Python Slice Notation with Comma/List
Question: I have come across some python code with slice notation that I am having
trouble figuring out. It looks like slice notation but uses a comma and a
list:

    
    
    list[:, [1, 2, 3]]
    

Is this syntax valid? If so what does it do?

**edit** looks like it is a 2D numpy array


Answer: Assuming that the object is really a `numpy` array, this is known as [advanced
indexing](http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#advanced-
indexing), and picks out the specified columns:

    
    
    >>> import numpy as np
    >>> a = np.arange(12).reshape(3,4)
    >>> a
    array([[ 0,  1,  2,  3],
           [ 4,  5,  6,  7],
           [ 8,  9, 10, 11]])
    >>> a[:, [1,2,3]]
    array([[ 1,  2,  3],
           [ 5,  6,  7],
           [ 9, 10, 11]])
    >>> a[:, [1,3]]
    array([[ 1,  3],
           [ 5,  7],
           [ 9, 11]])
    

Note that this won't work with the standard Python list:

    
    
    >>> a.tolist()
    [[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11]]
    >>> a.tolist()[:,[1,2,3]]
    Traceback (most recent call last):
      File "<ipython-input-17-7d77de02047a>", line 1, in <module>
        a.tolist()[:,[1,2,3]]
    TypeError: list indices must be integers, not tuple
    

