IterableStringOrSequence.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>The <code>for</code> statement is designed to allow you to iterate over the elements of a
sequence or other iterable object. Strings in Python are iterable, and often used as such.
However, they are also often considered, not as sequences of characters, but as atomic entities.
</p>

<p>
One source of defects in Python is mistakenly iterating over a non-iterable object such as an integer.
This sort of defect is easily detected as a <code>TypeError</code> will be raised. However, if a string
is mistakenly used as the iterable in a <code>for</code> statement, which also receives other sequences
(such as lists) then the code will iterate over the string one character at a time.
This is probably not what the programmer intended and results in errors that are hard to find. 
</p>

</overview>
<recommendation>

<p>Since this defect usually indicates a logical error, it is not possible to give a general method
for addressing the defect. However, adding a guard that checks that the iterator is not a string
could be worthwhile.
</p>

</recommendation>
<example>
<p>
In this example, the loop may iterate over <code>"Hello"</code> producing one character per line,
as well as over <code>[ "Hello", "World" ]</code>
It is likely that the programmer forgot to wrap the <code>"Hello"</code> in brackets.
</p>
<sample src="IterableStringOrSequence.py" />

</example>
<references>

<li>Python Language Reference: <a href="http://docs.python.org/reference/compound_stmts.html#the-for-statement">The for statement</a>,
 <a href="http://docs.python.org/2.7/reference/datamodel.html#object.__iter__">object.__iter__</a>.</li>
<li>Python Standard Library: <a href="http://docs.python.org/dev/library/stdtypes.html#iterator-types">Iterator types</a>.</li>
<li>Scipy lecture notes: <a href="http://scipy-lectures.github.io/advanced/advanced_python/#iterators">Iterators,
generator expressions and generators</a>.</li>


</references>
</qhelp>