add python collection deepcopy til

python/creating-collection-deepcopies.md

@@ -0,0 +1,33 @@
+# Creating Deep Copies of Collections
+
+Python has a [built-in `copy` module](https://docs.python.org/3/library/copy.html#module-copy):
+
+> "Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other. This module provides generic shallow and deep copy operations."
+
+This binding is especially useful for solving in-place modification of lists within a user-defined function:
+
+```python
+def rotate_list_plus_one_index(list):
+    list.insert(0, list.pop())
+
+nums = [1, 2, 3, 4, 5]
+rotate_list_plus_one_index(nums)
+print(nums)
+# [5, 1, 2, 3, 4]
+```
+
+However, if we want to recursively create new copies of all the nested elements contained within a mutable object (vs. modifying in place), we can use the `deepcopy` method:
+
+```python
+import copy
+
+nums_2d = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
+nums_2d_deepcopy = copy.deepcopy(nums_2d)
+nums_2d_deepcopy[0][0] = 0
+
+print(nums_2d)
+# [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
+
+print(nums_2d_deepcopy)
+# [[0, 2, 3], [4, 5, 6], [7, 8, 9]]
+```