[Numpy-discussion] numpy array change notifier?

Christan K. ckkart@hoc....
Thu Oct 30 07:51:36 CDT 2008

Erik Tollerud <erik.tollerud <at> gmail.com> writes:

> Is there any straightforward way of notifying on change of a numpy
> array that leaves the numpy arrays still efficient?

You can instantiate the following ndarray derived class with a callable 
argument. Any change to the data will call the callback function.

In [5]: def notify(arg):
   ...:     print 'array changed'

In [6]: x = cbarray([1,2,3,4],cb=notify)

In [7]: x[2] = -1
array changed

alternatively I have used the pubsub module available as part of wxPython (see 
the commented line in _notify())

class cbarray(N.ndarray):
    def __new__(subtype, data, cb=None, dtype=None, copy=False):
        subtype.__defaultcb = cb

        if copy:
            data = N.array(data,dtype=dtype)
            data = N.asarray(data,dtype=dtype)

        data = data.view(subtype)
        return data

    def _notify(self):
        if self.cb is not None:
    def _get_shape(self):
        return super(cbarray, self).shape
    shape = property(_get_shape)

    def __setitem__(self, item, val):
        N.ndarray.__setitem__(self, item, val)
    def __array_finalize__(self,obj):
        if not hasattr(self, "cb"):
            # The object does not already have a `.cb` attribute
            self.cb = getattr(obj,'cb',self.__defaultcb)

    def __reduce__(self):
        object_state = list(N.ndarray.__reduce__(self))
        subclass_state = (self.cb,)
        object_state[2] = (object_state[2],subclass_state)
        return tuple(object_state)

    def __setstate__(self,state):
        nd_state, own_state = state
        cb, = own_state
        self.cb = cb

Hope thqat helps, Christian

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