Pyrefly

Pyrefly is a Python type checker and language server with built-in support for a number of Pydantic-specific features. This support works out-of-the-box with Pydantic and Pyrefly installed, with no additional configuration needed.

Pyrefly is available as both an IDE extension and a command-line type checker.

IDE extension

Pyrefly gives you IDE features such as go-to-definition and autocomplete on your Pydantic models.

Here’s an example of signature help, powered by Pyrefly’s understanding of the Pydantic-specific validation_alias keyword:

As you type Model(), Pyrefly hints that you need to use the name x_ to populate the x field.

Type checker

Pyrefly can also catch errors in your code before you run it. Consider the following example:

from pydantic import BaseModel, ConfigDict


class Model1(BaseModel, frozen=True):
    x: int


class Model2(BaseModel):
    model_config = ConfigDict(frozen=True)
    x: int


model1 = Model1(x=0)
model1.x = 1  # validation error: mutating a frozen field

model2 = Model2(x=0)
model2.x = 1  # validation error: mutating a frozen field

Since Model1 declares that it is frozen using a standard type system feature, any type checker of your choice will catch the validation error from mutating model1.x. However, a type checker without special support for the Pydantic ConfigDict class will miss the validation error from mutating model2.x.

Pyrefly catches both errors:

ERROR Cannot set field `x` [read-only]
  --> foo.py:11:1
   |
11 | model1.x = 1  # validation error: mutating a frozen field
   | ^^^^^^^^
   |
  This field belongs to a frozen Pydantic model
ERROR Cannot set field `x` [read-only]
  --> foo.py:14:1
   |
14 | model2.x = 1  # validation error: mutating a frozen field
   | ^^^^^^^^
   |
  This field belongs to a frozen Pydantic model

See the Pyrefly documentation for more information.