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POCommon pitfalls in Python
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copied!<blockquote> <p><strong>Possible Duplicate:</strong><br> <a href="https://stackoverflow.com/questions/530530/python-2-x-gotchas-and-landmines">Python 2.x gotcha’s and landmines</a> </p> </blockquote> <p>Today I was bitten again by mutable default arguments after many years. I usually don't use mutable default arguments unless needed, but I think with time I forgot about that. Today in the application I added tocElements=[] in a PDF generation function's argument list and now "Table of Contents" gets longer and longer after each invocation of "generate pdf". :)</p> <p>What else should I add to my list of things to MUST avoid?</p> <ul> <li><p>Always import modules the same way, e.g. <code>from y import x</code> and <code>import x</code> are <a href="https://stackoverflow.com/questions/1459236/module-reimported-if-imported-from-different-path">treated as different modules</a>.</p></li> <li><p>Do not use <em>range</em> in place of lists because <code>range()</code> will become an iterator anyway, the following will fail:</p> <pre><code>myIndexList = [0, 1, 3] isListSorted = myIndexList == range(3) # will fail in 3.0 isListSorted = myIndexList == list(range(3)) # will not </code></pre> <p>Same thing can be mistakenly done with <em>xrange:</em></p> <pre><code>myIndexList == xrange(3) </code></pre></li> <li><p>Be careful catching multiple exception types:</p> <pre><code>try: raise KeyError("hmm bug") except KeyError, TypeError: print TypeError </code></pre> <p>This prints "hmm bug", though it is not a bug; it looks like we are catching exceptions of both types, but instead we are catching KeyError only as variable <em>TypeError,</em> use this instead:</p> <pre><code>try: raise KeyError("hmm bug") except (KeyError, TypeError): print TypeError </code></pre></li> </ul>

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