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add example about trying to swap keys and values in a dictionary with mutable values

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Mark Pilgrim
2009-11-06 14:44:10 +01:00
parent 50e5f30368
commit f02cd3e406
1 changed files with 10 additions and 0 deletions
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comprehensions.html
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@@ -326,6 +326,16 @@ body{counter-reset:h1 3}
<samp class=p>>>> </samp><kbd class=pp>{value:key for key, value in a_dict.items()}</kbd>
<samp class=pp>{1: 'a', 2: 'b', 3: 'c'}</samp></pre>
<p>Of course, this only works if the values of the dictionary are immutable, like strings or tuples. If you try this with a dictionary that contains lists, it will fail most spectacularly.
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>a_dict = {'a': [1, 2, 3], 'b': 4, 'c': 5}</kbd>
<samp class=p>>>> </samp><kbd class=pp>{value:key for key, value in a_dict.items()}</kbd>
<samp class=traceback>Traceback (most recent call last):
File "&lt;stdin>", line 1, in &lt;module>
File "&lt;stdin>", line 1, in &lt;dictcomp>
TypeError: unhashable type: 'list'</samp></pre>
<p class=a>&#x2042;
<h2 id=setcomprehension>Set Comprehensions</h2>