Slicing of SolutionArray raises error

[ischoegl]

System information

• Cantera version: 2.5.0a4
• OS: Ubuntu
• Python/MATLAB version: Python 3.6.8

Expected behavior

Slicing of SolutionArray objects should work as usual (as indexing is implemented).

Actual behavior

Slicing does not work.

To Reproduce

Steps to reproduce the behavior:

In [1]: import cantera as ct
   ...: gas = ct.Solution('gri30.yaml')
   ...: arr = ct.SolutionArray(gas, 10)

In [2]: arr[0]
Out[2]: <cantera.composite.SolutionArray at 0x7ffa61a364e0>

In [3]: arr[:]
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-47ed00660ce3> in <module>()
----> 1 arr[:]

/usr/local/lib/python3.6/dist-packages/cantera/composite.py in __getitem__(self, index)
    534     def __getitem__(self, index):
    535         states = self._states[index]
--> 536         shape = states.shape[:-1]
    537         return SolutionArray(self._phase, shape, states)
    538 

AttributeError: 'list' object has no attribute 'shape'

Additional context

The error message clearly points at the issue.

[sin-ha]

@ischoegl I am new and wish to contribute to Cantera learning about its codebase kindly review my PR

[ischoegl]

@ischoegl I am new and wish to contribute to Cantera learning about its codebase kindly review my PR

No problem. Welcome! Regarding review - the solution is virtually identical to 2f8f685, so I don't have anything to add other than there shouldn't be a second newline below the method.

[bryanwweber]

Interestingly, a zero-length slice still raises an error even after #816 is merged:

In [8]: import cantera as ct
In [9]: gas = ct.Solution('gri30.yaml')
In [10]: arr = ct.SolutionArray(gas, 10)
In [11]: arr[slice(0)]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-95d215c2766e> in <module>
----> 1 arr[slice(0)]

~/GitHub/cantera/build/python/cantera/composite.py in __getitem__(self, index)
    548         if(isinstance(states, list)):
    549             num_rows = len(states)
--> 550             return SolutionArray(self._phase, num_rows, states)
    551         else:
    552             shape = states.shape[:-1]

~/GitHub/cantera/build/python/cantera/composite.py in __init__(self, phase, shape, states, extra)
    512         else:
    513             self._indices = list(np.ndindex(self._shape))
--> 514             self._output_dummy = self._states[..., 0]
    515
    516         reserved = self.__dir__()

TypeError: list indices must be integers or slices, not tuple

whereas that returns a zero-length list/array for a plain Python list and a NumPy array. So I think there is one more edge case to handle here.

[ischoegl]

@bryanwweber / @sin-ha ... didn’t anticipate this one either: good catch.

[sin-ha]

the problem extends for this case too i.e. for all arr[x:x]

>>>arr[2:2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/cantera/composite.py", line 540, in __getitem__
    return SolutionArray(self._phase, num_rows, states)
  File "/usr/local/lib/python3.7/site-packages/cantera/composite.py", line 514, in __init__
    self._output_dummy = self._states[..., 0]
TypeError: list indices must be integers or slices, not tuple

The problem exists in basic handling of shapes in the init class where the len(shapes) == 0, I will update the PR
thanks @bryanwweber @ischoegl