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    <p>The only problem with <code>heapq</code> is that it doesn't provide a <code>key</code> function like everything else in the stdlib does. (If you're curious why, Raymond Hettinger explains in <a href="http://code.activestate.com/lists/python-list/162387/" rel="noreferrer">this email</a>. He's right that <code>heapq</code> couldn't provide the same interface as other sort functions—but the reasons don't affect your use case, where <code>key</code> would just be <code>lambda x: -x</code>.)</p> <p>The usual workaround is to decorate-heap-undecorate. That is, put a modified version of your values into the heap that sorts by <code>key</code>. Normally, this means one of the following:</p> <ul> <li>Storing <code>key(x)</code> instead of <code>x</code>, and then accessing <code>unkey(value)</code> instead of <code>value</code> (assuming <code>key</code> is reversible).</li> <li>Storing <code>(key(x), x)</code> instead of <code>x</code>, and then accessing <code>value[1]</code>. (This can break stability, but <code>heapq</code> doesn't promise stability anyway.)</li> <li>Writing a wrapper class that implements a custom <code>__le__</code> method, then storing <code>Wrapper(x)</code> instead of <code>x</code> and accessing <code>value.value</code> instead of <code>value</code>.</li> </ul> <p>In your case, the key function is reversible. So, just store <code>-x</code>, and access <code>-value</code>. That's about as trivial as decoration gets.</p> <p>Still, regardless of how simple it is, you should probably write a wrapper, or you will screw it up at some point. For example, you could write a <code>maxheap</code> that wraps the minheap in <code>heapq</code> like this:</p> <pre><code>import heapq def heapify(x): for i in range(len(x)): x[i] = -x[i] heapq.heapify(x) def heappush(heap, item): heapq.heappush(heap, -item) def heappop(heap): return -heapq.heappop(heap) </code></pre> <p>… and so on for any other functions you need. It may be a bit of a pain, but it's a lot less work than implementing the whole thing from scratch.</p> <p>While you're at it, you may want to wrap the heap in an object-oriented API so you can do <code>heap.push(x)</code> instead of <code>heapq.heappush(heap, x)</code>, etc.</p> <pre><code>import heapq class MaxHeap(object): def __init__(self, x): self.heap = [-e for e in x] heapq.heapify(self.heap) def push(self, value): heapq.heappush(self.heap, -value) def pop(self): return -heapq.heappop(self.heap) </code></pre> <p>…</p> <p>If you take a quick look around the recipes at ActiveState or the modules on PyPI, you should find that others have already done most of the work for you.</p> <p>Alternatively, you could copy and paste the <code>heapq</code> source (it's pure Python) as <code>maxheapq.py</code> and just replace the <code>cmp_lt</code> function with its opposite. (Of course if you're doing that, it's probably just as easy, and certainly a lot clearer, to modify <code>cmp_lt</code> to take a <code>key</code> argument in the first place, and modify all the other functions to pass the <code>key</code> through—bearing in mind that it won't be as generally applicable anymore, because it can't make the usual guarantee that <code>key</code> is only called once.)</p> <p>If you really want to live dangerously (you shouldn't), you could even monkeypatch it:</p> <pre><code>import heapq def cmp_gt(x, y): return y &lt; x if hasattr(y, '__lt__') else not (x &lt;= y) heapq.cmp_lt = cmp_gt </code></pre> <p>But you don't want to do that in real code.</p>
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    1. POpython topN max heap, use heapq or self implement?
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    1. COWhere can I find the source code for heapq?
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      1. USScott David Tesler
    2. CO@ScottDavidTesler: If you look at [the documentation for the module](http://docs.python.org/3/library/heapq.html), there's a link to the source code right there at the top. Just follow it.
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    3. CO@ScottDavidTesler: Or just `import heapq; print(heapq.__file__)` and you'll usually get the path to a local copy on your system. (This won't work if your Python installation puts the whole stdlib into a zipfile, or keeps .pyc files only, or various other tricks that are common on embedded systems.)
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