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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Idioms and Anti-Idioms in Python &#8212; Python 2.7.16 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.16 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="Functional Programming HOWTO" href="functional.html" /> <link rel="prev" title="Descriptor HowTo Guide" href="descriptor.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/2/howto/doanddont.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="functional.html" title="Functional Programming HOWTO" accesskey="N">next</a> |</li> <li class="right" > <a href="descriptor.html" title="Descriptor HowTo Guide" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> &#187;</li> <li> <a href="../index.html">Python 2.7.16 documentation</a> &#187; </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Python HOWTOs</a> &#187;</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="idioms-and-anti-idioms-in-python"> <h1>Idioms and Anti-Idioms in Python<a class="headerlink" href="#idioms-and-anti-idioms-in-python" title="Permalink to this headline">¶</a></h1> <table class="docutils field-list" frame="void" rules="none"> <col class="field-name" /> <col class="field-body" /> <tbody valign="top"> <tr class="field-odd field"><th class="field-name">Author:</th><td class="field-body">Moshe Zadka</td> </tr> </tbody> </table> <p>This document is placed in the public domain.</p> <div class="topic"> <p class="topic-title first">Abstract</p> <p>This document can be considered a companion to the tutorial. It shows how to use Python, and even more importantly, how <em>not</em> to use Python.</p> </div> <div class="section" id="language-constructs-you-should-not-use"> <h2>Language Constructs You Should Not Use<a class="headerlink" href="#language-constructs-you-should-not-use" title="Permalink to this headline">¶</a></h2> <p>While Python has relatively few gotchas compared to other languages, it still has some constructs which are only useful in corner cases, or are plain dangerous.</p> <div class="section" id="from-module-import"> <h3>from module import *<a class="headerlink" href="#from-module-import" title="Permalink to this headline">¶</a></h3> <div class="section" id="inside-function-definitions"> <h4>Inside Function Definitions<a class="headerlink" href="#inside-function-definitions" title="Permalink to this headline">¶</a></h4> <p><code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span> <span class="pre">*</span></code> is <em>invalid</em> inside function definitions. While many versions of Python do not check for the invalidity, it does not make it more valid, no more than having a smart lawyer makes a man innocent. Do not use it like that ever. Even in versions where it was accepted, it made the function execution slower, because the compiler could not be certain which names were local and which were global. In Python 2.1 this construct causes warnings, and sometimes even errors.</p> </div> <div class="section" id="at-module-level"> <h4>At Module Level<a class="headerlink" href="#at-module-level" title="Permalink to this headline">¶</a></h4> <p>While it is valid to use <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span> <span class="pre">*</span></code> at module level it is usually a bad idea. For one, this loses an important property Python otherwise has — you can know where each toplevel name is defined by a simple “search” function in your favourite editor. You also open yourself to trouble in the future, if some module grows additional functions or classes.</p> <p>One of the most awful questions asked on the newsgroup is why this code:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s2">&quot;www&quot;</span><span class="p">)</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">()</span> </pre></div> </div> <p>does not work. Of course, it works just fine (assuming you have a file called “www”.) But it does not work if somewhere in the module, the statement <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">os</span> <span class="pre">import</span> <span class="pre">*</span></code> is present. The <a class="reference internal" href="../library/os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module has a function called <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> which returns an integer. While it is very useful, shadowing a builtin is one of its least useful properties.</p> <p>Remember, you can never know for sure what names a module exports, so either take what you need — <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span> <span class="pre">name1,</span> <span class="pre">name2</span></code>, or keep them in the module and access on a per-need basis — <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">module;print</span> <span class="pre">module.name</span></code>.