def row(self, r, v=None):
        if r < 0 or r > 3: raise IndexError, 'row index out of range'
        if v is None: return V(self._v[r*4:(r+1)*4])
        e = v
        if isinstance(v, V): e = v._v
        if len(e) != 4: raise ValueError, 'new row must include 4 values'
        self._v[r*4:(r+1)*4] = e
        return v
Returns or replaces a vector representing a row of the matrix.
dataset/ETHPy150Open hhatto/autopep8/test/vectors_example.py/M.row