find parent array of numpy slice array

Question

Is it possible to find the parent array of a slice ie. the array that the slice is taken from? I would like to do this so I can add functionality to matplotlib plots which allows you to change which slice of an array you are viewing interactively in a plot. For instance, if I do this

plt.pcolormesh(myArray[0,:,:])

I would like to be able to run some code to change the plot to

plt.pcolormesh(myArray[1,:,:])

but to do that I need to know that myArray[0,:,:] is a slice of myArray.

Thanks Niall

mgilson · Accepted Answer · 2013-05-30 12:35:20Z

4

With simple slices, you can look at the base attribute:

a = np.arange(50)
b = a[10:20]
print (b.base is a)

However, I don't believe that this is guaranteed to work in all circumstances...(depending on contiguousness of a, etc.)

answered May 30, 2013 at 12:35

mgilson

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6 Comments

seberg Over a year ago

Basically, there is another problem with this. It could be that b.base is a.base and not b.base is a also. There is np.may_share_memory, which will tell you that the two must have some common base, but there is no way to find out if one is a slice of the other, since two arrays may be incompatible slices of the same array.

mgilson Over a year ago

@seberg -- Precisely. This will only work in very simple circumstances. However, I think that it's a common enough simple circumstance to be worth mentioning.

seberg Over a year ago

Yeah, just wanted to warn, especially since there is a change with version 1.7. here. In his case, and if you also throw in strides information, you should be able to get quite far though.

nrob Over a year ago

Unfortunately, as helpful as this is, its doesn't seem to work with matplotlib.cm.get_array() - the base of that array is just the same array, but the looks oft things.

Joe Kington Over a year ago

@nrob - For what you're wanting to do, there are simpler ways. Just write a wrapper class that holds on to the original data and sets the viewed data to a new slice on a button press or slider update. As a quick example: gist.github.com/joferkington/5682690

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