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 <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>
<dl class="field-list simple">
<dt class="field-odd">Author</dt>
<dd class="field-odd">Moshe Zadka
</dd>
</dl>
This document is placed in the public domain.
<div class="topic">
Abstract
This document can be considered a companion to the tutorial. It shows how to use
Python, and even more importantly, how not to use Python.
</div>
<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>
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.
<section id="from-module-import">
<h3>from module import *<a class="headerlink" href="#from-module-import" title="Permalink to this headline">¶</a></h3>
<section id="inside-function-definitions">
<h4>Inside Function Definitions<a class="headerlink" href="#inside-function-definitions" title="Permalink to this headline">¶</a></h4>
<code class="docutils literal notranslate">from module import *</code> is invalid 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.
</section>
<section id="at-module-level">
<h4>At Module Level<a class="headerlink" href="#at-module-level" title="Permalink to this headline">¶</a></h4>
While it is valid to use <code class="docutils literal notranslate">from module import *</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.
One of the most awful questions asked on the newsgroup is why this code:
<div class="highlight-default notranslate"><div class="highlight"><pre>f = open(&quot;www&quot;)
f.read()
</pre></div>
</div>
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">from
os import *</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">os</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">open()</code></a> which returns an integer. While it is very useful, shadowing a
builtin is one of its least useful properties.
Remember, you can never know for sure what names a module exports, so either
take what you need — <code class="docutils literal notranslate">from module import name1, name2</code>, or keep them in the
module and access on a per-need basis — <code class="docutils literal notranslate">import module;print module.name</code>.
</section>
<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>
There are situations in which <code class="docutils literal notranslate">from module import *</code> is just fine:
<ul class="simple">
<li>The interactive prompt. For example, <code class="docutils literal notranslate">from math import *</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">from import *</code> safe.</li>
</ul>
</section>
</section>
<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">exec</code></a>, <a class="reference internal" href="../library/functions.html#execfile" title="execfile"><code class="xref py py-func docutils literal notranslate">execfile()</code></a> and friends<a class="headerlink" href="#unadorned-exec-execfile-and-friends" title="Permalink to this headline">¶</a></h3>
The word “unadorned” refers to the use without an explicit dictionary, in which
case those constructs evaluate code in the current environment. This is
dangerous for the same reasons <code class="docutils literal notranslate">from import *</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.
Bad examples:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; for name in sys.argv[1:]:
&gt;&gt;&gt; exec &quot;%s=1&quot; % name
&gt;&gt;&gt; def func(s, **kw):
&gt;&gt;&gt; for var, val in kw.items():
&gt;&gt;&gt; exec &quot;s.%s=val&quot; % var # invalid!
&gt;&gt;&gt; execfile(&quot;handler.py&quot;)
&gt;&gt;&gt; handle()
</pre></div>
</div>
Good examples:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; d = {}
&gt;&gt;&gt; for name in sys.argv[1:]:
&gt;&gt;&gt; d[name] = 1
&gt;&gt;&gt; def func(s, **kw):
&gt;&gt;&gt; for var, val in kw.items():
&gt;&gt;&gt; setattr(s, var, val)
&gt;&gt;&gt; d={}
&gt;&gt;&gt; execfile(&quot;handle.py&quot;, d, d)
&gt;&gt;&gt; handle = d[&#39;handle&#39;]
&gt;&gt;&gt; handle()
</pre></div>
</div>
</section>
<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>
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.
Bad example:
<div class="highlight-default notranslate"><div class="highlight"><pre># foo.py
a = 1

# bar.py
from foo import a
if something():
 a = 2 # danger: foo.a != a
</pre></div>
</div>
Good example:
<div class="highlight-default notranslate"><div class="highlight"><pre># foo.py
a = 1

