Sync hashtable with the updated tutorial (Python 3.15) (#923)

* Sync hashtable with the updated tutorial (Python 3.15)

- BLANK and DELETED are built-in sentinel() objects instead of object()
- HashTable.pairs in 05_separate_chaining unpacks the buckets in a set
  comprehension (PEP 798)
- requirements.txt: pytest 9.1.1, pytest-unordered 0.8.0 on Python 3.15
- README: the code requires Python 3.15 or later
- pyproject.toml: exclude hashtable from ruff, like the python315-*
  folders (ruff 0.14.1 targets py314: sentinel is F821 there, and
  unpacking in a comprehension is a syntax error)

* hashtable: a deep copy of the pairs in the defensive-copying step

A shallow copy of the list still shares mutable values, so a caller could
change the table through it (reported by a reader in 2022). From the next
step on, .pairs is a set, whose members must be hashable, so a shallow
copy is enough there.

---------

Co-authored-by: Bartosz Zaczyński <bartosz@realpython.com>

Author: realpython-bot

hashtable/01_hashtable_prototype/02_insert_a_key_value_pair/hashtable.py

@@ -1,6 +1,6 @@
 # hashtable.py
 
-BLANK = object()
+BLANK = sentinel("BLANK")
 
 
 class HashTable:

hashtable/01_hashtable_prototype/03_find_a_value_by_key/hashtable.py

@@ -1,6 +1,6 @@
 # hashtable.py
 
-BLANK = object()
+BLANK = sentinel("BLANK")
 
 
 class HashTable:

hashtable/01_hashtable_prototype/04_delete_a_key_value_pair/hashtable.py

@@ -1,6 +1,6 @@
 # hashtable.py
 
-BLANK = object()
+BLANK = sentinel("BLANK")
 
 
 class HashTable:

hashtable/01_hashtable_prototype/05_update_the_value_of_an_existing_pair/hashtable.py

@@ -1,6 +1,6 @@
 # hashtable.py
 
-BLANK = object()
+BLANK = sentinel("BLANK")
 
 
 class HashTable:

hashtable/01_hashtable_prototype/07_use_defensive_copying/hashtable.py

@@ -1,5 +1,6 @@
 # hashtable.py
 
+import copy
 from typing import Any, NamedTuple
 
 
@@ -46,7 +47,7 @@ def get(self, key, default=None):
 
     @property
     def pairs(self):
-        return [pair for pair in self._pairs if pair]
+        return copy.deepcopy([pair for pair in self._pairs if pair])
 
     def _index(self, key):
         return hash(key) % len(self)

hashtable/02_linear_probing/hashtable.py

@@ -2,7 +2,7 @@
 
 from typing import Any, NamedTuple
 
-DELETED = object()
+DELETED = sentinel("DELETED")
 
 
 class Pair(NamedTuple):

hashtable/03_autoresize/hashtable.py

@@ -2,7 +2,7 @@
 
 from typing import Any, NamedTuple
 
-DELETED = object()
+DELETED = sentinel("DELETED")
 
 
 class Pair(NamedTuple):

hashtable/04_load_factor/hashtable.py

@@ -2,7 +2,7 @@
 
 from typing import Any, NamedTuple
 
-DELETED = object()
+DELETED = sentinel("DELETED")
 
 
 class Pair(NamedTuple):

hashtable/05_separate_chaining/hashtable.py

@@ -91,7 +91,7 @@ def get(self, key, default=None):
 
     @property
     def pairs(self):
-        return {pair for bucket in self._buckets for pair in bucket}
+        return {*bucket for bucket in self._buckets}
 
     @property
     def values(self):

hashtable/README.md

@@ -4,10 +4,12 @@ Code supplementing the [tutorial on the hash table](https://realpython.com/pytho
 
 ## Installation
 
+The code requires Python 3.15 or later. It uses the built-in `sentinel()` and unpacking in comprehensions, which are new in Python 3.15.
+
 To get started, create and activate a new virtual environment, and then install the required dependencies into it:
 
 ```shell
-$ python3 -m venv venv --prompt=hashtable
+$ python3.15 -m venv venv --prompt=hashtable
 $ source venv/bin/activate
 (hashtable) $ python -m pip install -r requirements.txt
 ```