User guide

NumPy User Guide, Release 1.9.0

self type is <class ’C’>

obj type is <class ’C’>

The signature of __array_finalize__ is:

def __array_finalize__(self, obj):

ndarray.__new__ passes __array_finalize__ the new object, of our own class (self) as well as the object

from which the view has been taken (obj). As you can see from the output above, the self is always a newly created

instance of our subclass, and the type of obj differs for the three instance creation methods:

• When called from the explicit constructor, obj is None

• When called from view casting, obj can be an instance of any subclass of ndarray, including our own.

• When called in new-from-template, obj is another instance of our own subclass, that we might use to update

the new self instance.

Because __array_finalize__ is the only method that always sees new instances being created, it is the sensible

place to ﬁll in instance defaults for new object attributes, among other tasks.

This may be clearer with an example.

2.8.7 Simple example - adding an extra attribute to ndarray

import numpy as np

class InfoArray(np.ndarray):

def __new__(subtype, shape, dtype=float, buffer=None, offset=0,

strides=None, order=None, info=None):

# Create the ndarray instance of our type, given the usual

# ndarray input arguments. This will call the standard

# ndarray constructor, but return an object of our type.

# It also triggers a call to InfoArray.__array_finalize__

obj = np.ndarray.__new__(subtype, shape, dtype, buffer, offset, strides,

order)

# set the new ’info’ attribute to the value passed

obj.info = info

# Finally, we must return the newly created object:

return obj

def __array_finalize__(self, obj):

# ‘‘self‘‘ is a new object resulting from

# ndarray.__new__(InfoArray, ...), therefore it only has

# attributes that the ndarray.__new__ constructor gave it -

# i.e. those of a standard ndarray.

# We could have got to the ndarray.__new__ call in 3 ways:

# From an explicit constructor - e.g. InfoArray():

# obj is None

# (we’re in the middle of the InfoArray.__new__

# constructor, and self.info will be set when we return to

# InfoArray.__new__)

if obj is None: return

# From view casting - e.g arr.view(InfoArray):

# obj is arr

# (type(obj) can be InfoArray)

# From new-from-template - e.g infoarr[:3]

2.8. Subclassing ndarray 39