</p> </div> <div class="section" id="when-it-is-just-fine"> <h4>When It Is Just Fine<a class="headerlink" href="#when-it-is-just-fine" title="Permalink to this headline">¶</a></h4> <p>There are situations in which <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span> <span class="pre">*</span></code> is just fine:</p> <ul class="simple"> <li>The interactive prompt. For example, <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">math</span> <span class="pre">import</span> <span class="pre">*</span></code> makes Python an amazing scientific calculator.</li> <li>When extending a module in C with a module in Python.</li> <li>When the module advertises itself as <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">import</span> <span class="pre">*</span></code> safe.</li> </ul> </div> </div> <div class="section" id="unadorned-exec-execfile-and-friends"> <h3>Unadorned <a class="reference internal" href="../reference/simple_stmts.html#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a>, <a class="reference internal" href="../library/functions.html#execfile" title="execfile"><code class="xref py py-func docutils literal notranslate"><span class="pre">execfile()</span></code></a> and friends<a class="headerlink" href="#unadorned-exec-execfile-and-friends" title="Permalink to this headline">¶</a></h3> <p>The word “unadorned” refers to the use without an explicit dictionary, in which case those constructs evaluate code in the <em>current</em> environment. This is dangerous for the same reasons <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">import</span> <span class="pre">*</span></code> is dangerous — it might step over variables you are counting on and mess up things for the rest of your code. Simply do not do that.</p> <p>Bad examples:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">for</span> <span class="n">name</span> <span class="ow">in</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">:]:</span> <span class="gp">&gt;&gt;&gt; </span> <span class="n">exec</span> <span class="s2">&quot;</span><span class="si">%s</span><span class="s2">=1&quot;</span> <span class="o">%</span> <span class="n">name</span> <span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="o">**</span><span class="n">kw</span><span class="p">):</span> <span class="gp">&gt;&gt;&gt; </span> <span class="k">for</span> <span class="n">var</span><span class="p">,</span> <span class="n">val</span> <span class="ow">in</span> <span class="n">kw</span><span class="o">.</span><span class="n">items</span><span class="p">():</span> <span class="gp">&gt;&gt;&gt; </span> <span class="n">exec</span> <span class="s2">&quot;s.</span><span class="si">%s</span><span class="s2">=val&quot;</span> <span class="o">%</span> <span class="n">var</span> <span class="c1"># invalid!</span> <span class="gp">&gt;&gt;&gt; </span><span class="n">execfile</span><span class="p">(</span><span class="s2">&quot;handler.py&quot;</span><span class="p">)</span> <span class="gp">&gt;&gt;&gt; </span><span class="n">handle</span><span class="p">()</span> </pre></div> </div> <p>Good examples:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">d</span> <span class="o">=</span> <span class="p">{}</span> <span class="gp">&gt;&gt;&gt; </span><span class="k">for</span> <span class="n">name</span> <span class="ow">in</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">:]:</span> <span class="gp">&gt;&gt;&gt; </span> <span class="n">d</span><span class="p">[</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span> <span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="o">**</span><span class="n">kw</span><span class="p">):</span> <span class="gp">&gt;&gt;&gt; </span> <span class="k">for</span> <span class="n">var</span><span class="p">,</span> <span class="n">val</span> <span class="ow">in</span> <span class="n">kw</span><span class="o">.</span><span class="n">items</span><span class="p">():</span> <span class="gp">&gt;&gt;&gt; </span> <span class="nb">setattr</span><span class="p">(</span><span class="n">s</span><span class="p">,</span> <span class="n">var</span><span class="p">,</span> <span class="n">val</span><span class="p">)</span> <span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="o">=</span><span class="p">{}</span> <span class="gp">&gt;&gt;&gt; </span><span class="n">execfile</span><span class="p">(</span><span class="s2">&quot;handle.py&quot;</span><span class="p">,</span> <span class="n">d</span><span class="p">,</span> <span class="n">d</span><span class="p">)</span> <span class="gp">&gt;&gt;&gt; </span><span class="n">handle</span> <span class="o">=</span> <span class="n">d</span><span class="p">[</span><span class="s1">&#39;handle&#39;</span><span class="p">]</span> <span class="gp">&gt;&gt;&gt; </span><span class="n">handle</span><span class="p">()</span> </pre></div> </div> </div> <div class="section" id="from-module-import-name1-name2"> <h3>from module import name1, name2<a class="headerlink" href="#from-module-import-name1-name2" title="Permalink to this headline">¶</a></h3> <p>This is a “don’t” which is much weaker than the previous “don’t”s but is still something you should not do if you don’t have good reasons to do that. The reason it is usually a bad idea is because you suddenly have an object which lives in two separate namespaces. When the binding in one namespace changes, the binding in the other will not, so there will be a discrepancy between them. This happens when, for example, one module is reloaded, or changes the definition of a function at runtime.