# Evaluator

Abstract base class for creating custom evaluators in Fiddler Evals.

The Evaluator class provides a flexible framework for creating builtin and custom evaluators that can assess LLM outputs against various criteria. Each evaluator is responsible for a single, specific evaluation task (e.g., hallucination detection, answer relevance, exact match, etc.).

Parameter Mapping: : Evaluators can define their own parameter mappings using score\_fn\_kwargs\_mapping in the constructor. These mappings specify how data from the evaluation context (inputs, outputs, expected\_outputs) should be passed to the evaluator's score method.\
Mapping Priority (highest to lowest):

1. Evaluator-level score\_fn\_kwargs\_mapping (set in constructor)
2. Evaluation-level kwargs\_mapping (passed to evaluate function)
3. Default parameter resolution<br>

This allows evaluators to define sensible defaults while still permitting customization at the evaluation level.

Creating Custom Evaluators: : To create a custom evaluator, inherit from this class and implement the score method with parameters specific to your evaluation needs:\
Example - Custom evaluator with parameter mapping: class ExactMatchEvaluator(Evaluator):<br>

> def **init**(self, output\_key: str = "answer", score\_name\_prefix: str = None): : super().**init**( : score\_name\_prefix=score\_name\_prefix, score\_fn\_kwargs\_mapping={"output": output\_key}\
> )
>
> \
> def score(self, output: str, expected\_output: str) -> Score: : is\_match = output.strip().lower() == expected\_output.strip().lower() return Score(\
> \> name=f"{self.score\_name\_prefix}exact\_match", > value=1.0 if is\_match else 0.0, > reasoning=f"Match: {is\_match}"\
> )

## Parameters

| Parameter                 | Type                               | Required | Default | Description                                                                                                                                                                                                           |
| ------------------------- | ---------------------------------- | -------- | ------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `score_name_prefix`       | `str \| None`                      | ✗        | `None`  | Optional prefix to prepend to score names. Useful for distinguishing scores when using multiple instances of the same evaluator on different fields or with different configurations.                                 |
| `score_fn_kwargs_mapping` | `ScoreFnKwargsMappingType \| None` | ✗        | `None`  | Optional mapping for parameter transformation. Maps parameter names to either string keys or transformation functions. This mapping takes precedence over evaluation-level mappings when running the evaluate method. |

{% hint style="info" %}
The score method signature is intentionally flexible using \*args and \*\*kwargs to allow each evaluator to define its own parameter requirements. This design enables maximum flexibility while maintaining a consistent interface across all evaluators in the framework.
{% endhint %}

Initialize the evaluator with parameter mapping configuration.

## Parameters

| Parameter                 | Type                                                        | Required | Default | Description                                                                                                                                                                                                           |
| ------------------------- | ----------------------------------------------------------- | -------- | ------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `score_name_prefix`       | `str \| None`                                               | ✗        | `None`  | Optional prefix to prepend to score names. Useful for distinguishing scores when using multiple instances of the same evaluator on different fields or with different configurations.                                 |
| `score_fn_kwargs_mapping` | `Dict[str, str \| Callable[[Dict[str, Any]], Any]] \| None` | ✗        | `None`  | Optional mapping for parameter transformation. Maps parameter names to either string keys or transformation functions. This mapping takes precedence over evaluation-level mappings when running the evaluate method. |

## Example

```python
# Simple string mapping
evaluator = MyEvaluator(score_fn_kwargs_mapping={"output": "answer"})

# Complex transformation function
evaluator = MyEvaluator(score_fn_kwargs_mapping={
        "question": lambda x: x["inputs"]["question"],
        "response": "answer"
    })

    # Using score name prefix for multiple instances
    evaluator1 = RegexSearch(r"\d+", score_name_prefix="question")
    evaluator2 = RegexSearch(r"\d+", score_name_prefix="answer")
    # Results in scores named "question_has_number" and "answer_has_number"
```

## Raises

**ScoreFunctionInvalidArgs** – If the mapping contains invalid parameter names that don't match the evaluator's score method signature. **Return type:** None

## *property* name *: str*

## *abstractmethod* score(\*args, \*\*kwargs)

Evaluate inputs and return a score or list of scores.

This method must be implemented by all concrete evaluator classes. Each evaluator can define its own parameter signature based on what it needs for evaluation.

Common parameter patterns:

* Output-only: score(self, output: str) -> Score
* Input-Output: score(self, input: str, output: str) -> Score
* Comparison: score(self, output: str, expected\_output: str) -> Score
* All parameters: score(self, input: str, output: str, context: list\[str]) -> Score

### Parameters

| Parameter  | Type  | Required | Default | Description                                             |
| ---------- | ----- | -------- | ------- | ------------------------------------------------------- |
| `*args`    | `Any` | ✗        | `-`     | Positional arguments specific to the evaluator's needs. |
| `**kwargs` | `Any` | ✗        | `-`     | Keyword arguments specific to the evaluator's needs.    |

### Returns

A single Score object or list of Score objects : representing the evaluation results. Each Score should include:

* name: The score name (e.g., "has\_zipcode")
* evaluator\_name: The evaluator name (e.g., "RegexMatch")
* value: The score value (typically 0.0 to 1.0)
* status: SUCCESS, FAILED, or SKIPPED
* reasoning: Optional explanation of the score
* error: Optional error information if evaluation failed

**Return type:** Score | list\[Score]

### Raises

* **ValueError** – If required parameters are missing or invalid.
* **TypeError** – If parameters have incorrect types.
* **Exception** – Any other evaluation-specific errors.


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