Python Slice Examples: Start, Stop and Step

Slice. A slice extracts elements, based on a start and stop.

We take slices on many types in Python. The special syntax for this operation is at first confusing.

But with practice, slicing becomes easy. We specify an optional first index, an optional last index, and an optional step. When we omit a value, a default is used.

Based on:

Python 3

Slice syntax forms:

	     Get elements from...
values[1:3]  Index 1 through index 3.
values[2:-1] Index 2 through index one from last.
values[:2]   Start through index 2.
values[2:]   Index 2 through end.
values[::2]  Start through end, skipping ahead 2 places each time.

List. This program takes a slice of a list. We first create a list with five numeric elements. We specify in the slice two values: 1 and 3. At index 1, we have the value 200.

Note: Python elements begin at index 0. So the first index (for slices or indexes) is zero.

Here: We continue up to (but not including) index 3, which is the value 400. So our result list has 200 and 300 in it.

Python program that slices list

values = [100, 200, 300, 400, 500]
# Get elements from second index to third index.
slice = values[1:3]
print(slice)

Output

[200, 300]

Resize list. A slice can be used to resize a list. We can remove elements past a certain length. This makes the list smaller (reduces its size). We can also pad a list with new elements.

Resize List

Negative. The second index in slice notation may be negative. This means counting begins from the last index. So a negative one means the same as "length minus one."

So: You can reduce an array length by one by using a second index of negative one. Leave the first index undefined.

Python program that slices, negative second index

values = [100, 200, 300, 400, 500]
# Slice from third index to index one from last.
slice = values[2:-1]
print(slice)

Output

[300, 400]

Start, end. Are you lazy? This could be a positive thing. In slicing, if you omit the first or second index, it means "start" or "end." This is clearer than specifying 0 or the length.

Tip: Programs with less confusing syntax tend to develop fewer bugs over time. So lazy programmers are sometimes the best ones.

Python program that slices, starts and ends

values = [100, 200, 300, 400, 500]
# Slice from start to second index.
slice = values[:2]
print(slice)

# Slice from second index to end.
slice = values[2:]
print(slice)

Output

[100, 200]
[300, 400, 500]

Strings. We use the slice notation on strings. In this example, we omit the first index to start at the beginning, and then consume four characters total. We extract the first four letters.

Substring

Python program that slices string

word = "something"
# Get first four characters.
part = word[:4]
print(part)

Output

some

Between. Sometimes we must use additional logic to get a slice. We can slice characters between two other strings with find and rfind.

Between, Before, After

Copy. With slicing, we can copy sequences like lists. We assign a new list variable to a slice with no specified start or stop values. So the slice copies the entire list and returns it.

Copy List

Step. There is a third, optional index used in slicing: the step. If the step is two, we advance two places after an element is copied. So we can skip over elements.

Tip: This allows us to quickly access all odd or even elements in an array. No complicated loops are required.

Python program that uses step, slices

values = "AaBbCcDdEe"

# Starting at 0 and continuing until end, take every other char.
evens = values[::2]
print(evens)

Output

ABCDE

Invalid. Slicing is safe. It will cause no errors assuming you use integers as arguments. Often when programming, a tiny error will cause an extreme disaster and the program will implode.

However: Slicing alleviates some of these hassles. But this too can be a problem. Our program might silently fail, causing confusion later.

Python program that uses invalid indexes

values = [9, 8, 7, 6]

# This causes no error.
test = values[555:555:5555]
print(test)

TypeError. Arguments to slice must be of a valid type. They cannot be strings or collections. If an object has an __index__ method, it can be used in place of an integer as a slice index.

Exceptions

Python that causes TypeError

values = ["carrot", "rat"]

# Must use indexes or None.
test = values["friend":"dog"]
print(test)

Output

Traceback (most recent call last):
  File "...", line 7, in <module>
    test = values["friend":"dog"]
TypeError: slice indices must be integers or None
    or have an __index__ method

Tuple. Tuples too may be sliced. Slicing notation is standardized throughout Python 3 objects. So once you learn it on lists or strings, you can slice tuples without much trouble.

Tuple

Object. There is a slice built-in method. It returns a slice object. As the documentation states, slice objects are not used in core types, but are sometimes needed in numerical libraries.

So: If you are trying to slice a string, list or other built-in type, pay no attention to the slice method. It won't help you.

Python that creates slice object

# Create a slice object.
example = slice(1, 10, 0)
print(example.start, example.stop, example.step)

Output

1 10 0

A syntax form. Slicing is convenient. Built into the Python language, it is often unavailable in other languages, making copies and extractions hard. We take substrings with slice notation.

Other operations. With slices, we copy lists and resize lists. This syntax is versatile and expressive. It is compact and provides useful defaults when values are omitted.

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