</p> <p>Bad example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># foo.py</span> <span class="n">a</span> <span class="o">=</span> <span class="mi">1</span> <span class="c1"># bar.py</span> <span class="kn">from</span> <span class="nn">foo</span> <span class="k">import</span> <span class="n">a</span> <span class="k">if</span> <span class="n">something</span><span class="p">():</span> <span class="n">a</span> <span class="o">=</span> <span class="mi">2</span> <span class="c1"># danger: foo.a != a</span> </pre></div> </div> <p>Good example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># foo.py</span> <span class="n">a</span> <span class="o">=</span> <span class="mi">1</span> <span class="c1"># bar.py</span> <span class="kn">import</span> <span class="nn">foo</span> <span class="k">if</span> <span class="n">something</span><span class="p">():</span> <span class="n">foo</span><span class="o">.</span><span class="n">a</span> <span class="o">=</span> <span class="mi">2</span> </pre></div> </div> </div> <div class="section" id="except"> <h3>except:<a class="headerlink" href="#except" title="Permalink to this headline">¶</a></h3> <p>Python has the <code class="docutils literal notranslate"><span class="pre">except:</span></code> clause, which catches all exceptions. Since <em>every</em> error in Python raises an exception, using <code class="docutils literal notranslate"><span class="pre">except:</span></code> can make many programming errors look like runtime problems, which hinders the debugging process.</p> <p>The following code shows a great example of why this is bad:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="n">foo</span> <span class="o">=</span> <span class="n">opne</span><span class="p">(</span><span class="s2">&quot;file&quot;</span><span class="p">)</span> <span class="c1"># misspelled &quot;open&quot;</span> <span class="k">except</span><span class="p">:</span> <span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="s2">&quot;could not open file!&quot;</span><span class="p">)</span> </pre></div> </div> <p>The second line triggers a <a class="reference internal" href="../library/exceptions.html#exceptions.NameError" title="exceptions.NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a>, which is caught by the except clause. The program will exit, and the error message the program prints will make you think the problem is the readability of <code class="docutils literal notranslate"><span class="pre">&quot;file&quot;</span></code> when in fact the real error has nothing to do with <code class="docutils literal notranslate"><span class="pre">&quot;file&quot;</span></code>.</p> <p>A better way to write the above is</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="n">foo</span> <span class="o">=</span> <span class="n">opne</span><span class="p">(</span><span class="s2">&quot;file&quot;</span><span class="p">)</span> <span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span> <span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="s2">&quot;could not open file&quot;</span><span class="p">)</span> </pre></div> </div> <p>When this is run, Python will produce a traceback showing the <a class="reference internal" href="../library/exceptions.html#exceptions.NameError" title="exceptions.NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a>, and it will be immediately apparent what needs to be fixed.</p> <p id="index-0">Because <code class="docutils literal notranslate"><span class="pre">except:</span></code> catches <em>all</em> exceptions, including <a class="reference internal" href="../library/exceptions.html#exceptions.SystemExit" title="exceptions.SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a>, <a class="reference internal" href="../library/exceptions.html#exceptions.KeyboardInterrupt" title="exceptions.KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a>, and <a class="reference internal" href="../library/exceptions.html#exceptions.GeneratorExit" title="exceptions.GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> (which is not an error and should not normally be caught by user code), using a bare <code class="docutils literal notranslate"><span class="pre">except:</span></code> is almost never a good idea. In situations where you need to catch all “normal” errors, such as in a framework that runs callbacks, you can catch the base class for all normal exceptions, <a class="reference internal" href="../library/exceptions.html#exceptions.Exception" title="exceptions.Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a>. Unfortunately in Python 2.x it is possible for third-party code to raise exceptions that do not inherit from <a class="reference internal" href="../library/exceptions.html#exceptions.Exception" title="exceptions.Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a>, so in Python 2.x there are some cases where you may have to use a bare <code class="docutils literal notranslate"><span class="pre">except:</span></code> and manually re-raise the exceptions you don’t want to catch.