fix: import errors for models with optional imports (#1384)

* fix: import errors for models with optional imports

* Addressed comments: added return type, changed error message

* Addressed comments: updated imports

Author: mehtarac

src/strands/models/__init__.py

@@ -3,8 +3,63 @@
 This package includes an abstract base Model class along with concrete implementations for specific providers.
 """
 
+from typing import Any
+
 from . import bedrock, model
 from .bedrock import BedrockModel
 from .model import Model
 
-__all__ = ["bedrock", "model", "BedrockModel", "Model"]
+__all__ = [
+    "bedrock",
+    "model",
+    "BedrockModel",
+    "Model",
+]
+
+
+def __getattr__(name: str) -> Any:
+    """Lazy load model implementations only when accessed.
+
+    This defers the import of optional dependencies until actually needed.
+    """
+    if name == "AnthropicModel":
+        from .anthropic import AnthropicModel
+
+        return AnthropicModel
+    if name == "GeminiModel":
+        from .gemini import GeminiModel
+
+        return GeminiModel
+    if name == "LiteLLMModel":
+        from .litellm import LiteLLMModel
+
+        return LiteLLMModel
+    if name == "LlamaAPIModel":
+        from .llamaapi import LlamaAPIModel
+
+        return LlamaAPIModel
+    if name == "LlamaCppModel":
+        from .llamacpp import LlamaCppModel
+
+        return LlamaCppModel
+    if name == "MistralModel":
+        from .mistral import MistralModel
+
+        return MistralModel
+    if name == "OllamaModel":
+        from .ollama import OllamaModel
+
+        return OllamaModel
+    if name == "OpenAIModel":
+        from .openai import OpenAIModel
+
+        return OpenAIModel
+    if name == "SageMakerAIModel":
+        from .sagemaker import SageMakerAIModel
+
+        return SageMakerAIModel
+    if name == "WriterModel":
+        from .writer import WriterModel
+
+        return WriterModel
+    raise AttributeError(f"cannot import name '{name}' from '{__name__}' ({__file__})")