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BUG: Fix typecasting problem in scipy.sparse.lil_matrix truediv #19408

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merged 5 commits into from
Nov 8, 2023
Merged

BUG: Fix typecasting problem in scipy.sparse.lil_matrix truediv #19408

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7b6dc44
Fix typecasting problem
vetschn Oct 19, 2023
c8fb9a8
Update scipy/sparse/_lil.py
vetschn Oct 20, 2023
642259e
Added test
vetschn Oct 23, 2023
2c38d3d
Rename and format
vetschn Oct 23, 2023
3447c36
Modify new lil_array's dtype directly
vetschn Oct 25, 2023
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4 changes: 4 additions & 0 deletions scipy/sparse/_lil.py
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Original file line number Diff line number Diff line change
Expand Up @@ -287,6 +287,10 @@ def _mul_scalar(self, other):
def __truediv__(self, other): # self / other
if isscalarlike(other):
new = self.copy()
new_dtype = np.result_type(self.dtype, np.array(other).dtype)
if new_dtype != self.dtype:
new.data = new.data.astype(new_dtype)
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This won't work. The data member of LIL-format objects is a numpy array of Python lists, with dtype=object.

To change the dtype, we need to do something like this:

for i, rowvals in enumerate(new.data):
    new.data[i] = rowvals.astype(new_dtype)

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Sorry, I should have noticed this. Isn't just the following what we really want?

new.dtype =  np.result_type(self, other)

The list data is actually modified correctly. Things only start breaking when trying to convert to a different storage format, for example.

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Hmmm... sorry that my solution is actually quite wrong in a number of ways.

@vetschn is correct that the values in new.data are correct (complex values).
It is the dtype that is not updated. The values are actually python objects. So they morph to what they need to become to hold the correct value.

It is definitely true that we need a test. That will show us what will work. :)
I think @vetschn 's solution should work. I have tried it at an interactive interpreter.
But there are many subtleties with dtype that I haven't experienced. :)

new.dtype = new_dtype
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# Divide every element by this scalar
for j, rowvals in enumerate(new.data):
new.data[j] = [val/other for val in rowvals]
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