Is there a simple way to create an immutable NumPy array?
If one has to derive a class from ndarray to do this, what's the minimum set of methods that one has to override to achieve immutability?
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Why do you need immutability?kennytm– kennytm2011-04-04 16:20:23 +00:00Commented Apr 4, 2011 at 16:20
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55@KennyTM To avoid coding errors caused by accidentally modifying something that's assumed invariant.NPE– NPE2011-04-04 16:22:12 +00:00Commented Apr 4, 2011 at 16:22
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3 Answers
You can make a numpy array unwriteable:
a = np.arange(10)
a.flags.writeable = False
a[0] = 1
# Gives: ValueError: assignment destination is read-only
Also see the discussion in this thread:
http://mail.scipy.org/pipermail/numpy-discussion/2008-December/039274.html
and the documentation:
http://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.flags.html
13 Comments
lafras
Alternatively,
a.setflags(write=False).lafras
@aix: A quick look at the documentation would make it seem as if the two approaches are identical. Personally, I prefer using a method to set attributes.
endolith
Does this also make it memoizable?
James Parker
IMPORTANT!! Numpy DOES NOT have an immutable array. Arrays with
.flags.writeable = False are still not immutable. If x is an array, y = x[:]; x.flags.writeable = False; y[0] = 5 updates the first element of x to 5.
Damon Maria
@JamesParker It seems the flags come with the array when you slice. So if the
writable flag is set to False before you slice then then y above would fail on update. |
Setting the flag directly didn't work for me, but using ndarray.setflags did work:
a = np.arange(10)
a.setflags(write=False)
a[0] = 1 # ValueError
Comments
I have a subclass of Array at this gist: https://gist.github.com/sfaleron/9791418d7023a9985bb803170c5d93d8
It makes a copy of its argument and marks that as read-only, so you should only be able to shoot yourself in the foot if you are very deliberate about it. My immediate need was for it to be hashable, so I could use them in sets, so that works too. It isn't a lot of code, but about 70% of the lines are for testing, so I won't post it directly.
Note that it's not a drop-in replacement; it won't accept any keyword args like a normal Array constructor. Instances will behave like Arrays, though.
