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Added new example: deep copy list #225

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44 changes: 44 additions & 0 deletions README.md
Original file line number Diff line number Diff line change
Expand Up @@ -59,6 +59,7 @@ So, here we go...
+ [▶ The disappearing variable from outer scope](#-the-disappearing-variable-from-outer-scope)
+ [▶ The mysterious key type conversion](#-the-mysterious-key-type-conversion)
+ [▶ Let's see if you can guess this?](#-lets-see-if-you-can-guess-this)
+ [▶ Copy DEEP without deep copy](#-copy-deep-without-deep-copy)
* [Section: Slippery Slopes](#section-slippery-slopes)
+ [▶ Modifying a dictionary while iterating over it](#-modifying-a-dictionary-while-iterating-over-it)
+ [▶ Stubborn `del` operation](#-stubborn-del-operation)
Expand Down Expand Up @@ -1898,6 +1899,49 @@ a, b = a[b] = {}, 5
True
```

---
### ▶ Copy DEEP without deep copy

```py
>>> new_list = [1,2,4,5,6]
>>> new_list_copy = new_list
>>> new_list_copy.append(10)
```

**Output:**
```py
>>> new_list
[1, 2, 4, 5, 6, 10]
>>> new_list_copy
[1, 2, 4, 5, 6, 10]
```
***But....***

```py
>>> new_list = [1,2,4,5,6]
>>> new_list_dcopy = new_list[:]
>>> new_list_dcopy.append(20)
```

**Output:**
```py
>>> new_list
[1, 2, 4, 5, 6]
>>> new_list_dcopy
[1, 2, 4, 5, 6, 20]
```

_What did just happen then?_

#### 💡 Explanation:

In Python, we have concept of [deep copy](https://docs.python.org/3/library/copy.html) and [shallow copy](https://docs.python.org/3/library/copy.html).
In the first example, `new_list` is the original list and `new_list_copy` is the shallow copy of the original list. As a result changes made to the copied list has reflected back to the original list.
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@OrangeChannel OrangeChannel Oct 1, 2020

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This isn't quite right, new_list_copy is not a copy of new_list at all. It is simply a reference to it, a simple assignment in Python never creates a copy. You can compare the two lists with is to test.


The second example shows that `new_list_dcopy` is a deep copy of the original list `new_list`, where changes made in the deep copied list doesn't reflect back to the original list.

One can easily deep copy a list without using the *copy.deepcopy(new_list)* just by using `slicing` , where any changes made to the copied list won't reflect back to the original list. Trick!!
Comment on lines +1941 to +1943

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Actually, slicing a list using an empty slice will result in a shallow copy of the list, meaning other mutable objects in the new list are not copies of themselves.


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