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fixed #tuples-in-a-boolean-context, explain how to create a tuple of one item

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Mark Pilgrim
2009-08-05 08:47:51 -07:00
parent 26cbe2ebc7
commit 3206f2a72d
1 changed files with 21 additions and 16 deletions
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native-datatypes.html
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@@ -186,9 +186,9 @@ ZeroDivisionError: Fraction(0, 0)</samp></pre>
<pre class=screen>
<a><samp class=p>>>> </samp><kbd class=pp>def is_it_true(anything):</kbd> <span class=u>&#x2460;</span></a>
<samp class=p>... </samp><kbd class=pp> if anything:</kbd>
<samp class=p>... </samp><kbd class=pp> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd class=pp> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd class=pp> else:</kbd>
<samp class=p>... </samp><kbd class=pp> print('no, it's false')</kbd>
<samp class=p>... </samp><kbd class=pp> print("no, it's false")</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true(1)</kbd> <span class=u>&#x2461;</span></a>
<samp>yes, it's true</samp>
@@ -423,9 +423,9 @@ IndexError: pop from empty list</samp></pre>
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd class=pp> if anything:</kbd>
<samp class=p>... </samp><kbd class=pp> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd class=pp> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd class=pp> else:</kbd>
<samp class=p>... </samp><kbd class=pp> print('no, it's false')</kbd>
<samp class=p>... </samp><kbd class=pp> print("no, it's false")</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true([])</kbd> <span class=u>&#x2460;</span></a>
<samp>no, it's false</samp>
@@ -506,20 +506,25 @@ AttributeError: 'tuple' object has no attribute 'remove'</samp>
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd class=pp> if anything:</kbd>
<samp class=p>... </samp><kbd class=pp> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd class=pp> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd class=pp> else:</kbd>
<samp class=p>... </samp><kbd class=pp> print('no, it's false')</kbd>
<samp class=p>... </samp><kbd class=pp> print("no, it's false")</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true(())</kbd> <span class=u>&#x2460;</span></a>
<samp>no, it's false</samp>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true(('a'))</kbd> <span class=u>&#x2461;</span></a>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true(('a', 'b'))</kbd> <span class=u>&#x2461;</span></a>
<samp>yes, it's true</samp>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true((False))</kbd> <span class=u>&#x2462;</span></a>
<samp>yes, it's true</samp></pre>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true((False,))</kbd> <span class=u>&#x2462;</span></a>
<samp>yes, it's true</samp>
<a><samp class=p>>>> </samp><kbd class=pp>type((False))</kbd> <span class=u>&#x2463;</span></a>
<samp class=pp>&lt;class 'bool'></samp>
<samp class=p>>>> </samp><kbd class=pp>type((False,))</kbd>
<samp class=pp>&lt;class 'tuple'></samp></pre>
<ol>
<li>In a boolean context, an empty tuple is false.
<li>Any tuple with at least one item is true.
<li>Any tuple with at least one item is true. The value of the items is irrelevant.
<li>Any tuple with at least one item is true. The value of the items is irrelevant. But what&#8217;s that comma doing there?
<li>To create a tuple of one item, you need a comma after the value. Without the comma, Python just assumes you have an extra pair of parentheses, which is harmless, but it doesn&#8217;t create a tuple.
</ol>
<h3 id=multivar>Assigning Multiple Values At Once</h3>
@@ -789,9 +794,9 @@ KeyError: 'pop from an empty set'</samp></pre>
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd class=pp> if anything:</kbd>
<samp class=p>... </samp><kbd class=pp> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd class=pp> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd class=pp> else:</kbd>
<samp class=p>... </samp><kbd class=pp> print('no, it's false')</kbd>
<samp class=p>... </samp><kbd class=pp> print("no, it's false")</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true(set())</kbd> <span class=u>&#x2460;</span></a>
<samp>no, it's false</samp>
@@ -889,9 +894,9 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
<pre class=screen>
<samp class=p>>>> </samp><kbd class=pp>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd class=pp> if anything:</kbd>
<samp class=p>... </samp><kbd class=pp> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd class=pp> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd class=pp> else:</kbd>
<samp class=p>... </samp><kbd class=pp> print('no, it's false')</kbd>
<samp class=p>... </samp><kbd class=pp> print("no, it's false")</kbd>
<samp class=p>...</samp>
<a><samp class=p>>>> </samp><kbd class=pp>is_it_true({})</kbd> <span class=u>&#x2460;</span></a>
<samp>no, it's false</samp>
@@ -929,9 +934,9 @@ KeyError: 'db.diveintopython3.org'</samp></pre>
<pre class='nd screen'>
<samp class=p>>>> </samp><kbd class=pp>def is_it_true(anything):</kbd>
<samp class=p>... </samp><kbd class=pp> if anything:</kbd>
<samp class=p>... </samp><kbd class=pp> print('yes, it's true')</kbd>
<samp class=p>... </samp><kbd class=pp> print("yes, it's true")</kbd>
<samp class=p>... </samp><kbd class=pp> else:</kbd>
<samp class=p>... </samp><kbd class=pp> print('no, it's false')</kbd>
<samp class=p>... </samp><kbd class=pp> print("no, it's false")</kbd>
<samp class=p>...</samp>
<samp class=p>>>> </samp><kbd class=pp>is_it_true(None)</kbd>
<samp>no, it's false</samp>