On 14 Feb 2015 08:57, "Alexander Belopolsky" <alexander.belopolsky@gmail.com> wrote:
On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar <mistersheik@gmail.com>
Interesting:
http://stackoverflow.com/questions/5490824/should-constructors-comply-with-t...
Let me humbly conjecture that the people who wrote the top answers have background in less capable languages than Python.
Not every language allows you to call self.__class__(). In the languages
wrote: that don't you can get away with incompatible constructor signatures.
However, let me try to focus the discussion on a specific issue before we
go deep into OOP theory.
With python's standard datetime.date we have:
from datetime import * class Date(date): ... pass ... Date.today() Date(2015, 2, 13) Date.fromordinal(1) Date(1, 1, 1)
Both .today() and .fromordinal(1) will break in a subclass that redefines
__new__ as follows:
class Date2(date): ... def __new__(cls, ymd): ... return date.__new__(cls, *ymd) ... Date2.today() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: __new__() takes 2 positional arguments but 4 were given Date2.fromordinal(1) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: __new__() takes 2 positional arguments but 4 were given
Why is this acceptable, but we have to sacrifice the convenience of
having Date + timedelta
return Date to make it work with Date2:
Date2((1,1,1)) + timedelta(1) datetime.date(1, 1, 2)
Coupling alternative constructors to the default constructor signature is pretty normal - it just means that if you override the signature of the default constructor, you may need to update the alternative ones accordingly. Cheers, Nick.
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