Get column of a matrix python with index operator

Question

How can I refactor this code to generalize the argument parsing? I feel like this code can be better but I can't find a way. The usage syntax of the index operator is similar to the NumPy indexing syntax.

    def __getitem__(self, arg):
    """
    [] operator to index elements in the matrix
    Usage: 
    >> mat[0] returns first row
    >> mat[3, :] return the 4th row
    >> mat[:, 7] return the 8th column
    >> mat[0, 0] returns first element
    """
    if isinstance(arg, int):
        return self.matrix[arg]
    elif isinstance(arg, tuple):
        if isinstance(arg[0], int) and isinstance(arg[1], int):
            y, x = arg
            return self.matrix[x][y]
        elif isinstance(arg[0], slice) and isinstance(arg[1], int):
            return self.get_column(arg[1])
        elif isinstance(arg[0], int) and isinstance(arg[1], slice):
            return self.get_column(arg[1])
    else :
        raise TypeError('Invalid indexing arguments type', arg)

Also, how can I make a setitem function that uses the [] operator to set columns without rewriting the getitem function inside it? Here is my current implementation without the possibility of setting a column.

    def __setitem__(self, arg, value):
    if isinstance(value, Fraction):
        self[arg] = value
    elif isinstance(value, list) and all(isinstance(elem, Fraction) for elem in value):
        self[arg] = value
    else:
        raise TypeError('Invalid value type', value)

Gareth Rees · Accepted Answer · 2018-03-15 13:53:51Z

Before improving the code it would be worth fixing the following problems.

The docstring says:
```
>> mat[3, :] return the 4th row
```
but the implementation tries to return a column:
```
elif isinstance(arg[0], int) and isinstance(arg[1], slice):
    return self.get_column(arg[1])
```
but since arg[1] is a slice object, this will fail in some way. (I can't say exactly how since you didn't show us the code for get_column.)
The indexing is inconsistent. As it says in the docstring:
```
>> mat[3, :] return the 4th row
```
but to get the first element of the fourth row, you have to write:
```
mat[0, 3]
```
because the indexes get swapped:
```
y, x = arg
return self.matrix[x][y]
```
This is contrary to the way indexing works in NumPy and so seems very likely to lead to confusion.

If you pass a tuple of length 1 then this causes an IndexError:

>>> mat[:,]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "cr189408.py", line 21, in __getitem__
    elif isinstance(arg[0], slice) and isinstance(arg[1], int):
IndexError: tuple index out of range

If you pass a pair of slices then this is ignored and __getitem__ returns None.
```
>>> mat[:,:] is None
True
```
In NumPy this would return the whole matrix.
The code only checks whether the argument is a slice or not, and if it is a slice, it assumes that the whole row or column is required. This is right if a bare : was passed, but misleading if some other slice was passed. In NumPy mat[0:2, 0] gets the first two element of column 0, but the code in the post returns the whole of column 0.

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