__call__ considered harmful or indispensable?
tjreedy at udel.edu
Fri Aug 3 20:49:49 CEST 2007
Skip Montanaro a écrit :
>>> In this case there was a bug. Depending on inputs, sometimes obj
>>> initialized to a class, sometimes an instance of that class. (I fixed
>>> that too while I was at it.) The problem was that the use of __call__
>>> obscured the underlying bug by making the instance as well as the class
>> I don't quite get the point here. A concrete example would be welcome.
> The bug went something like this:
> obj = some.default_class
> if some_other_rare_condition_met:
> ... several lines ...
> obj = some.other_class()
Should that have been some.other_class (without the ()s?). It is in the
nature of Python's dynamic typing that mis-typing errors sometimes show up
later than one would wish. Hence the need for unit testing. Consider
i = condition and 1 or '2' # whoops, should have been 2, without the 's
k = j * i # j an int; whoops, bad i value passes silently
use of k as string raises exception
I do not think __call__ should be specifically blamed for this general
downside of Python's design.
Terry Jan Reedy
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