Calling parent constructor with different argument list

[pinkisntwell]

class Vertex(tuple):
pass

class Positioned_Vertex(Vertex):

def __init__(self, a, b):
Vertex.__init__(a)

a=Positioned_Vertex((0,0,0), 1)

This gives:

TypeError: tuple() takes at most 1 argument (2 given)

It looks like the explicit call to Vertex.__init__ is never made and
Vertex.__init__ is implicitly called when a Positioned_Vertex is
created. Is there a way to work around this and call the constructor
with the intended argument list?

 

[Mark Lawrence]

class Vertex(tuple):
pass

class Positioned_Vertex(Vertex):

def __init__(self, a, b):

def __init__(self, a): # just take out b

Vertex.__init__(a)

a=Positioned_Vertex((0,0,0), 1)

a=Positioned_Vertex( ( (0,0,0), 1) ) # and add a pair of brackets
print a

This gives:

TypeError: tuple() takes at most 1 argument (2 given)

It looks like the explicit call to Vertex.__init__ is never made and
Vertex.__init__ is implicitly called when a Positioned_Vertex is
created. Is there a way to work around this and call the constructor
with the intended argument list?

Simplest way to get it to work is above using Python 2.6.2 on Windows.
I'm sure there are variations depending on your use case, but I'll leave
that to the experts.

 

[Gabriel Genellina]

class Vertex(tuple):
pass

class Positioned_Vertex(Vertex):

def __init__(self, a, b):
Vertex.__init__(a)

a=Positioned_Vertex((0,0,0), 1)

This gives:

TypeError: tuple() takes at most 1 argument (2 given)

It looks like the explicit call to Vertex.__init__ is never made and
Vertex.__init__ is implicitly called when a Positioned_Vertex is
created. Is there a way to work around this and call the constructor
with the intended argument list?

The tuple constructor (like numbers, strings, and other immutable objects)
never calls __init__. You have to override __new__ instead:

py> class Point3D(tuple):
.... def __new__(cls, x, y, z):
.... obj = super(Point3D, cls).__new__(cls, (x,y,z))
.... return obj
....
py> a = Point3D(10, 20, 30)
py> a
(10, 20, 30)
py> type(a)
<class '__main__.Point3D'>

See http://docs.python.org/reference/datamodel.html#basic-customization