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clarify which kinds of tuples can be used as dictionary keys

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Mark Pilgrim
2009-10-06 11:31:13 -04:00
parent a7df51eab7
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@@ -490,7 +490,7 @@ AttributeError: 'tuple' object has no attribute 'remove'</samp>
<ul>
<li>Tuples are faster than lists. If you&#8217;re defining a constant set of values and all you&#8217;re ever going to do with it is iterate through it, use a tuple instead of a list.
<li>It makes your code safer if you &#8220;write-protect&#8221; data that doesn&#8217;t need to be changed. Using a tuple instead of a list is like having an implied <code>assert</code> statement that shows this data is constant, and that special thought (and a specific function) is required to override that.
<li>Some tuples can be used as dictionary keys, as you&#8217;ll see later in this chapter. (Lists can never be used as dictionary keys.)
<li>Some tuples can be used as dictionary keys (specifically, tuples that contain <i>immutable</i> values like strings, numbers, and other tuples). Lists can never be used as dictionary keys, because lists are not immutable.
</ul>
<blockquote class=note>