[Term Entry] Python deque: append()

content/python/concepts/deque/terms/append/append.md

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+---
+Title: 'append()'
+Description: 'Adds an element to the right end of a deque.'
+Subjects:
+  - 'Computer Science'
+  - 'Data Structures'
+Tags:
+  - 'Collections'
+  - 'Deque'
+  - 'Python'
+CatalogContent:
+  - 'learn-python-3'
+  - 'paths/computer-science'
+---
+
+The **`append()`** method of a `collections.deque` object adds an element to the right end (back) of the deque efficiently. This operation runs in O(1) time and is ideal for queue or stack-like use cases.
+
+## Syntax
+
+```pseudo
+deque_object.append(value)
+```
+
+**Parameters:**
+
+- `value`: The element to be added to the right end of the deque.
+
+**Return value:**
+
+Does not return a `value` (returns None). The deque is modified in place.
+
+## Example 1
+
+In this example, an element is added to an initially empty deque, then another element is appended and the contents are displayed:
+
+```py
+from collections import deque
+
+dq = deque()
+dq.append('a')
+print(dq)
+
+dq.append('b')
+print(dq)
+```
+
+The output of this code is:
+
+```shell
+deque(['a'])
+deque(['a', 'b'])
+```
+
+## Example 2
+
+In this example, an element is appended to a deque with a fixed maximum length, showing how older elements are automatically dropped from the opposite end when full:
+
+```py
+from collections import deque
+
+dq = deque([1, 2, 3], maxlen=3)
+print(dq)
+
+dq.append(4)
+print(dq)
+```
+
+The output of this code is:
+
+```shell
+deque([1, 2, 3], maxlen=3)
+deque([2, 3, 4], maxlen=3)
+```
+
+## Codebyte Example
+
+In this example, multiple items are appended to a deque, then elements are popped from the left while printing the current state and size:
+
+```codebyte/python
+from collections import deque
+
+dq = deque()
+for ch in ['x', 'y', 'z']:
+  dq.append(ch)
+  print(f"After append('{ch}'): {dq}")
+
+while dq:
+  print(f"Popping left: {dq.popleft()} | Current deque: {dq}")
+```
+
+## Frequently Asked Questions
+
+### 1. How do you append to a deque in Python?
+
+Use the `append(value)` method on the deque object to add the element `value` to its right end.
+
+### 2. What is the `append()` function used for in Python?
+
+In the context of a `deque`, `append()` adds an element at the back end of the container, supporting efficient queue- or stack-like operations.
+
+### 3. What is the deque in Python?
+
+The `deque` class in the `collections` module is a double-ended queue that supports fast appends and pops from both ends, making it useful for queues, stacks, and history buffers.