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added example of human-readable message in assert statement

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Mark Pilgrim
2009-07-15 10:08:37 -04:00
parent 428353aec6
commit 8843481267
2 changed files with 14 additions and 11 deletions
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advanced-iterators.html
+13 -10
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@@ -51,7 +51,7 @@ import itertools
def solve(puzzle):
words = re.findall('[A-Z]+', puzzle.upper())
unique_characters = set(join(words))
assert len(unique_characters) <= 10
assert len(unique_characters) <= 10, 'Too many letters'
first_letters = {word[0] for word in words}
n = len(first_letters)
sorted_characters = ''.join(first_letters) + \
@@ -161,28 +161,31 @@ if __name__ == '__main__':
<p>Like many programming languages, Python has an <code>assert</code> statement. Here&#8217;s how it works.
<pre class=screen>
<a><samp class=p>>>> </samp><kbd class=pp>assert 1 + 1 == 2</kbd> <span class=u>&#x2460;</span></a>
<a><samp class=p>>>> </samp><kbd class=pp>assert 1 + 1 == 3</kbd> <span class=u>&#x2461;</span></a>
<a><samp class=p>>>> </samp><kbd class=pp>assert 1 + 1 == 2</kbd> <span class=u>&#x2460;</span></a>
<a><samp class=p>>>> </samp><kbd class=pp>assert 1 + 1 == 3</kbd> <span class=u>&#x2461;</span></a>
<samp class=traceback>Traceback (most recent call last):
File "&lt;stdin>", line 1, in <module>
AssertionError</samp></pre>
AssertionError</samp>
<a><samp class=p>>>> </samp><kbd class=pp>assert 2 + 2 == 5, "Only for very large values of 2"</kbd> <span class=u>&#x2462;</span></a>
<samp class=traceback>Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError: Only for very large values of 2</samp></pre>
<ol>
<li>The <code>assert</code> statement is followed by any valid Python expression. In this case, the expression <code>1 + 1 == 2</code> evaluates to <code>True</code>, so the <code>assert</code> statement does nothing.
<li>However, if the Python expression evaluates to <code>False</code>, the <code>assert</code> statement will raise an <code>AssertionError</code>.
<li>You can also include a human-readable message that is printed if the <code>AssertionError</code> is raised.
</ol>
<p>Therefore, this line of code:
<pre class=nd><code class=pp>assert len(unique_characters) <= 10</code></pre>
<pre class=nd><code class=pp>assert len(unique_characters) <= 10, 'Too many letters'</code></pre>
<p>&hellip;is equivalent to&hellip;
<p>&hellip;is equivalent to this:
<pre class=nd><code class=pp>if len(unique_characters) > 10:
raise AssertionError</code></pre>
raise AssertionError('Too many letters')</code></pre>
<p>But a bit easier to read and write.
<p>The alphametics solver uses this exact <code>assert</code> statement to bail out early if the puzzle contains more than ten unique letters. Since each letter is assigned a unique digit, and there are only ten digits, a puzzle with more than ten unique letters is unsolvable.
<p>The alphametics solver uses this exact <code>assert</code> statement to bail out early if the puzzle contains more than ten unique letters. Since each letter is assigned a unique digit, and there are only ten digits, a puzzle with more than ten unique letters can not possibly have a solution.
<p class=a>&#x2042;
examples/alphametics.py
+1 -1
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@@ -10,7 +10,7 @@ import itertools
def solve(puzzle):
words = re.findall('[A-Z]+', puzzle.upper())
unique_characters = set(''.join(words))
assert len(unique_characters) <= 10
assert len(unique_characters) <= 10, 'Too many letters'
first_letters = {word[0] for word in words}
n = len(first_letters)
sorted_characters = ''.join(first_letters) + \