Some of the classes in Qahirah, my Cairo binding
<https://github.com/ldo/qahirah> I found handy to reuse elsewhere, for example
in my binding for Pixman <https://github.com/ldo/python_pixman>. Subclassing is
easy, but then you need to ensure that operations inherited from the superclass
return instances of the right class. This means that superclass methods must
never refer directly to the class by name for constructing new objects; they
need to obtain the current class by more indirect means.
(“isinstance” checks on the superclass name are fine, since they will succeed
on subclasses as well.)
For example, consider the qahirah.Matrix class. Here is how I define the
multiplication operator:
def __mul__(m1, m2) :
"returns concatenation with another Matrix, or mapping of a Vector."
if isinstance(m2, Matrix) :
result = m1.__class__ \
(
xx = m1.xx * m2.xx + m1.xy * m2.yx,
yx = m1.yx * m2.xx + m1.yy * m2.yx,
xy = m1.xx * m2.xy + m1.xy * m2.yy,
yy = m1.yx * m2.xy + m1.yy * m2.yy,
x0 = m1.xx * m2.x0 + m1.xy * m2.y0 + m1.x0,
y0 = m1.yx * m2.x0 + m1.yy * m2.y0 + m1.y0,
)
elif isinstance(m2, Vector) :
result = m2.__class__ \
(
x = m2.x * m1.xx + m2.y * m1.xy + m1.x0,
y = m2.x * m1.yx + m2.y * m1.yy + m1.y0
)
else :
result = NotImplemented
#end if
return \
result
#end __mul__
The idea behind this is that you can do “matrix * matrix” to do matrix
multiplication, or “matrix * vector” to transform a Vector by a Matrix.
In Pixman, the corresponding class names are Transform instead of Matrix, and
Point instead of Vector (following the usual pixman terminology). But the
inherited multiplication operation still works on them.
For another example, consider the qahirah.Vector.from_tuple method, which
allows easy passing of a simple pair of (x, y) coordinates wherever a Vector is
wanted, with only a little extra work:
@classmethod
def from_tuple(celf, v) :
"converts a tuple of 2 numbers to a Vector. Can be used to ensure that"
\
" v is a Vector."
if not isinstance(v, celf) :
v = celf(*v)
#end if
return \
v
#end from_tuple
The key thing here is to avoid staticmethods and use classmethods instead, so
you get passed the class object and can use it to construct new instances.
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