# python – Understanding slice notation

## python – Understanding slice notation

Its pretty simple really:

```
a[start:stop] # items start through stop-1
a[start:] # items start through the rest of the array
a[:stop] # items from the beginning through stop-1
a[:] # a copy of the whole array
```

There is also the `step`

value, which can be used with any of the above:

```
a[start:stop:step] # start through not past stop, by step
```

The key point to remember is that the `:stop`

value represents the first value that is *not* in the selected slice. So, the difference between `stop`

and `start`

is the number of elements selected (if `step`

is 1, the default).

The other feature is that `start`

or `stop`

may be a *negative* number, which means it counts from the end of the array instead of the beginning. So:

```
a[-1] # last item in the array
a[-2:] # last two items in the array
a[:-2] # everything except the last two items
```

Similarly, `step`

may be a negative number:

```
a[::-1] # all items in the array, reversed
a[1::-1] # the first two items, reversed
a[:-3:-1] # the last two items, reversed
a[-3::-1] # everything except the last two items, reversed
```

Python is kind to the programmer if there are fewer items than you ask for. For example, if you ask for `a[:-2]`

and `a`

only contains one element, you get an empty list instead of an error. Sometimes you would prefer the error, so you have to be aware that this may happen.

### Relation to `slice()`

object

The slicing operator `[]`

is actually being used in the above code with a `slice()`

object using the `:`

notation (which is only valid within `[]`

), i.e.:

```
a[start:stop:step]
```

is equivalent to:

```
a[slice(start, stop, step)]
```

Slice objects also behave slightly differently depending on the number of arguments, similarly to `range()`

, i.e. both `slice(stop)`

and `slice(start, stop[, step])`

are supported.

To skip specifying a given argument, one might use `None`

, so that e.g. `a[start:]`

is equivalent to `a[slice(start, None)]`

or `a[::-1]`

is equivalent to `a[slice(None, None, -1)]`

.

While the `:`

-based notation is very helpful for simple slicing, the explicit use of `slice()`

objects simplifies the programmatic generation of slicing.

The Python tutorial talks about it (scroll down a bit until you get to the part about slicing).

The ASCII art diagram is helpful too for remembering how slices work:

```
+---+---+---+---+---+---+
| P | y | t | h | o | n |
+---+---+---+---+---+---+
0 1 2 3 4 5 6
-6 -5 -4 -3 -2 -1
```

One way to remember how slices work is to think of the indices as pointing

betweencharacters, with the left edge of the first character numbered 0. Then the right edge of the last character of a string ofncharacters has indexn.

#### python – Understanding slice notation

Enumerating the possibilities allowed by the grammar:

```
>>> seq[:] # [seq[0], seq[1], ..., seq[-1] ]
>>> seq[low:] # [seq[low], seq[low+1], ..., seq[-1] ]
>>> seq[:high] # [seq[0], seq[1], ..., seq[high-1]]
>>> seq[low:high] # [seq[low], seq[low+1], ..., seq[high-1]]
>>> seq[::stride] # [seq[0], seq[stride], ..., seq[-1] ]
>>> seq[low::stride] # [seq[low], seq[low+stride], ..., seq[-1] ]
>>> seq[:high:stride] # [seq[0], seq[stride], ..., seq[high-1]]
>>> seq[low:high:stride] # [seq[low], seq[low+stride], ..., seq[high-1]]
```

Of course, if `(high-low)%stride != 0`

, then the end point will be a little lower than `high-1`

.

If `stride`

is negative, the ordering is changed a bit since were counting down:

```
>>> seq[::-stride] # [seq[-1], seq[-1-stride], ..., seq[0] ]
>>> seq[high::-stride] # [seq[high], seq[high-stride], ..., seq[0] ]
>>> seq[:low:-stride] # [seq[-1], seq[-1-stride], ..., seq[low+1]]
>>> seq[high:low:-stride] # [seq[high], seq[high-stride], ..., seq[low+1]]
```

Extended slicing (with commas and ellipses) are mostly used only by special data structures (like NumPy); the basic sequences dont support them.

```
>>> class slicee:
... def __getitem__(self, item):
... return repr(item)
...
>>> slicee()[0, 1:2, ::5, ...]
(0, slice(1, 2, None), slice(None, None, 5), Ellipsis)
```