Python Catch-All Unpacking Over Slicing

Often I used indexing and slicing to access items from Python lists and this can be error prone. We can prevent this by using less error-prone and cleaner way of catch-all unpacking.

Simple Example

fruits = ["apple", "banana", "strawberry", "orange", "kiwi"]
first, second, *rest = fruits
print(first, second, rest)
>>>
apple banana ['strawberry', 'orange', 'kiwi']

Advantages of using catch-all unpacking

• The code becomes shorter, easier to read, and less error-prone.
• “A starred expression may appear in any position”

first, *rest, last = fruits
print(first, rest, last)
>>>
apple ['banana', 'strawberry', 'orange'] kiwi

Notes

• The catch-all unpacking must have at least one required part. Otherwise, a SyntaxError will be raised.

  *rest = fruits
  >>>
  File "<stdin>", line 1
  SyntaxError: starred assignment target must be in a list or tuple
  

• Multiple catch-all unpacking expressions in a single-level are not allowed

  first, *middle, *second_middle, last = fruits
  >>>
  File "<stdin>", line 1
  SyntaxError: two starred expressions in assignment
  

• Unpacked values from starred expression always become list instances. If there are no leftover elements from the sequence being unpacked, the catch-all unpack will be an empty list.

  fruits = ["apple", "banana"]
  first, second, *rest = fruits
  print(first, second, rest)
  >>>
  apple banana []
  

• It is possible to unpack values of iterators (ex: generator).

  def generate_csv():
      yield('Year', 'Title', 'Genre')
      ...
  
  it = generate_csv()
  header, *rows = it
  print('CSV Header:', header)
  print('Row count:', len(rows))
  >>>
  CSV Header: ('Year', 'Title', 'Genre')
  Row Count: 200
  

  As we noticed, we can get all the rows via *rows instead of converting iterators to list and accessing by indexing. Similarly, we can unpack header easily.

  • Important Note: A starred expression always becomes list so it may be risky to unpack an iterator because it may use up all memory on your machine and cause program to crash.

References

• Effective Python: Item 13: Prefer Catch-All Unpacking Over Slicing