Like overloading __init__(), but how?

[John M. Gabriele]

I know that Python doesn't do method overloading like
C++ and Java do, but how am I supposed to do something
like this:

--------------------- incorrect ------------------------
#!/usr/bin/python

class Point3d:
pass

class Vector3d:
"""A vector in three-dimensional cartesian space."""

def __init__( self ):
"""Create a Vector3d with some reasonable default value."""
x, y, z = 0.0, 0.0, 0.0


def __init__( self, x_from, y_from, z_from,
x_to, y_to, z_to ):
"""Create a Vector3d from x-y-z coords."""
# ...
pass


def __init__( self, point_from, point_to ):
"""Create a Vector3d from two Point3d objects."""
# ...
pass


def __init__( self, same_as_this_ve c ):
"""Create a Vector3d from a copy of another one."""
# ...
pass

p = Point3d()
p2 = Point3d()
# v = Vector3d( p2, p ) -- Nope. Only the last __init__() counts.

---------------------- /incorrect -------------------------------

Thanks.

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[Steven Bethard]

Re: Like overloading __init__(), but how?

John M. Gabriele wrote:[color=blue]
> class Vector3d:
> def __init__(self):
> ...
> def __init__(self, x_from, y_from, z_from, x_to, y_to, z_to):
> ...
> def __init__(self, point_from, point_to):
> ...
> def __init__(self, same_as_this_ve c):
> ...[/color]

My preferred option is to break these into different methods. I pick
the xyz-coords (and the empty coords) __init__ as the most basic cases,
but if you find one of the other cases to be more basic, the
modification should be pretty simple.

class Vector3d(object ):
# define init to take only xyz coords (missing coords default to 0)
# this covers your first two cases, I think
def __init__(self, x_from=0.0, y_from=0.0, z_from=0.0,
x_to=0.0, y_to=0.0, z_to=0.0):
...

# define a classmethod that creates a Vector3d from points
# this covers your third case
@classmethod
def from_points(cls , point_from, point_to):
...
return cls(x_from=..., y_from=..., ...)

# define copy as an instance method, not a constructor
# this covers your fourth case
def copy(self):
...
return type(self)(x_fr om=..., y_from=..., ...)


Another possibility is to play around with *args:

class Vector3d(object ):
def __init__(self, *args):
if not args:
# constructor with no arguments
elif len(args) == 6:
# constructor with xyz coords
elif len(args) == 2:
# constructor with points
elif len(args) == 1:
# copy constructor
else:
raise TypeError('expe cted 0, 1, 2 or 6 args, got %i' %
len(args))

But this can get really ugly really quick.

HTH,

STeVe

[djw]

Re: Like overloading __init__(), but how?

This was discussed only days ago:

http://groups-beta.google.com/group/comp.lang.python/browse_thread/thread/41a6c0e1e260cd72/fc1c924746532316?q=multiple+constructors+python&_done=%2Fgroups%3Fq%3Dmultiple+constructors+python%26&_doneTitle=Back+to+Search&&d#fc1c924746532316

-Don


John M. Gabriele wrote:[color=blue]
> I know that Python doesn't do method overloading like
> C++ and Java do, but how am I supposed to do something
> like this:
>
> --------------------- incorrect ------------------------
> #!/usr/bin/python
>
> class Point3d:
> pass
>
> class Vector3d:
> """A vector in three-dimensional cartesian space."""
>
> def __init__( self ):
> """Create a Vector3d with some reasonable default value."""
> x, y, z = 0.0, 0.0, 0.0
>
>
> def __init__( self, x_from, y_from, z_from,
> x_to, y_to, z_to ):
> """Create a Vector3d from x-y-z coords."""
> # ...
> pass
>
>
> def __init__( self, point_from, point_to ):
> """Create a Vector3d from two Point3d objects."""
> # ...
> pass
>
>
> def __init__( self, same_as_this_ve c ):
> """Create a Vector3d from a copy of another one."""
> # ...
> pass
>
> p = Point3d()
> p2 = Point3d()
> # v = Vector3d( p2, p ) -- Nope. Only the last __init__() counts.
>
> ---------------------- /incorrect -------------------------------
>
> Thanks.
>[/color]

[Kent Johnson]

Re: Like overloading __init__(), but how?

John M. Gabriele wrote:[color=blue]
> I know that Python doesn't do method overloading like
> C++ and Java do, but how am I supposed to do something
> like this:[/color]

This was just discussed. See
http://tinyurl.com/6zo3g

Kent
[color=blue]
>
> --------------------- incorrect ------------------------
> #!/usr/bin/python
>
> class Point3d:
> pass
>
> class Vector3d:
> """A vector in three-dimensional cartesian space."""
>
> def __init__( self ):
> """Create a Vector3d with some reasonable default value."""
> x, y, z = 0.0, 0.0, 0.0
>
>
> def __init__( self, x_from, y_from, z_from,
> x_to, y_to, z_to ):
> """Create a Vector3d from x-y-z coords."""
> # ...
> pass
>
>
> def __init__( self, point_from, point_to ):
> """Create a Vector3d from two Point3d objects."""
> # ...
> pass
>
>
> def __init__( self, same_as_this_ve c ):
> """Create a Vector3d from a copy of another one."""
> # ...
> pass
>
> p = Point3d()
> p2 = Point3d()
> # v = Vector3d( p2, p ) -- Nope. Only the last __init__() counts.
>
> ---------------------- /incorrect -------------------------------
>
> Thanks.
>[/color]

[John M. Gabriele]

Re: Like overloading __init__(), but how?

On Wed, 23 Feb 2005 21:32:52 -0700, Steven Bethard wrote:
[color=blue]
> [snip]
> Another possibility is to play around with *args:
>
> class Vector3d(object ):
> def __init__(self, *args):
> if not args:
> # constructor with no arguments
> elif len(args) == 6:
> # constructor with xyz coords
> elif len(args) == 2:
> # constructor with points
> elif len(args) == 1:
> # copy constructor
> else:
> raise TypeError('expe cted 0, 1, 2 or 6 args, got %i' %
> len(args))
>
> But this can get really ugly really quick.
>
> HTH,
>
> STeVe[/color]

Thanks STeVe. I think, for now, I'll probably stick with
the easier but less elegant way of checking *args. I guess
I could compare args[0].__class__.__na me__ against 'Point3d',
'Vector3d', or just 'float'.

Didn't realize this topic came up recently. Sorry for the
duplication.

---J

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