Re: python bug when subclassing list?

[Steve Holden]

Hamish McKenzie wrote:

I want to write a Vector class and it makes the most sense to just
subclass list. I also want to be able to instantiate a vector using either:
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Vector( 1, 2, 3 )
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OR
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Vector( [1, 2, 3] )
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so I have this:
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class Vector(list):
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def __new__( cls, *a ):
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try:
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print a
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return list.__new__(cl s, a)
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except:
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print 'broken'
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return list.__new__(cl s, list(a))
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doing Vector( 1, 2, 3 ) on this class results in a TypeError – which
doesn’t seem to get caught by the try block (ie “broken” never gets
printed, and it never tries to
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I can do pretty much the exact same code but inheriting from tuple
instead of list and it works fine.
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is this a python bug? or am I doing something wrong?
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Vector(1, 2, 3) fails for exactly the same reasons as list:

>>list(1, 2, 3)

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: list() takes at most 1 argument (3 given)

So the behavior you want cannot be inherited from list, since list
doesn't implement that behavior!

As toy our assertion that you can subclass tuple that way, I am inclined
to doubt it because of this:

>>tuple(1, 2, 3)

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: tuple() takes at most 1 argument (3 given)

regards
Steve
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