt;&gt; 3 JUMP_ABSOLUTE 3 &gt;&gt; 6 LOAD_CONST 0 (None) 9 RETURN_VALUE while True ---------------------------- 8 0 SETUP_LOOP 3 (to 6) 9 &gt;&gt; 3 JUMP_ABSOLUTE 3 &gt;&gt; 6 LOAD_CONST 0 (None) 9 RETURN_VALUE </code></pre> <p>Is there a change in python3 to the way booleans are evaluated?</p>
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    1. COFYI, the parens around your conditions are not required and violate the python style guide. You should remove them.
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    2. COWorse than violating style guides, they announce to the world that you just got off the java/c/perl/etc. boat yesterday
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    3. COThank you for asking this question with such a well-written example of what was happening under the hood. I learned something as a result.
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