Slice. A Python slice extracts elements, based on a start and stop. We specify an optional first index, an optional last index, and an optional step.
Understanding slice syntax in Python is important. It allows us not just to slice arrays, but also take substrings. This can be used in many programs.
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.
values = [100, 200, 300, 400, 500]
# Get elements from second index to third index.
slice = values[1:3]
print(slice)[200, 300]
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.
values = [100, 200, 300, 400, 500]
# Slice from third index to index one from last.
slice = values[2:-1]
print(slice)[300, 400]
Start, end. 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.
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)[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.
word = "something"# Get first four characters.
part = word[:4]
print(part)some
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.
values = "AaBbCcDdEe"# Starting at 0 and continuing until end, take every other char.
evens = values[::2]
print(evens)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.
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.
values = ["carrot", "rat"]
# Must use indexes or None.
test = values["friend":"dog"]
print(test)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
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.
# Create a slice object.
example = slice(1, 10, 0)
print(example.start, example.stop, example.step)1 10 0
Syntax review. The special syntax for this operation is at first confusing. But with practice, slicing becomes easy. When we omit a value, a default is used.
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.
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.
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.
Summary. Slices are used throughout Python programs—we take substrings, copy lists and resize lists. This syntax is versatile. It provides useful defaults when values are omitted.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Aug 29, 2022 (simplify).
