Numpy Array vs. Numpy Masked Array -- Madness

gistfile1.py

import numpy as np
import numpy.ma as ma

# Create a 3x3 array in regular numpy
a = np.arange(9).reshape((3,3))

# Get the middle row
b = a[1]

# Change the middle value in the middle row
b[1] = 20

# The value at a[1,1] will be 20
print 'Row slice: {0}'.format(b)
print 'Regular array:\n{0}'.format(a)

# Create a 3x3 masked array in numpy.ma
a = ma.masked_array(np.arange(9).reshape((3,3)), mask=np.ones((3,3)))

# Get the middle row
b = a[1]

# Change the middle value in the middle row
b[1] = 20

# The value at a[1,1] will still be marked as missing.
print 'Row slice: {0}'.format(b)
print 'Masked array:\n{0}'.format(a)