[Thomas Lehmann]
> In C++, programming STL you will use the insert method which always
> provides a position and a flag which indicates whether the position
> results from a new insertion or an exisiting element. Idea is to have
> one search only.
>
> <code>
> if data.has_key(key):
> value = data[key]
> </code>
>
> But this does mean (does it?) that the dictionary is searched two
> times! If so, can somebody show me how to do this in one step?
Several thoughts for you:
* Python isn't C++
* Dict lookups in Python are ubiquitous
* Trying to avoid them is often an exercise in futility
* Because of cache effects, double lookups are *very* cheap
* If this particular fragment is critical, consider using get():
data_get = data.get
...
# inner-loop code
value = data_get(key) # assigns None for missing values
* Another alternative is a try-block:
try: # setup is cheap
value = data[key]
except KeyError: # matching is expensive
...
* Or you can use collections.defaultdict() or a dict subclass that
defines __missing__().
* In general though, think of dicts as one of Python's most optimized
structures, one that should be embraced rather than avoided.
* Tim Peter's take on the subject from year's ago (paraphrased):
"Anything written using Python dictionaries is a gazillion times
faster than C".
Raymond
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