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

Alexander Belopolsky alexander.belopolsky at gmail.com
Fri Feb 13 23:55:37 CET 2015

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) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20150213/94d87e0d/attachment.html>

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