Thanks all for the discussion. Actually I am trying to use something like numpy ndarray indexing in the function. Like when I call:

func(a,'1:3,:,2:4'), it knows I want to retrieve a[1:3,:,2:4], and func(a,'1:3,:,4') for a[1:3,:,4] ect.
I am very close now.

#so this function changes the string to list of slice objects.
def convert_string_to_slice(slice_string):
    """
    provide slice_string as '2:3,:', it will return [slice(2, 3, None), slice(None, None, None)]
    """
    slice_list=[]
    split_slice_string_list=slice_string.split(',')
    for sub_slice_string in split_slice_string_list:
        split_sub=sub_slice_string.split(':')
        if len(split_sub)==1:
            sub_slice=slice(int(split_sub[0]))
        else:
            if split_sub[0]=='':
                sub1=None
            else:
                sub1=int(split_sub[0])
            if split_sub[1]=='':
                sub2=None
            else:
                sub2=int(split_sub[1])
            sub_slice=slice(sub1,sub2)
        slice_list.append(sub_slice)
    return slice_list

In [119]: a=np.arange(3*4*5).reshape(3,4,5)

for this it works fine.
In [120]: convert_string_to_slice('1:3,:,2:4')
Out[120]: [slice(1, 3, None), slice(None, None, None), slice(2, 4, None)]

In [121]: a[slice(1, 3, None), slice(None, None, None), slice(2, 4, None)]==a[1:3,:,2:4]
Out[121]:
array([[[ True,  True],
        [ True,  True],
        [ True,  True],
        [ True,  True]],

       [[ True,  True],
        [ True,  True],
        [ True,  True],
        [ True,  True]]], dtype=bool)

And problems happens when I want to retrieve a single number along a given dimension:
because it treats 1:3,:,4 as 1:3,:,:4, as shown below:

In [122]: convert_string_to_slice('1:3,:,4')
Out[122]: [slice(1, 3, None), slice(None, None, None), slice(None, 4, None)]

In [123]: a[1:3,:,4]
Out[123]:
array([[24, 29, 34, 39],
       [44, 49, 54, 59]])

In [124]: a[slice(1, 3, None), slice(None, None, None), slice(None, 4, None)]
Out[124]:
array([[[20, 21, 22, 23],
        [25, 26, 27, 28],
        [30, 31, 32, 33],
        [35, 36, 37, 38]],

       [[40, 41, 42, 43],
        [45, 46, 47, 48],
        [50, 51, 52, 53],
        [55, 56, 57, 58]]])


Then I have a function:

#this function retrieves data from ndarray a by specifying slice_string:
def retrieve_data(a,slice_string):
    slice_list=convert_string_to_slice(slice_string)
    return a[*slice_list]

In the list line of the fuction "retrieve_data" I have problem, I get an invalid syntax error.

    return a[*slice_list]
             ^
SyntaxError: invalid syntax

I hope it's not too long, please comment as you like. Thanks a lot!!!!

Chao