# bar.py
import foo
if something():
 foo.a = 2
</pre></div>
</div>
</section>
<section id="except">
<h3>except:<a class="headerlink" href="#except" title="Permalink to this headline">¶</a></h3>
Python has the <code class="docutils literal notranslate">except:</code> clause, which catches all exceptions. Since every
error in Python raises an exception, using <code class="docutils literal notranslate">except:</code> can make many
programming errors look like runtime problems, which hinders the debugging
process.
The following code shows a great example of why this is bad:
<div class="highlight-default notranslate"><div class="highlight"><pre>try:
 foo = opne(&quot;file&quot;) # misspelled &quot;open&quot;
except:
 sys.exit(&quot;could not open file!&quot;)
</pre></div>
</div>
The second line triggers a <code class="xref py py-exc docutils literal notranslate">NameError</code>, 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">&quot;file&quot;</code> when in fact
the real error has nothing to do with <code class="docutils literal notranslate">&quot;file&quot;</code>.
A better way to write the above is
<div class="highlight-default notranslate"><div class="highlight"><pre>try:
 foo = opne(&quot;file&quot;)
except IOError:
 sys.exit(&quot;could not open file&quot;)
</pre></div>
</div>
When this is run, Python will produce a traceback showing the <code class="xref py py-exc docutils literal notranslate">NameError</code>,
and it will be immediately apparent what needs to be fixed.
Because <code class="docutils literal notranslate">except:</code> catches all exceptions, including <code class="xref py py-exc docutils literal notranslate">SystemExit</code>,
<code class="xref py py-exc docutils literal notranslate">KeyboardInterrupt</code>, and <code class="xref py py-exc docutils literal notranslate">GeneratorExit</code> (which is not an error and
should not normally be caught by user code), using a bare <code class="docutils literal notranslate">except:</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, <code class="xref py py-exc docutils literal notranslate">Exception</code>. Unfortunately in Python 2.x it is
possible for third-party code to raise exceptions that do not inherit from
<code class="xref py py-exc docutils literal notranslate">Exception</code>, so in Python 2.x there are some cases where you may have to
use a bare <code class="docutils literal notranslate">except:</code> and manually re-raise the exceptions you don’t want
to catch.
</section>
</section>
<section id="exceptions">
<h2>Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2>
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.
The following is a very popular anti-idiom
<div class="highlight-default notranslate"><div class="highlight"><pre>def get_status(file):
 if not os.path.exists(file):
 print &quot;file not found&quot;
 sys.exit(1)
 return open(file).readline()
</pre></div>
</div>
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">os.path.exists()</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">open()</code></a> is called. In that
case the last line will raise an <code class="xref py py-exc docutils literal notranslate">IOError</code>. The same thing would happen
if file 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
<code class="xref py py-exc docutils literal notranslate">IOError</code> (or perhaps some other <code class="xref py py-exc docutils literal notranslate">EnvironmentError</code>) escapes to the
user, who gets to watch the ugly traceback.
Here is a somewhat better way to do it.
<div class="highlight-default notranslate"><div class="highlight"><pre>def get_status(file):
 try:
 return open(file).readline()
 except EnvironmentError as err:
 print &quot;Unable to open file: {}&quot;.format(err)
 sys.exit(1)
</pre></div>
</div>
In this version, either 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.
However, even this version of <code class="xref py py-func docutils literal notranslate">get_status()</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
<div class="highlight-default notranslate"><div class="highlight"><pre>try:
 status = get_status(log)
except SystemExit:
 status = None
</pre></div>
</div>
But there is a better way. You should try to use as few <code class="docutils literal notranslate">except</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.
So, an even better version of <code class="xref py py-func docutils literal notranslate">get_status()</code> is probably
<div class="highlight-default notranslate"><div class="highlight"><pre>def get_status(file):
 return open(file).readline()
</pre></div>
</div>
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 its
caller.
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.
The best version of this function uses the <code class="docutils literal notranslate">open()</code> call as a context
manager, which will ensure that the file gets closed as soon as the
function returns:
<div class="highlight-default notranslate"><div class="highlight"><pre>def get_status(file):
 with open(file) as fp:
 return fp.readline()
</pre></div>
</div>
</section>
<section id="using-the-batteries">
<h2>Using the Batteries<a class="headerlink" href="#using-the-batteries" title="Permalink to this headline">¶</a></h2>
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.
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">os.path</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.
Compare:
<div class="highlight-default notranslate"><div class="highlight"><pre># ugh!
return dir+&quot;/&quot;+file
# better
return os.path.join(dir, file)
</pre></div>
</div>
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">os.path</code></a>: <code class="xref py py-func docutils literal notranslate">basename()</code>, <code class="xref py py-func docutils literal notranslate">dirname()</code> and
<code class="xref py py-func docutils literal notranslate">splitext()</code>.
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">min()</code></a> and <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate">max()</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">max()</code></a>/<a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate">min()</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">reduce()</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:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; n = 4
&gt;&gt;&gt; import operator
&gt;&gt;&gt; reduce(operator.mul, range(1, n+1))
24
</pre></div>
</div>
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">float()</code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate">int()</code></a> and
<a class="reference internal" href="../library/functions.html#long" title="long"><code class="xref py py-func docutils literal notranslate">long()</code></a> all accept string arguments and will reject ill-formed strings
by raising an <code class="xref py py-exc docutils literal notranslate">ValueError</code>.
</section>
<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>
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:
<div class="highlight-default notranslate"><div class="highlight"><pre>if foo.bar()[&#39;first&#39;][0] == baz.quux(1, 2)[5:9] and \
 calculate_number(10, 20) != forbulate(500, 360):
 pass
</pre></div>
</div>
You should realize that this is dangerous: a stray space after the <code class="docutils literal notranslate">\</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:
<div class="highlight-default notranslate"><div class="highlight"><pre>value = foo.bar()[&#39;first&#39;][0]*baz.quux(1, 2)[5:9] \
 + calculate_number(10, 20)*forbulate(500, 360)
</pre></div>
</div>
then it would just be subtly wrong.
It is usually much better to use the implicit continuation inside parenthesis:
This version is bulletproof:
<div class="highlight-default notranslate"><div class="highlight"><pre>value = (foo.bar()[&#39;first&#39;][0]*baz.quux(1, 2)[5:9]
 + calculate_number(10, 20)*forbulate(500, 360))
</pre></div>
</div>
</section>
</section>

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 <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="xref std std-keyword docutils literal notranslate">exec</code>, <code class="xref py py-func docutils literal notranslate">execfile()</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>

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