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