On 15-02-13, Neil Girdhar wrote:

Interesting: http://stackoverflow.com/questions/5490824/should-constructors-comply-with-t...

You'd better believe I read that thread not 20 minutes ago. :)

On Fri, Feb 13, 2015 at 3:37 PM, Isaac Schwabacher <alexander.belopolsky@gmail.com <ischwabacher@wisc.edu')" target="1">ischwabacher@wisc.edu> wrote:

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On 15-02-13, Guido van Rossum wrote:

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Are you willing to wait 10 days for an answer? I'm out of round tuits for a while.

IIUC, the argument is that the Liskov Substitution Principle is a statement about how objects of a subtype behave relative to objects of a supertype, and it doesn't apply to constructors because they aren't behaviors of existing objects. So other overriding methods *should* be able to handle the same inputs that the respective overridden methods do, but constructors don't need to. Even though __init__ is written as an instance method, it seems like it's "morally" a part of the class method __new__ that's only split off for convenience.

If this message is unclear, it's because I don't really understand this myself and I'm trying to articulate my best understanding of what's been said on this thread and those it links to.

ijs

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On Fri, Feb 13, 2015 at 10:22 AM, Alexander Belopolsky <http://bugs.python.org/issue2267#msg125979(javascript:main.compose('new', 't=alexander.belopolsky@gmail.com>(java_script:main.compose()> wrote:

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On Fri, Feb 13, 2015 at 1:19 PM, Alexander Belopolsky <alexander.belopolsky@gmail.com <alexander.belopolsky@gmail.com>(java_script:main.compose()> wrote:

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FWIW you're wrong when you claim that "a constructor is no different from any other method". Someone else should probably explain this (it's an old argument that's been thoroughly settled).

Well, the best answer I've got in the past [1] was "ask on python-dev since Guido called the operator overriding expectation." :-)

And let me repost this bit of history [1]:

Here is the annotated pre-r82065 code:

39876 gvanrossum def __add__(self, other): 39876 gvanrossum if isinstance(other, timedelta): 39928 gvanrossum return self.__class__(self.__days + other.__days, 39876 gvanrossum self.__seconds + other.__seconds, 39876 gvanrossum self.__microseconds + other.__microseconds) 40207 tim_one return NotImplemented 39876 gvanrossum

[1] > --Guido van Rossum (python.org/~guido(http://python.org/~guido)(http://python.org/~guido(http://python.org/~guido)))

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