---
title: Validation Pattern
category: patterns
---
# Validation Pattern

## Rules

- Validate inputs early and raise ValueError or domain-specific exceptions (ProviderError) for invalid contract inputs.
- Tests should assert that invalid inputs raise the expected exceptions.
- Use explicit checks for types and shapes on public APIs (e.g., ensure text is str before embedding).

## Examples

### ai_provider.py - Type validation

```python
if not isinstance(text, str):
    raise ProviderError("text must be a string")
```

### pipeline/ai_provider_wrapper.py - Defensive empty handling

```python
if not texts:
    return []
if motion_ids is None:
    motion_ids = [None for _ in texts]
```

## Anti-Patterns

### Bad: Invalid values into computation

**Problem**: Allowing invalid values to propagate into heavy computation (e.g., non-string into embedding pipeline).

**Remediation**: Fail fast with a typed exception and add unit tests to cover validations.