Add Use __post_init__ For dataclass Validations as a Python TIL

python/use-post-init-for-dataclass-validations.md

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+# Use `__post_init__` For `dataclass` Validations
+
+The [`dataclass`](https://docs.python.org/3/library/dataclasses.html) construct
+is a handy stdlib way of modeling some data with many improvements over a `dict`
+such as named attributes and type visibility.
+
+```python
+from dataclasses import dataclass
+from typing import ClassVar
+
+@dataclass
+class BPEConfig:
+    BASE_VOCAB_SIZE: ClassVar[int] = 256
+
+    vocab_size: int
+    special_tokens: list[str]
+```
+
+I want to enhance `BPEConfig` a little by validating the `vocab_size` which
+cannot be less than the `BASE_VOCAB_SIZE`. The
+[`__post_init__`](https://docs.python.org/3/library/dataclasses.html#dataclasses.__post_init__)
+method is a good place for this kind of validation.
+
+```python
+from dataclasses import dataclass
+from typing import ClassVar
+
+@dataclass
+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)
+```
+
+With this in place, my program will fail fast if I try to use an invalid
+`vocab_size`:
+
+```python
+>>> BPEConfig(22, [])
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+  File "<string>", line 5, in __init__
+  File "/Users/lastword/dev/misc/build-an-llm/chapter_02/bpe_tokenizer.py", line 24, in __post_init__
+    raise ValueError(msg)
+ValueError: vocab_size (22) must be greater than or equal to BASE_VOCAB_SIZE (256)
+```
+
+This example is pulled directly from [the `BPETokenizer` I'm building](https://github.com/jbranchaud/build-an-llm-from-scratch/blob/d3fd0acd65c3e7419b2d15a64c8d74266d0488f6/chapter_02/bpe_tokenizer.py#L14-L24).