function.md•1.59 kB
# `pydantic_ai.models.function`
A model controlled by a local function.
[`FunctionModel`][pydantic_ai.models.function.FunctionModel] is similar to [`TestModel`](test.md),
but allows greater control over the model's behavior.
Its primary use case is for more advanced unit testing than is possible with `TestModel`.
Here's a minimal example:
```py {title="function_model_usage.py" call_name="test_my_agent" noqa="I001"}
from pydantic_ai import Agent
from pydantic_ai import ModelMessage, ModelResponse, TextPart
from pydantic_ai.models.function import FunctionModel, AgentInfo
my_agent = Agent('openai:gpt-4o')
async def model_function(
messages: list[ModelMessage], info: AgentInfo
) -> ModelResponse:
print(messages)
"""
[
ModelRequest(
parts=[
UserPromptPart(
content='Testing my agent...',
timestamp=datetime.datetime(...),
)
]
)
]
"""
print(info)
"""
AgentInfo(
function_tools=[], allow_text_output=True, output_tools=[], model_settings=None
)
"""
return ModelResponse(parts=[TextPart('hello world')])
async def test_my_agent():
"""Unit test for my_agent, to be run by pytest."""
with my_agent.override(model=FunctionModel(model_function)):
result = await my_agent.run('Testing my agent...')
assert result.output == 'hello world'
```
See [Unit testing with `FunctionModel`](../../testing.md#unit-testing-with-functionmodel) for detailed documentation.
::: pydantic_ai.models.function