[Numpy-discussion] Regression: in-place operations (possibly intentional)

Benjamin Root ben.root at ou.edu
Mon Sep 17 09:42:08 EDT 2012

Consider the following code:

import numpy as np
a = np.array([1, 2, 3, 4, 5], dtype=np.int16)
a *= float(255) / 15

In v1.6.x, this yields:
array([17, 34, 51, 68, 85], dtype=int16)

But in master, this throws an exception about failing to cast via same_kind.

Note that numpy was smart about this operation before, consider:
a = np.array([1, 2, 3, 4, 5], dtype=np.int16)
a *= float(128) / 256

array([0, 1, 1, 2, 2], dtype=int16)

Of course, this is different than if one does it in a non-in-place manner:
np.array([1, 2, 3, 4, 5], dtype=np.int16) * 0.5

which yields an array with floating point dtype in both versions.  I can
appreciate the arguments for preventing this kind of implicit casting
between non-same_kind dtypes, but I argue that because the operation is
in-place, then I (as the programmer) am explicitly stating that I desire to
utilize the current array to store the results of the operation, dtype and
all.  Obviously, we can't completely turn off this rule (for example, an
in-place addition between integer array and a datetime64 makes no sense),
but surely there is some sort of happy medium that would allow these sort
of operations to take place?

Lastly, if it is determined that it is desirable to allow in-place
operations to continue working like they have before, I would like to see
such a fix in v1.7 because if it isn't in 1.7, then other libraries (such
as matplotlib, where this issue was first found) would have to change their
code anyway just to be compatible with numpy.

Ben Root
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