</p> </div> </div> <div class="section" id="exceptions"> <h2>Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2> <p>Exceptions are a useful feature of Python. You should learn to raise them whenever something unexpected occurs, and catch them only where you can do something about them.</p> <p>The following is a very popular anti-idiom</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_status</span><span class="p">(</span><span class="n">file</span><span class="p">):</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">file</span><span class="p">):</span> <span class="nb">print</span> <span class="s2">&quot;file not found&quot;</span> <span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="k">return</span> <span class="nb">open</span><span class="p">(</span><span class="n">file</span><span class="p">)</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span> </pre></div> </div> <p>Consider the case where the file gets deleted between the time the call to <a class="reference internal" href="../library/os.path.html#os.path.exists" title="os.path.exists"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.path.exists()</span></code></a> is made and the time <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> is called. In that case the last line will raise an <a class="reference internal" href="../library/exceptions.html#exceptions.IOError" title="exceptions.IOError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IOError</span></code></a>. The same thing would happen if <em>file</em> exists but has no read permission. Since testing this on a normal machine on existent and non-existent files makes it seem bugless, the test results will seem fine, and the code will get shipped. Later an unhandled <a class="reference internal" href="../library/exceptions.html#exceptions.IOError" title="exceptions.IOError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IOError</span></code></a> (or perhaps some other <a class="reference internal" href="../library/exceptions.html#exceptions.EnvironmentError" title="exceptions.EnvironmentError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">EnvironmentError</span></code></a>) escapes to the user, who gets to watch the ugly traceback.</p> <p>Here is a somewhat better way to do it.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_status</span><span class="p">(</span><span class="n">file</span><span class="p">):</span> <span class="k">try</span><span class="p">:</span> <span class="k">return</span> <span class="nb">open</span><span class="p">(</span><span class="n">file</span><span class="p">)</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span> <span class="k">except</span> <span class="ne">EnvironmentError</span> <span class="k">as</span> <span class="n">err</span><span class="p">:</span> <span class="nb">print</span> <span class="s2">&quot;Unable to open file: </span><span class="si">{}</span><span class="s2">&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">err</span><span class="p">)</span> <span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> </pre></div> </div> <p>In this version, <em>either</em> the file gets opened and the line is read (so it works even on flaky NFS or SMB connections), or an error message is printed that provides all the available information on why the open failed, and the application is aborted.</p> <p>However, even this version of <code class="xref py py-func docutils literal notranslate"><span class="pre">get_status()</span></code> makes too many assumptions — that it will only be used in a short running script, and not, say, in a long running server. Sure, the caller could do something like</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="n">status</span> <span class="o">=</span> <span class="n">get_status</span><span class="p">(</span><span class="n">log</span><span class="p">)</span> <span class="k">except</span> <span class="ne">SystemExit</span><span class="p">:</span> <span class="n">status</span> <span class="o">=</span> <span class="kc">None</span> </pre></div> </div> <p>But there is a better way. You should try to use as few <code class="docutils literal notranslate"><span class="pre">except</span></code> clauses in your code as you can — the ones you do use will usually be inside calls which should always succeed, or a catch-all in a main function.</p> <p>So, an even better version of <code class="xref py py-func docutils literal notranslate"><span class="pre">get_status()</span></code> is probably</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_status</span><span class="p">(</span><span class="n">file</span><span class="p">):</span> <span class="k">return</span> <span class="nb">open</span><span class="p">(</span><span class="n">file</span><span class="p">)</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span> </pre></div> </div> <p>The caller can deal with the exception if it wants (for example, if it tries several files in a loop), or just let the exception filter upwards to <em>its</em> caller.