a slice of python

1.py

In [118]: alpha
Out[118]: ['a', 'b', 'c', 'd', 'e', 'f']

In [119]: alpha[20]
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-119-c5b145def39d> in <module>()
----> 1 alpha[20]

IndexError: list index out of range

In [120]: alpha[20:]
Out[120]: []

2.py

In [122]: alpha = ['a', 'b', 'c', 'd', 'e', 'f']

In [123]: alpha
Out[123]: ['a', 'b', 'c', 'd', 'e', 'f']

In [124]: alpha[0]
Out[124]: 'a'

In [127]: alpha[0] = 'A'

In [128]: alpha
Out[128]: ['A', 'b', 'c', 'd', 'e', 'f']

In [129]: alpha[0,1]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-129-c7eb16585371> in <module>()
----> 1 alpha[0,1]

TypeError: list indices must be integers, not tuple

3.py

In [130]: alpha[0:1]
Out[130]: ['A']

In [131]: alpha[0:1] = 'a'

In [132]: alpha
Out[132]: ['a', 'b', 'c', 'd', 'e', 'f']

In [133]: alpha[0:2] = ['A', 'B']

In [134]: alpha
Out[134]: ['A', 'B', 'c', 'd', 'e', 'f']

In [135]: alpha[2:2] = ['x', 'xx']

In [136]: alpha
Out[136]: ['A', 'B', 'x', 'xx', 'c', 'd', 'e', 'f']

4.py

In [137]: alpha = ['a', 'b', 'c', 'd', 'e', 'f']

In [142]: alpha[1:5:2] 
Out[142]: ['b', 'd']

In [143]: alpha[-1:-5:-2]
Out[143]: ['f', 'd']

In [144]: alpha[1:5:-2]
Out[144]: []

In [145]: alpha[-1:-5:2]      
Out[145]: []

5.py

def py_slice_get_indices_ex(obj, start=None, stop=None, step=None):

    length = len(obj)

    if step is None:
        step = 1
    if step == 0:
        raise Exception("Step cannot be zero.")

    if start is None:
        start = 0 if step > 0 else length - 1
    else:
        if start < 0:
            start += length
        if start < 0:
            start = 0 if step > 0 else -1
        if start >= length:
            start = length if step > 0 else length - 1

    if stop is None:
        stop = length if step > 0 else -1
    else:
        if stop < 0:
            stop += length
        if stop < 0:
            stop = 0 if step > 0 else -1
        if stop >= length:
            stop = length if step > 0 else length - 1

    if (step < 0 and stop >= start) or (step > 0 and start >= stop):
        slice_length = 0
    elif step < 0:
        slice_length = (stop - start + 1)/(step) + 1
    else:
        slice_length = (stop - start - 1)/(step) + 1
    
    return (start, stop, step, slice_length)

6.py

In [21]: alpha = ['a', 'b', 'c', 'd', 'e', 'f']

In [22]: s = slice(None, None, None)

In [23]: s
Out[23]: slice(None, None, None)

In [24]: s.indices(len(alpha)) 
Out[24]: (0, 6, 1)

In [25]: range(*s.indices(len(alpha)))
Out[25]: [0, 1, 2, 3, 4, 5]

In [26]: s = slice(None, None, -1) 

In [27]: range(*s.indices(len(alpha)))
Out[27]: [5, 4, 3, 2, 1, 0]

In [28]: s = slice(None, 3, -1)        

In [29]: range(*s.indices(len(alpha)))
Out[29]: [5, 4]

7.py

In [170]: alpha
Out[170]: ['a', 'b', 'c', 'd', 'e', 'f']

In [171]: alpha[]
  File "<ipython-input-171-a17d5cfb2c2f>", line 1
    alpha[]
          ^
SyntaxError: invalid syntax

In [172]: alpha[:]
Out[172]: ['a', 'b', 'c', 'd', 'e', 'f']

In [173]: alpha[::]
Out[173]: ['a', 'b', 'c', 'd', 'e', 'f']

In [174]: alpha[:-1:]
Out[174]: ['a', 'b', 'c', 'd', 'e']

In [175]: alpha[:2:-1]
Out[175]: ['f', 'e', 'd']

In [176]: alpha[:10000000000000000000000000000000000000000000000000000000]  
Out[176]: ['a', 'b', 'c', 'd', 'e', 'f']

In [177]: alpha[::1000000000]
Out[177]: ['a']

In [178]: alpha[::-1]
Out[178]: ['f', 'e', 'd', 'c', 'b', 'a']