[GitHub] [arrow] spenczar opened a new issue, #35747: [Python] Add from_numpy_ndarray and to_numpy_ndarray to ListArray types

["spenczar (via GitHub)" <gi...@apache.org>]

spenczar opened a new issue, #35747:
URL: https://github.com/apache/arrow/issues/35747

   ### Describe the enhancement requested
   
   Interoperation between numpy `ndarray`s and Arrow's ListArray types (ListArray, LargeListArray, FixedSizeListArray) is a bit tricky. 
   
   It's hard to construct values: one must convert to a Python list-of-lists first, which is unnecessarily expensive:
   
   ```py
   >>> import numpy as np
   >>> import pyarrow as pa
   >>> np_values = np.ones((3, 2), np.float64())
   >>> pa_dtype = pa.list_(pa.float64())
   >>> pa_values= pa.array(np_values, type=pa_dtype)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "pyarrow/array.pxi", line 323, in pyarrow.lib.array
     File "pyarrow/array.pxi", line 83, in pyarrow.lib._ndarray_to_array
     File "pyarrow/error.pxi", line 100, in pyarrow.lib.check_status
   pyarrow.lib.ArrowInvalid: only handle 1-dimensional arrays
   >>> pa_values = pa.array(np_values.tolist(), type=pa_dtype)
   <pyarrow.lib.ListArray object at 0x11ba433a0>
   [
     [
       1,
       1
     ],
     [
       1,
       1
     ],
     [
       1,
       1
     ]
   ]
   ```
   
   Likewise, converting to a numpy ndarray from a Pyarrow ListArray type is tricky, as described in https://github.com/apache/arrow/issues/35622. That issue describes trickiness with FixedSizeListArrays, but the same is true of ListArrays, which often _might_ have equal-length lists in every entry, making them amenable to presentation as an ndarray.
   
   I'd like to propose the following 6 new methods:
    - `FixedSizeListArray.from_numpy_ndarray(values, type)`: 
      Constructs a new FixedSizeListArray from `values`, which must be a numpy ndarray with `ndim == 2`. 
      `type` is optional; it will be looked up from the ndarray's `dtype` if unset. 
      If `type` is set, values of the ndarray's dtype must be convertible to the provided type.
      
    - `FixedSizeListArray.to_numpy_ndarray(self)`: 
      Returns the FixedSizeListArray's values as a numpy ndarray with a shape of `(len(self), self.type.list_size)`. 
    
      If any of the FixedSizeListArray's values are `null`, raises an error.
      
      If any of the FixedSizeListArray's values _contain_ a `null`, then returns a `ndarray` with `nan` in the null spots, and with `dtype` set to `float64`, or `None` in the null spots and `dtype` of `object` if a conversion to `float64` is not possible. This matches the behavior of `Array.to_numpy` for primitive types.
      
   - `ListArray.from_numpy_ndarray(values, type)`: 
     Works just like `FixedSizeListArray.from_numpy_ndarray`.
   - `ListArray.to_numpy_ndarray(self)`: 
     Works like `FixedSizeListArray.to_numpy_ndarray`, with an additional check that all list elements are of equal length. If any are different, then raises an error.
   
   and same for LargeListArray as for ListArray, bringing the total to 6.
   
   The FixedSizeListArray methods already have an implementation in the `FixedShapeTensor` extension type. Those implementation are actually a bit more complicated because of tensors' support for permutations:
   https://github.com/apache/arrow/blob/95c33d82e67c9500b08ea7086a46b447d65bc14b/python/pyarrow/array.pxi#L3149
   https://github.com/apache/arrow/blob/95c33d82e67c9500b08ea7086a46b447d65bc14b/python/pyarrow/array.pxi#L3164
   
   ### Component(s)
   
   Python


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Re: [I] [Python] Add from_numpy_ndarray and to_numpy_ndarray to ListArray types [arrow]

["thisisnic (via GitHub)" <gi...@apache.org>]

thisisnic commented on issue #35747:
URL: https://github.com/apache/arrow/issues/35747#issuecomment-3921742488

   This issue has been marked as stale because it has had no activity in the past 2 years. Please remove the stale label or comment below, or this issue will be closed in 14 days.


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