</p> <p>But the last version still has a serious problem — due to implementation details in CPython, the file would not be closed when an exception is raised until the exception handler finishes; and, worse, in other implementations (e.g., Jython) it might not be closed at all regardless of whether or not an exception is raised.</p> <p>The best version of this function uses the <code class="docutils literal notranslate"><span class="pre">open()</span></code> call as a context manager, which will ensure that the file gets closed as soon as the function returns:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_status</span><span class="p">(</span><span class="n">file</span><span class="p">):</span> <span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">file</span><span class="p">)</span> <span class="k">as</span> <span class="n">fp</span><span class="p">:</span> <span class="k">return</span> <span class="n">fp</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span> </pre></div> </div> </div> <div class="section" id="using-the-batteries"> <h2>Using the Batteries<a class="headerlink" href="#using-the-batteries" title="Permalink to this headline">¶</a></h2> <p>Every so often, people seem to be writing stuff in the Python library again, usually poorly. While the occasional module has a poor interface, it is usually much better to use the rich standard library and data types that come with Python than inventing your own.</p> <p>A useful module very few people know about is <a class="reference internal" href="../library/os.path.html#module-os.path" title="os.path: Operations on pathnames."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os.path</span></code></a>. It always has the correct path arithmetic for your operating system, and will usually be much better than whatever you come up with yourself.</p> <p>Compare:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># ugh!</span> <span class="k">return</span> <span class="nb">dir</span><span class="o">+</span><span class="s2">&quot;/&quot;</span><span class="o">+</span><span class="n">file</span> <span class="c1"># better</span> <span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="nb">dir</span><span class="p">,</span> <span class="n">file</span><span class="p">)</span> </pre></div> </div> <p>More useful functions in <a class="reference internal" href="../library/os.path.html#module-os.path" title="os.path: Operations on pathnames."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os.path</span></code></a>: <code class="xref py py-func docutils literal notranslate"><span class="pre">basename()</span></code>, <code class="xref py py-func docutils literal notranslate"><span class="pre">dirname()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">splitext()</span></code>.</p> <p>There are also many useful built-in functions people seem not to be aware of for some reason: <a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a> and <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a> can find the minimum/maximum of any sequence with comparable semantics, for example, yet many people write their own <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a>/<a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a>. Another highly useful function is <a class="reference internal" href="../library/functions.html#reduce" title="reduce"><code class="xref py py-func docutils literal notranslate"><span class="pre">reduce()</span></code></a> which can be used to repeatedly apply a binary operation to a sequence, reducing it to a single value. For example, compute a factorial with a series of multiply operations:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">n</span> <span class="o">=</span> <span class="mi">4</span> <span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">operator</span> <span class="gp">&gt;&gt;&gt; </span><span class="n">reduce</span><span class="p">(</span><span class="n">operator</span><span class="o">.</span><span class="n">mul</span><span class="p">,</span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="n">n</span><span class="o">+</span><span class="mi">1</span><span class="p">))</span> <span class="go">24</span> </pre></div> </div> <p>When it comes to parsing numbers, note that <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-func docutils literal notranslate"><span class="pre">float()</span></code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a> and <a class="reference internal" href="../library/functions.html#long" title="long"><code class="xref py py-func docutils literal notranslate"><span class="pre">long()</span></code></a> all accept string arguments and will reject ill-formed strings by raising an <a class="reference internal" href="../library/exceptions.html#exceptions.ValueError" title="exceptions.ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>.