Python: The Dictionary Playbook

You mentioned it a bit, but I want to make it clear that, even if you don't use 2.7, you can still count by doing:<p><pre><code> from collections import defaultdict counter = defaultdict(int) </code></pre> There is a difference though, because you have to count manually, i.e:<p><pre><code> for i in 'supercalifragilisticexpialidocius': counter[i] += 1 </code></pre> Also, because defaultdict accepts any callable, you can have a dict of counters by doing:<p><pre><code> counters = defaultdict(lambda: defaultdict(int)) for word in ['apple', 'berry', 'grape']: for letter in word: counters[word][letter] += 1 </code></pre> This is not very obvious, so I don't use it a lot, but sometimes it's the most elegant solution.

You know a guide is good when it makes you want to go back and refactor old code. Great information, thanks :)

This is a really nice guide. I've been up to my eyes in python dicts over the last few days so already had most of these figured out.<p>setdefault is new to me, which is cool. Unfortunately I can only see one place to use it in my code and it would be inefficient [0]. Best stash it away for later use :)<p>[0] r = re_subs.setdefault(s, re.compile(s))

Good advice. I've got one to add:<p><pre><code> #x and y are dictionaries z = dict(x.items() + y.items()) </code></pre> It merges two dictionaries, giving precedence to the second (in Python 2 - Python 3 is a bit more nasty: <a href="http://stackoverflow.com/questions/38987/how-can-i-merge-union-two-python-dictionaries-in-a-single-expression" rel="nofollow">http://stackoverflow.com/questions/38987/how-can-i-merge-uni...</a>).

Love defaultdict. It and dict/set/list compressions are a big part of what makes Python so fast to write in.<p>Great practice for 2.7 that's probably quashed in 3.0. For large dicts, no need to create a giant set en route when iterating over keys, values, or both. Use "for k in d.iterkeys()", "for v in d.itervals", "for k,v in d.iteritems."<p>While I'm at it-- if you're ever finding yourself using a huge amount of awfully rigid objects from a single class, use __slots__ to allocate needed variables! Python will otherwise define the object's namespace in a dict (called __dict__) which allocates a whole kilobyte per object. Bad news if you have several hundred thousand... Guessing this is why Guido loves namedtuples so much for basic attributed storage.

I love curiosity sparking posts like this.<p>&#62;&#62;key in dct<p>is much better than<p>key in dct.keys()<p>Of course, that got me curios to find out if there is a magic method out there that takes advantage of keyword "in". Turns out __contains__ does that.<p>Always exciting to stumble upon new stuff in my favorite language.

I hate to be that guy but what about dictionary comprehensions?

One my favorites, safe deep searching: by returning a dict you can run another .get<p>var = {'a' : 'b' , 'c' : {'d' : 'f'}}<p>print var.get('c', {}).get('d') print var.get('DNE', {}).get('d')

"not key in dct", or "not (key in dct)", is never slower than "key not in dct" because when key is found in dct, the expression is immediately true.

In item 3, the "boilerplate" and the "awesome way" are not equivalent. The boilerplate does the equivalent of setdefault, which is mentioned later<p><pre><code> dct[key] = dct.setdefault(key, 0) + 1</code></pre>

In most circumstances for (key, val) in data won't work, as the default dictionary iterable only contains the keys. You want for key, val in data.items():

Nice! Some good information here. Good for beginners, good for intermediates, some expert will probably say it's good for them too.

Thanks for writing this; it's very useful.<p>Are you planning on doing similar posts about other parts of Python in the future?

Can logical operator in Python be used to conditionally initialize an item in a dict? For example, can:<p><pre><code> group = dct.setdefault(key, []) group.append(value) </code></pre> be replaced with some equivalent of this Ruby snippet:<p><pre><code> (dict[key] ||= []) &#60;&#60; value ?</code></pre>

Thanks, I didn't know about "x not in D"! On the same note:<p><a href="http://dacavtricks.wordpress.com/2011/05/23/python-default-values-in-a-dictionary/" rel="nofollow">http://dacavtricks.wordpress.com/2011/05/23/python-default-v...</a>

I also find the combination of lambda and defaultdict quite useful:<p><pre><code> d = defaultdict(lambda: False) </code></pre> or<p><pre><code> d = defaultdict(lambda: {'foo':set(), 'bar':False}) d['baz']['foo'].add(1)</code></pre>

Pretty cool, bookmarked. I always enjoy different forms of presenting the info, playbook was a nice touch :)

my Bitdefender is blocking the site; says it's insecure.

nice! very useful, thanks for putting this together.

This is ... wait for it.. Legendary. :)

Rocking it out! Awesome! Getting down! Make it happen!