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copied!<p>It may help to print out what you get from that extra step:</p> <pre><code>>>> months_abbvs {'apr': 'April', 'aug': 'August', 'dec': 'December', 'feb': 'February', 'jan': 'January', 'jul': 'July', 'jun': 'June', 'mar': 'March', 'may': 'May', 'nov': 'November', 'oct': 'October', 'sep': 'September'} </code></pre> <p>So, you have a mapping of lowercased abbreviations to full month names.</p> <p>How does that work? Well, first, let's look at what the expression does:</p> <pre><code>>>> m = 'December' >>> m[:3].lower() 'dec' >>> m[:3].lower(), m ('dec', 'December') </code></pre> <hr> <p>So, the comprehension just does that for each month:</p> <pre><code>>>> [(m[:3].lower(), m) for m in months] [('jan', 'January'), ('feb', 'February'), ('mar', 'March'), ('apr', 'April'), ('may', 'May'), ('jun', 'June'), ('jul', 'July'), ('aug', 'August'), ('sep', 'September'), ('oct', 'October'), ('nov', 'November'), ('dec', 'December')] </code></pre> <p>As explained in more detail in <a href="http://docs.python.org/2/tutorial/datastructures.html#list-comprehensions" rel="nofollow">the tutorial</a>, a comprehension is basically shorthand for a loop. In particular, this:</p> <pre><code>>>> m2 = [<expression with m> for m in months] </code></pre> <p>… is equivalent to:</p> <pre><code>>>> m2 = [] >>> for m in months: ... m2.append(<expression with m>) </code></pre> <hr> <p>Using a generator expression instead of a list comprehension just means the sequence is built as a lazy iterator instead of a list.</p> <p>And then passing the result (either way) to <code>dict</code> builds a dictionary mapping the first value of each tuple to the second.</p> <hr> <p>You could write this all a bit more readably as a dictionary comprehension:</p> <pre><code>months_abbvs = {m[:3].lower(): m for m in months} </code></pre> <p>Even better, instead of writing <code>m[:3].lower()</code> repeatedly, give it a nice name and use that:</p> <pre><code>def abbreviate(m): return m[:3].lower() months_abbvs = {abbreviate(m): m for m in months} </code></pre> <p>And then:</p> <pre><code>def valid_month(month): if month: short_month = abbreviate(month) return month_abbvs.get(short_month) </code></pre> <hr> <p>Now, what you do with your input in the new version is:</p> <pre><code>short_month = month[:3].lower() return month_abbvs.get(short_month) </code></pre> <p>Since <code>month_abbvs</code> is a <code>dict</code> (you can tell that by printing it out, or just from the fact that it was created by calling <code>dict</code> on something), the <code>get</code> method is <a href="http://docs.python.org/2/library/stdtypes.html#dict.get" rel="nofollow"><code>dict.get</code></a>. So, as explained in the linked docs, <code>month_abbvs.get(short_month)</code> is the same as <code>months_abbvs[short_month]</code>, except that if the key <code>short_month</code> is not found, you will get <code>None</code>, instead of raising an exception.</p> <p>So, if given <code>'October'</code>, you'll set <code>short_month</code> to <code>'oct'</code>. And then you look it up in the abbreviation dictionary, which returns <code>'October'</code>. If given <code>'OCT'</code> or <code>'october'</code> or <code>'octal digit string'</code>, you'll also return the same thing. Since any non-empty string is truthy, if you've done something like <code>if valid_month('October'):</code>, it will be true.</p> <p>But, if given, say, <code>'Muhammed'</code>, you'll set <code>short_month</code> to <code>'muh'</code>. And then you look that up, and it isn't there. As explained above, the <code>get</code> method returns <code>None</code> for unknown keys, so you will return <code>None</code>. Since <code>None</code> is falsey, if you've done something like <code>if valid_month('Muhammed'):</code>, it will not be true.</p> <p>In other words, it makes the function more lenient—which may be an improvement, or may be a bad thing (or may be a little of both—maybe you wanted <code>'OCT'</code> to work, but not <code>'octal digit string'</code>).</p>

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