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Slicing Deep Dive

Slicing is one of the most useful syntax features in Python and one worth getting familiar with early. It can not only extract subsequences but also reverse, step, and shallow-copy. Many patterns become very natural once you write them enough.

Basic Syntax​

sequence[start:stop:step]
  • start: Start position, inclusive
  • stop: End position, exclusive
  • step: Step size

Most Common Patterns​

nums = [0, 1, 2, 3, 4, 5]

print(nums[1:4])   # [1, 2, 3]
print(nums[:3])    # [0, 1, 2]
print(nums[3:])    # [3, 4, 5]
print(nums[::2])   # [0, 2, 4]
print(nums[::-1])  # [5, 4, 3, 2, 1, 0]

Negative Indexing​

text = "abcdef"

print(text[-1])    # f
print(text[-3:])   # def

Negative indexing counts from the end, making it great for handling "the last few elements".

Slicing can not only read, but also modify​

nums = [0, 1, 2, 3, 4, 5]
nums[1:4] = [10, 20, 30]
print(nums)

You can also delete a segment:

del nums[2:4]

Common Uses​

1. Shallow copy a list​

copied = nums[:]

2. Reverse a sequence​

reversed_text = text[::-1]

3. Take every k-th element​

evens = nums[::2]

An easily overlooked point​

Slices usually return new objects, but whether they're "completely independent" depends on whether the elements in the sequence themselves are mutable. For more specific examples, see:

My quick-reference shorthand​

  • [:n]: First n elements
  • [n:]: From n to the end
  • [-n:]: Last n elements
  • [::k]: Every k-th element
  • [::-1]: Reverse