Python Data Structures: Strings, Lists, Tuples, Dicts, Sets, Stacks & Queues

Data Structures in Algorithms

Algorithms require appropriate in‑memory data structures to store temporary data during execution; choosing the right structure is crucial for efficiency.

Python Data Structures

String: ordered, immutable sequence of elements

List: ordered, mutable sequence

Tuple: ordered, immutable sequence

Dictionary: unordered collection of key‑value pairs

Set: unordered collection of unique elements

String

A string is an ordered, immutable sequence representing textual data.

<code># generate a string
astring = "hello 2023!"  # single or double quotes
# positive indexing (first index is 0)
astring[0]  # 'h'
astring[2]  # 'l'
# negative indexing (last element is -1)
astring[-1]  # '!'
# slicing (start inclusive, end exclusive)
astring[2:4]  # 'll'
</code>

List

In Python, a list stores a mutable sequence of elements, which can be of different types.

<code># generate a list
alist = [1, 2, True, 'Hello world', [1, 2, 3]]
# positive indexing
alist[0]  # 1
alist[2]  # True
# negative indexing
alist[-1]  # [1, 2, 3]
# slicing
alist[2:4]  # [True, 'Hello world']
</code>

Tuple

A tuple is an immutable (read‑only) sequence, similar to a list but cannot be modified after creation.

<code># generate a tuple
atuple = (1, 2, True, 'Hello world', [1, 2, 3])
# positive indexing
atuple[0]  # 1
atuple[2]  # True
# negative indexing
atuple[-1]  # [1, 2, 3]
# slicing
atuple[2:4]  # [True, 'Hello world']
</code>

List elements can be added, removed, or changed, which is not possible for tuples.

<code>>> alist[0] = 3  # modify first element
>>> alist[0]
3
>>> atuple[0] = 3  # raises TypeError
TypeError: 'tuple' object does not support item assignment
</code>

Dictionary

A dictionary stores data as key‑value pairs; the colon (:) separates keys from values.

<code># generate a dictionary
adict = {
  "manual_color": "Yellow",
  "approved_color": "Green",
  "refused_color": "Red"
}
# access values by key
adict['manual_color']  # 'Yellow'
adict.keys()   # dict_keys(['manual_color', 'approved_color', 'refused_color'])
adict.values() # dict_values(['Yellow', 'Green', 'Red'])
</code>

Set

A set contains unordered, unique elements.

<code># generate a set
aset = {1, 2, 2, 3, 'hello', (1, 2)}
# duplicate elements are removed
aset  # {(1, 2), 1, 2, 3, 'hello'}
# union
aset | {7, 8}  # {(1, 2), 1, 2, 3, 'hello', 7, 8}
# intersection
aset & {7, 8}  # set()
</code>

Below is a comparison of the data types mentioned above:

Stack

A stack is a linear data structure that stores items in a last‑in‑first‑out (LIFO) order. Python does not have a built‑in stack type.

Stack implementation:

<code>class Stack:
    def __init__(self):
        self.items = []
    def isEmpty(self):
        return self.items == []
    def push(self, item):
        self.items.append(item)
    def pop(self):
        return self.items.pop()
    def peek(self):
        return self.items[len(self.items)-1]
    def size(self):
        return len(self.items)
</code>

Example usage:

<code>stack = Stack()
stack.push('Red')
stack.push('Green')
stack.push('Blue')
stack.push('Yellow')
# stack now ['Red', 'Green', 'Blue', 'Yellow']
stack.pop()   # removes 'Yellow'
stack.size() # 3
</code>

Queue

A queue stores elements in a first‑in‑first‑out (FIFO) order.

<code>class Queue(object):
    def __init__(self):
        self.items = []
    def isEmpty(self):
        return self.items == []
    def enqueue(self, item):
        self.items.insert(0, item)
    def dequeue(self):
        return self.items.pop()
    def size(self):
        return len(self.items)
</code>

Example usage:

<code>queue = Queue()
queue.enqueue('Red')
queue.enqueue('Green')
queue.enqueue('Blue')
queue.enqueue('Yellow')
# queue now ['Yellow', 'Blue', 'Green', 'Red']
queue.dequeue()  # removes 'Red'
queue.size()    # 3
</code>

References

Imran Ahmad, "40 Algorithms Every Programmer Should Know: Hone your problem‑solving skills by learning different algorithms and their implementation in Python"

"真的这么神奇吗 《程序员必会的40种算法》", https://blog.csdn.net/lose_user___/article/details/122437832