[Python-Dev] subclassing builtin data structures (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Sat Feb 14 10:44:19 CET 2015

On 14 Feb 2015 08:57, "Alexander Belopolsky" <alexander.belopolsky at gmail.com> wrote:

On Fri, Feb 13, 2015 at 4:44 PM, Neil Girdhar <mistersheik at gmail.com> wrote:

Interesting: http://stackoverflow.com/questions/5490824/should-constructors-comply-with-the-liskov-substitution-principle 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 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 "", line 1, in TypeError: new() takes 2 positional arguments but 4 were given >>> Date2.fromordinal(1) Traceback (most recent call last): File "", line 1, in 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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