[Python-Dev] defaultdict and on_missing() (original) (raw)

Michael Chermside mcherm at mcherm.com
Fri Feb 24 03:39:56 CET 2006

Walter Dörwald writes:

I always thought that magic method calls are done by Python on objects it doesn't know about. The special method name ensures that it is indeed the protocol Python is talking about, not some random method (with next() being the exception). In the defaultdict case this isn't a problem, because defaultdict is calling its own method.

I, instead, felt that the xxx convention served a few purposes. First, it indicates that the method will be called in some means OTHER than by name (generally, the interpreter invokes it directly, although in this case it's a built-in method of dict that would invoke it). Secondly, it serves to flag the method as being special -- true newbies can safely ignore nearly all special methods aside from init(). And it serves to create a separate namespace... writers of Python code know that names beginning and ending with double-underscores are "reserved for the language". Of these, I always felt that special invocation was the most important feature. The next() method of iterators was an interesting object lesson. The original reasoning (I think) for using next() not next() was that sometimes the method was called directly by name (when stepping an iterator manually, which one frequently does for perfectly good reasons). Since it was sometimes invoked by name and sometimes by special mechanism, the choice was to use the unadorned name, but later experience showed that it would have been better the other way.

-- Michael Chermside

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