Python Deques

Deques, or double ended queues, are a handy collection type from the collections module that allow for highly efficient append and pop operations from either end of the collection. They also come with a very useful rotate method used for taking an item from one end of the deque, and appending it to the other end.

The collections module is part of Python's Standard Library, but we still need to import it to make use of deques.

from collections import deque

From here, we can create a deque by passing in an iterable to the deque class. This could be a list, a set, or even a string.

Note that to create a deque object containing a single string, we have to pass that string in as an element in another iterable object, otherwise each character will be a separate element in the resulting deque.

oops = deque("abc")  # deque(['a', 'b', 'c'])

Once we have a deque containing the items we want to work with, we can use many of the same methods available to us when using a regular list. This includes append, count, copy, clear, and several others. We do, however, also gain access to some new methods specific to deques.

The first method of note is appendleft. This works just like append, but places the new element at index 0. appendleft has a counterpart in the form of popleft, which functions exactly like pop(0) for the standard list. One limitations of deques is that their pop method doesn't accept any arguments, and always pops the final item in the collection.

base = deque([1, 2, 3])
    
x = base.pop()  # 3
base.appendleft(x)  # deque([3, 1, 2])
    
y = base.popleft()  # 3
base.append(y)  # deque([1, 2, 3])

Another very interesting method available to us when using deques is rotate. rotate allows us to pop an item from one end of the deque and append it to the opposite end.

base = deque([1, 2, 3])
base.rotate()  # deque([3, 1, 2])

In the example above, 3 was popped from the right and appended to the left.

We're not limited to a single rotation, however, and we can control the degree and direction of the rotation by passing in a number and associated sign as an argument. By default, a deque rotates to the right, but providing a negative rotation value will cause it to rotate left.

base = deque([1, 2, 3, 4, 5])
    
# rotates base 2 steps to the left
base.rotate(-2)  # deque([3, 4, 5, 1, 2])
    
# rotates base 3 steps to the right
base.rotate(3)   # deque([5, 1, 2, 3, 4])

Wrapping up

And that's it for our brief highlight of deques! If you want to learn more about deques, you can read about them in the official documentation, along with all the other cool collection types in the collections module.

We also cover stuff like this in our Complete Python Course, so be to check it out if you want to upgrade your Python skills even further.

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