Here’s a very simple example for Python that should supplement the pickle docs. class Foo(object): def __init__(self, val=2): self.val = val def __getstate__(self): print(“I’m being pickled”) self.val *= 2 return self.__dict__ def __setstate__(self, d): print(“I’m being unpickled with these values: ” + repr(d)) self.__dict__ = d self.val *= 3 import pickle f = Foo() f_data … Read more
Special methods (essentially anything with two underscores on each end) have to be defined on the class. The internal lookup procedure for special methods completely skips the instance dict. Among other things, this is so if you do class Foo(object): def __repr__(self): return ‘Foo()’ the __repr__ method you defined is only used for instances of … Read more
I don’t think there’s anything deeper going on than what you have originally said: it’s just syntactic sugar whereby the compiler converts f(a) into f.apply(a) as a special syntax case. This might seem like a specific rule, but only a few of these (for example, with update) allows for DSL-like constructs and libraries.
All you need to do is add “&” in front of your __get function to pass it as reference: public function &__get ( $index ) Struggled with this one for a while.
What do the exception messages mean, and how do they relate to the problem? As one might guess, a TypeError is an Error that has to do with the Type of something. In this case, the meaning is a bit strained: Python also uses this error type for function calls where the arguments (the things … Read more
[UPDATE] – added (yet another) explanation on for comprehensions The * method: This returns the default projection – which is how you describe: ‘all the columns (or computed values) I am usually interested’ in. Your table could have several fields; you only need a subset for your default projection. The default projection must match the … Read more
This example uses memoization, basically storing values in a table (dictionary in this case) so you can look them up later instead of recalculating them. Here we use a simple class with a __call__ method to calculate factorials (through a callable object) instead of a factorial function that contains a static variable (as that’s not … Read more
I almost posted this as a comment to the other answers but it’s really an answer in and of itself. To make your items sortable, they only need to implement __lt__. That’s the only method used by the built in sort. The other comparisons or functools.total_ordering are only needed if you actually want to use … Read more
I have been exactly in your case in the past. And I went for magic methods. This was a mistake, the last part of your question says it all : this is slower (than getters/setters) there is no auto-completion (and this is a major problem actually), and type management by the IDE for refactoring and … Read more
AFAIK, len is special in this respect and has historical roots. Here’s a quote from the FAQ: Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))? The major reason is history. Functions were used for those operations that were generic for a group of types and which were … Read more