Extending numpy arrays

Question

I'm new to Python/numpy.

I'm trying to extend numpy.array to give it some functions that make it nice for representing images (e.g. convert to greyscale etc).

import numpy as np
import cv2
from support import *
import matplotlib.pyplot as plt

class Frame(np.array):
    def __init__(self):
        print "new frame"


f = Frame()

currently this gives me:

  File "o.py", line 6, in <module>
    class Frame(np.array):
TypeError: Error when calling the metaclass bases
    cannot create 'builtin_function_or_method' instances

I don't understand why this is an issue for Python?

Just as a side note you should check out: PIL Python Imaging Library pythonware.com/products/pil/#pil117. It has these abilities and converting to/from ndarray is quite simple — odedsh
– odedsh, Commented Apr 16, 2014 at 16:44
And if you are going to go with PIL, you should try its fork pillow, which is a more updated version that is also compatible with Python 3. — Steven Rumbalski
– Steven Rumbalski, Commented Apr 16, 2014 at 16:55

JoshAdel · Accepted Answer · 2014-04-16 16:45:04Z

2

You want to be subclassing np.ndarray, not np.array, but it's a little more complicated than just swapping on out for the other in your example. It's probably worth taking a look at the documentation: http://docs.scipy.org/doc/numpy/user/basics.subclassing.html

answered Apr 16, 2014 at 16:45

JoshAdel

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1 Comment

cjm2671 Over a year ago

Thanks for this. In the end, I classed 'around' it, rather than extend it, looks like the safer option.

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