</p> </div> <div class="section" id="using-backslash-to-continue-statements"> <h2>Using Backslash to Continue Statements<a class="headerlink" href="#using-backslash-to-continue-statements" title="Permalink to this headline">¶</a></h2> <p>Since Python treats a newline as a statement terminator, and since statements are often more than is comfortable to put in one line, many people do:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">foo</span><span class="o">.</span><span class="n">bar</span><span class="p">()[</span><span class="s1">&#39;first&#39;</span><span class="p">][</span><span class="mi">0</span><span class="p">]</span> <span class="o">==</span> <span class="n">baz</span><span class="o">.</span><span class="n">quux</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">)[</span><span class="mi">5</span><span class="p">:</span><span class="mi">9</span><span class="p">]</span> <span class="ow">and</span> \ <span class="n">calculate_number</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">)</span> <span class="o">!=</span> <span class="n">forbulate</span><span class="p">(</span><span class="mi">500</span><span class="p">,</span> <span class="mi">360</span><span class="p">):</span> <span class="k">pass</span> </pre></div> </div> <p>You should realize that this is dangerous: a stray space after the <code class="docutils literal notranslate"><span class="pre">\</span></code> would make this line wrong, and stray spaces are notoriously hard to see in editors. In this case, at least it would be a syntax error, but if the code was:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">value</span> <span class="o">=</span> <span class="n">foo</span><span class="o">.</span><span class="n">bar</span><span class="p">()[</span><span class="s1">&#39;first&#39;</span><span class="p">][</span><span class="mi">0</span><span class="p">]</span><span class="o">*</span><span class="n">baz</span><span class="o">.</span><span class="n">quux</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">)[</span><span class="mi">5</span><span class="p">:</span><span class="mi">9</span><span class="p">]</span> \ <span class="o">+</span> <span class="n">calculate_number</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">)</span><span class="o">*</span><span class="n">forbulate</span><span class="p">(</span><span class="mi">500</span><span class="p">,</span> <span class="mi">360</span><span class="p">)</span> </pre></div> </div> <p>then it would just be subtly wrong.</p> <p>It is usually much better to use the implicit continuation inside parenthesis:</p> <p>This version is bulletproof:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">value</span> <span class="o">=</span> <span class="p">(</span><span class="n">foo</span><span class="o">.</span><span class="n">bar</span><span class="p">()[</span><span class="s1">&#39;first&#39;</span><span class="p">][</span><span class="mi">0</span><span class="p">]</span><span class="o">*</span><span class="n">baz</span><span class="o">.</span><span class="n">quux</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">)[</span><span class="mi">5</span><span class="p">:</span><span class="mi">9</span><span class="p">]</span> <span class="o">+</span> <span class="n">calculate_number</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">)</span><span class="o">*</span><span class="n">forbulate</span><span class="p">(</span><span class="mi">500</span><span class="p">,</span> <span class="mi">360</span><span class="p">))</span> </pre></div> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Idioms and Anti-Idioms in Python</a><ul> <li><a class="reference internal" href="#language-constructs-you-should-not-use">Language Constructs You Should Not Use</a><ul> <li><a class="reference internal" href="#from-module-import">from module import *</a><ul> <li><a class="reference internal" href="#inside-function-definitions">Inside Function Definitions</a></li> <li><a class="reference internal" href="#at-module-level">At Module Level</a></li> <li><a class="reference internal" href="#when-it-is-just-fine">When It Is Just Fine</a></li> </ul> </li> <li><a class="reference internal" href="#unadorned-exec-execfile-and-friends">Unadorned <code class="docutils literal notranslate"><span class="pre">exec</span></code>, <code class="docutils literal notranslate"><span class="pre">execfile()</span></code> and friends</a></li> <li><a class="reference internal" href="#from-module-import-name1-name2">from module import name1, name2</a></li> <li><a class="reference internal" href="#except">except:</a></li> </ul> </li> <li><a class="reference internal" href="#exceptions">Exceptions</a></li> <li><a class="reference internal" href="#using-the-batteries">Using the Batteries</a></li> <li><a class="reference internal" href="#using-backslash-to-continue-statements">Using Backslash to Continue Statements</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="descriptor.html" title="previous chapter">Descriptor HowTo Guide</a></p> <h4>Next topic</h4> <p class="topless"><a href="functional.html" title="next chapter">Functional Programming HOWTO</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/howto/doanddont.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" 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