Queue implementation using two Stacks

data_structures/queue/queue_on_two_stacks.py

@@ -0,0 +1,123 @@
+"""Queue implementation using two stacks"""
+
+from typing import Any
+
+
+class Queue:
+    def __init__(self) -> None:
+        self._stack1: list[Any] = []
+        self._stack2: list[Any] = []
+
+    def put(self, item: Any) -> None:
+        """
+        Put `item` into the Queue
+
+        >>> queue = Queue()
+        >>> queue.put(10)
+        >>> queue.put(20)
+        >>> len(queue) == 2
+        True
+        >>> str(queue)
+        '<[10, 20]>'
+        """
+
+        self._stack1.append(item)
+
+    def get(self) -> Any:
+        """
+        Get `item` from the Queue
+
+        >>> queue = Queue()
+        >>> for i in (10, 20, 30):
+        ...     queue.put(i)
+        >>> len(queue)
+        3
+        >>> queue.get()
+        10
+        >>> queue.get()
+        20
+        >>> len(queue) == 1
+        True
+        >>> queue.get()
+        30
+        >>> queue.get()
+        Traceback (most recent call last):
+            ...
+        IndexError: Queue is empty
+        """
+
+        # To reduce number of attribute look-ups in `while` loop.
+        stack1_pop = self._stack1.pop
+        stack2_append = self._stack2.append
+
+        if not self._stack2:
+            while self._stack1:
+                stack2_append(stack1_pop())
+
+        if not self._stack2:
+            raise IndexError("Queue is empty")
+        return self._stack2.pop()
+
+    def size(self) -> int:
+        """
+        Returns the length of the Queue
+
+        >>> queue = Queue()
+        >>> queue.size()
+        0
+        >>> queue.put(10)
+        >>> queue.put(20)
+        >>> queue.size()
+        2
+        >>> queue.get()
+        10
+        >>> queue.size() == 1
+        True
+        """
+
+        return len(self)
+
+    def __str__(self) -> str:
+        """
+        >>> queue = Queue()
+        >>> str(queue)
+        '<[]>'
+        >>> queue.put(10)
+        >>> str(queue)
+        '<[10]>'
+        >>> queue.put(20)
+        >>> str(queue)
+        '<[10, 20]>'
+        >>> queue.get()
+        10
+        >>> queue.put(30)
+        >>> str(queue) == '<[20, 30]>'
+        True
+        """
+
+        return f"<{self._stack2[::-1] + self._stack1}>"
+
+    def __len__(self) -> int:
+        """
+        >>> queue = Queue()
+        >>> for i in range(1, 11):
+        ...     queue.put(i)
+        ...
+        >>> len(queue) == 10
+        True
+        >>> for i in range(2):
+        ...   queue.get()
+        ...
+        1
+        2
+        >>> len(queue) == 8
+        True
+        """
+
+        return len(self._stack1) + len(self._stack2)
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()