More __init__ methods

[Terry Reedy]

Duncan Booth wrote:
> Mr.SpOOn <mr.spoon21 at gmail.com> wrote:
> 
>> Now I must pass a and b to the main constructor and calculate them in
>> the classmethods.
>>
>> class foo:
>>     def __init__(self, a, b):
>>          self.a = a
>>          self.b = b
>>
>>     @classmethod
>>     def from_string(self, ..):
>>           ...
>>           ...
>>
>> What I mean is: I can't use anymore __init__ as the default
>> constructor, but I always have to specify the way I'm creating my
>> object. Am I right? I'm asking just to be sure I have understood.
> 
> There is a really big advantage to being explicit in this situation: you no 
> longer have to make sure that all your constructors use a unique set of 
> types. Consider:
> 
> class Location(object):
>     def __init__(self, lat, long): ...
> 
>     @classmethod
>     def from_city(name): ...
> 
>     @classmethod
>     def from_postcode(name): ...
> 
> 'from_string' is a bad name here for your factory method: you should try to 
> make it clear what sort of string is expected.

One built-in model for .__init__ + .from_data is dict.  __init__ can 
take either one iterable or several keywords.  In either case, it gets a 
set of key:value pairs.  .from_keys take a set of keys and an optional 
value to override None that is matched with all keys.