Item 6: Avoid Using start, end, and stride in a Single Slice

In addition to basic slicing (see Item 5: “Know How to Slice Sequences”), Python has special syntax for the stride of a slice in the form somelist[start:end:stride]. This lets you take every nth item when slicing a sequence. For example, the stride makes it easy to group by even and odd indexes in a list.

a = [‘red’, ‘orange’, ‘yellow’, ‘green’, ‘blue’, ‘purple’] 
odds = a[::2]
evens = a[1::2] print(odds)
print(evens)

>>>

[‘red’, ‘yellow’, ‘blue’] 
[‘orange’, ‘green’, ‘purple’]

The problem is that the stride syntax often causes unexpected behavior that can introduce bugs. For example, a common Python trick for reversing a byte string is to slice the string with a stride of -1.

x = b’mongoose’ 
y = x[::-1] 
print(y)

>>> 
b’esoognom’

That works well for byte strings and ASCII characters, but it will break for Unicode characters encoded as UTF-8 byte strings.

w = ‘谢谢’
x = w.encode(‘utf-8’) 
y = x[::-1]
z = y.decode(‘utf-8’)


>>>

UnicodeDecodeError: ‘utf-8’ codec can’t decode byte 0x9d in position 0: invalid start byte

Are negative strides besides -1 useful? Consider the following examples.

a = [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’]
a[::2]  # [‘a’, ‘c’, ‘e’, ‘g’] 
a[::-2] # [‘h’, ‘f’, ‘d’, ‘b’]

Here, ::2 means select every second item starting at the beginning. Trickier, ::-2 means select every second item starting at the end and moving backwards.

What do you think 2::2 means? What about -2::-2 vs. -2:2:-2 vs. 2:2:-2?

a[2::2]     # [‘c’, ‘e’, ‘g’]
a[-2::-2]   # [‘g’, ‘e’, ‘c’, ‘a’] 
a[-2:2:-2]  # [‘g’, ‘e’]
a[2:2:-2]   # []

The point is that the stride part of the slicing syntax can be extremely confusing. Having three numbers within the brackets is hard enough to read because of its density. Then it’s not obvious when the start and end indexes come into effect relative to the stride value, especially when stride is negative.

To prevent problems, avoid using stride along with start and end indexes. If you must use a stride, prefer making it a positive value and omit start and end indexes. If you must use stride with start or end indexes, consider using one assignment to stride and another to slice.

b = a[::2]   # [‘a’, ‘c’, ‘e’, ‘g’] 
c = b[1:-1]  # [‘c’, ‘e’]

Slicing and then striding will create an extra shallow copy of the data. The first operation should try to reduce the size of the resulting slice by as much as possible. If your program can’t afford the time or memory required for two steps, consider using the itertools built-in module’s islice method (see Item 46: “Use Built-in Algorithms and Data Structures”), which doesn’t permit negative values for start, end, or stride.

Things to Remember

• Specifying start, end, and stride in a slice can be extremely confusing.

• Prefer using positive stride values in slices without start or end indexes. Avoid negative stride values if possible.

• Avoid using start, end, and stride together in a single slice. If you need all three parameters, consider doing two assignments (one to slice, another to stride) or using islice from the itertools built-in module.