Add Avoid Modification With Frozen Dataclass as a Python TIL

python/avoid-modification-with-frozen-dataclass.md

@@ -0,0 +1,47 @@
+# Avoid Modification With Frozen Dataclass
+
+The `@dataclass` decorator can be set as _frozen_ to prevent modification of
+values on instances of that `dataclass`.
+
+Without making it frozen, I can easily subvert validations by changing the value
+of attributes after the `__post_init__` validations are called.
+
+```python
+>>> config = BPEConfig(300, []) # passes validations
+>>> config.vocab_size = 22 # this is invalid, wish this was prevented
+```
+
+Here is the updated `@dataclass` declaration with `frozen=True` passed as a
+parameter.
+
+```python
+from dataclasses import dataclass
+from typing import ClassVar
+
+@dataclass(frozen=True)
+class BPEConfig:
+    BASE_VOCAB_SIZE: ClassVar[int] = 256
+
+    vocab_size: int
+    special_tokens: list[str]
+
+    def __post_init__(self):
+        if self.vocab_size < self.BASE_VOCAB_SIZE:
+            msg = f"vocab_size ({self.vocab_size}) must be greater than or equal to BASE_VOCAB_SIZE ({self.BASE_VOCAB_SIZE})"
+            raise ValueError(msg)
+```
+
+Now I am prevented from modifying a scalar value like `vocab_size` after the
+instance has been created.
+
+```python
+>>> config = BPEConfig(300, [])
+>>> config.vocab_size = 22
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "<string>", line 4, in __setattr__
+dataclasses.FrozenInstanceError: cannot assign to field 'vocab_size'
+```
+
+This doesn't prevent you from modifying the contents of attributes that are
+`list` or `dict` types.