IncorrectComparisonUsingIs.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>

<overview>
<p>When you compare two values using the <code>is</code> or <code>is not</code> operator, it is the
object identities of the two values that is tested rather than their equality.
 If the class of either of the values in the comparison redefines equality then the
 <code>is</code> operator may return <code>False</code> even though the objects compare as equal.
 Equality is defined by the <code>__eq__</code> or, in Python2, <code>__cmp__</code> method.
 To compare two objects for equality, use the <code>==</code> or <code>!=</code> operator instead.</p>

</overview>
<recommendation>

<p>When you want to compare the value of two literals, use the comparison operator <code>==</code> or
<code>!=</code> in place of <code>is</code> or <code>is not</code>.</p>

<p>If the uniqueness property or performance are important then use an object that does not redefine equality.</p>

</recommendation>
<example>

<p>In the first line of the following example the programmer tests the value of <code>value</code> against
<code>DEFAULT</code> using the <code>is</code> operator. Unfortunately, this may fail when the function
is called with the string <code>"default"</code>.</p>
<p>
To function correctly, change the expression <code>value is DEFAULT</code> to <code>value == DEFAULT</code>.
Alternatively, if the uniqueness property is desirable, then change the definition of <code>DEFAULT</code> to
either of the alternatives below.
</p>

<sample src="IncorrectComparisonUsingIs.py" />


</example>
<references>

     <li>Python Standard Library: <a href="http://docs.python.org/2/library/stdtypes.html#comparisons">Comparisons</a>.</li>

</references>
</qhelp>