## Change Summary

This means `pydantic/__init__.py` is now lazy about all imports using `__getattr__`.

Import performance changes:
### Before

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 29.7 msec per loop
```

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"
1 loop, best of 1: 30.4 msec per loop
```

### After

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 4.8 msec per loop
```

```
(env3112) code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"           
1 loop, best of 1: 18.7 msec per loop
```

Tuna output:

<img width="1421" alt="image" src="https://github.com/pydantic/pydantic/assets/4039449/0a57434e-fdda-4709-8793-d96c0a0ee762">

## Related issue number

As promised in https://github.com/pydantic/pydantic/pull/7423#issuecomment-1733361629.

## Checklist

* [x] The pull request title is a good summary of the changes - it will be used in the changelog
* [ ] Unit tests for the changes exist
* [x] Tests pass on CI
* [ ] Documentation reflects the changes where applicable
* [x] My PR is ready to review, **please add a comment including the phrase "please review" to assign reviewers**


Selected Reviewer: @sydney-runkle
Use `__getattr__` for all package imports, improve import time. ## Change Summary

This means `pydantic/__init__.py` is now lazy about all imports using `__getattr__`.

Import performance changes:
### Before

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 29.7 msec per loop
```

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"
1 loop, best of 1: 30.4 msec per loop
```

### After

```
code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "import pydantic"
1 loop, best of 1: 4.8 msec per loop
```

```
(env3112) code/pydantic 0 ➤ pdm run python -m timeit -n 1 -r 1 "from pydantic import BaseModel"           
1 loop, best of 1: 18.7 msec per loop
```

Tuna output:

<img width="1421" alt="image" src="https://github.com/pydantic/pydantic/assets/4039449/0a57434e-fdda-4709-8793-d96c0a0ee762">

## Related issue number

As promised in https://github.com/pydantic/pydantic/pull/7423#issuecomment-1733361629.

## Checklist

* [x] The pull request title is a good summary of the changes - it will be used in the changelog
* [ ] Unit tests for the changes exist
* [x] Tests pass on CI
* [ ] Documentation reflects the changes where applicable
* [x] My PR is ready to review, **please add a comment including the phrase "please review" to assign reviewers**


Selected Reviewer: @sydney-runkle
An alias is an alternative name for a field, used when serializing and deserializing data.

You can specify an alias in the following ways:

* `alias` on the [`Field`][pydantic.fields.Field]
* `validation_alias` on the [`Field`][pydantic.fields.Field]
* `serialization_alias` on the [`Field`][pydantic.fields.Field]
* `alias_generator` on the [`Config`][pydantic.config.ConfigDict.alias_generator]

## Alias Precedence

If you specify an `alias` on the [`Field`][pydantic.fields.Field], it will take precedence over the generated alias by default:

```py
from pydantic import BaseModel, ConfigDict, Field


def to_camel(string: str) -> str:
    return ''.join(word.capitalize() for word in string.split('_'))


class Voice(BaseModel):
    model_config = ConfigDict(alias_generator=to_camel)

    name: str
    language_code: str = Field(alias='lang')


voice = Voice(Name='Filiz', lang='tr-TR')
print(voice.language_code)
#> tr-TR
print(voice.model_dump(by_alias=True))
#> {'Name': 'Filiz', 'lang': 'tr-TR'}
```

### Alias Priority

You may set `alias_priority` on a field to change this behavior:

* `alias_priority=2` the alias will *not* be overridden by the alias generator.
* `alias_priority=1` the alias *will* be overridden by the alias generator.
* `alias_priority` not set, the alias will be overridden by the alias generator.

The same precedence applies to `validation_alias` and `serialization_alias`.
See more about the different field aliases under [field aliases](../concepts/fields.md#field-aliases).

An alias is an alternative name for a field, used when serializing and deserializing data.

You can specify an alias in the following ways:

* `alias` on the [`Field`][pydantic.fields.Field]
* `validation_alias` on the [`Field`][pydantic.fields.Field]
* `serialization_alias` on the [`Field`][pydantic.fields.Field]
* `alias_generator` on the [`Config`][pydantic.config.ConfigDict.alias_generator]

## Alias Precedence

If you specify an `alias` on the [`Field`][pydantic.fields.Field], it will take precedence over the generated alias by default:

```py
from pydantic import BaseModel, ConfigDict, Field


def to_camel(string: str) -> str:
    return ''.join(word.capitalize() for word in string.split('_'))


class Voice(BaseModel):
    model_config = ConfigDict(alias_generator=to_camel)

    name: str
    language_code: str = Field(alias='lang')


voice = Voice(Name='Filiz', lang='tr-TR')
print(voice.language_code)
#> tr-TR
print(voice.model_dump(by_alias=True))
#> {'Name': 'Filiz', 'lang': 'tr-TR'}
```

### Alias Priority

You may set `alias_priority` on a field to change this behavior:

* `alias_priority=2` the alias will *not* be overridden by the alias generator.
* `alias_priority=1` the alias *will* be overridden by the alias generator.
* `alias_priority` not set, the alias will be overridden by the alias generator.

The same precedence applies to `validation_alias` and `serialization_alias`.
See more about the different field aliases under [field aliases](../concepts/fields.md#field-aliases).
