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Pydantic Lax Mode Type Conversions

This page provides a complete reference for how Pyrefly converts types when working with Pydantic models in lax mode (the default). For background on how lax mode works, see the main Pydantic documentation.

Note: Types without a specific conversion rule (e.g., Callable, Any, custom classes) are converted to Any.

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Atomic Type Conversions

Named unions are used for atomic types to keep type signatures concise.

Input Type	Named Union	Expanded Type
int	LaxInt	int | bool | float | str | bytes | Decimal
float	LaxFloat	int | bool | float | str | bytes | Decimal
bool	LaxBool	bool | int | float | str | Decimal
Decimal	LaxDecimal	Decimal | int | float | str
str	LaxStr	str | bytes | bytearray
bytes	LaxBytes	str | bytes | bytearray
date	LaxDate	date | datetime | int | float | str | bytes | Decimal
datetime	LaxDatetime	date | datetime | int | float | str | bytes | Decimal
time	LaxTime	time | int | float | str | bytes | Decimal
timedelta	LaxTimedelta	timedelta | int | float | str | bytes | Decimal
Path	LaxPath	Path | str
UUID	LaxUuid	UUID | str
None	(no conversion)	None

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Compositional Type Conversions

Notation:

• T_converted means the type T is recursively converted using lax mode rules (e.g., int → LaxInt)
• T_flattened means the type T is converted and expanded (e.g., int → int | bool | float | str | bytes | Decimal)

Input Type/Container	Output Type/Container
type[T]	type[T_converted]
T1 | T2 | ...	T1_flattened | T2_flattened | ...
list[T], set[T], frozenset[T],
Sequence[T], Iterable[T], deque[T],
tuple[T, ...]	Iterable[T_converted]
tuple[T1, T2, ...]	Iterable[T1_flattened | T2_flattened | ...]
dict[K, V]	Mapping[K, V_converted]

Examples:

• Type wrapper: type[int] → type[LaxInt]
• Union types: Each member is converted and flattened. int | bool → int | bool | float | str | bytes | Decimal
• Single-element containers and unbounded tuples: Named unions are preserved. list[int] → Iterable[LaxInt]
• Concrete tuples: Element types are expanded and flattened. tuple[int, str] → Iterable[int | bool | float | str | bytes | bytearray | Decimal]
• Dictionaries: Only values are converted; keys remain unchanged. dict[str, int] → Mapping[str, LaxInt]