Add Deduplicate A List Into A Tuple as a Python TIL

README.md

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 - [Control Passing Of Time In Tests](python/control-passing-of-time-in-tests.md)
 - [Create A Dummy DataFrame In Pandas](python/create-a-dummy-dataframe-in-pandas.md)
 - [Create A Range Of Descending Values](python/create-a-range-of-descending-values.md)
+- [Deduplicate A List Into A Tuple](python/deduplicate-a-list-into-a-tuple.md)
 - [Dunder Methods](python/dunder-methods.md)
 - [Easy Key-Value Aggregates With defaultdict](python/easy-key-value-aggregates-with-defaultdict.md)
 - [Install With PIP For Specific Interpreter](python/install-with-pip-for-specific-interpreter.md)

python/deduplicate-a-list-into-a-tuple.md

@@ -0,0 +1,46 @@
+# Deduplicate A List Into A Tuple
+
+A `list` is not hashable which means you can't use it for things like `dict`
+keys or cache keys. Instead you need to convert it into something like a `set`
+or a `tuple`.
+
+Here is an example list:
+
+```python
+>>> l1 = [3,4,1,2,5,4,1]
+```
+
+Turning this list into a `set` or `frozenset` is straightforward:
+
+```python
+>>> set(l1)
+{1, 2, 3, 4, 5}
+>>> frozenset(l1)
+frozenset({1, 2, 3, 4, 5})
+```
+
+If you're trying to preserve the order after deduplicating, then you'll want to
+use a `tuple` instead of a `set`. In order to deduplicate while maintaining the
+ordering, you can exploit the fact that `dict` keys maintain their order. A
+`list` can be transformed into the keys of a `dict` with
+[`dict.fromkeys`](https://docs.python.org/3/library/stdtypes.html#dict.fromkeys):
+
+```python
+>>> dict.fromkeys(l1)
+{3: None, 4: None, 1: None, 2: None, 5: None}
+```
+
+And here is your `tuple` which extracts the keys of the `dict`:
+
+```python
+>>> tuple(dict.fromkeys(l1))
+(3, 4, 1, 2, 5)
+```
+
+By comparison, here is the `tuple` transformed directly from the `list` without
+deduplication.
+
+```python
+>>> tuple(l1)
+(3, 4, 1, 2, 5, 4, 1)
+```