Lazy-import third-party backends (#794)

python/sglang/__init__.py

+import importlib
+
+
+class LazyImport:
+    def __init__(self, module_name, class_name):
+        self.module_name = module_name
+        self.class_name = class_name
+        self._module = None
+
+    def _load(self):
+        if self._module is None:
+            module = importlib.import_module(self.module_name)
+            self._module = getattr(module, self.class_name)
+        return self._module
+
+    def __getattr__(self, name):
+        module = self._load()
+        return getattr(module, name)
+
+    def __call__(self, *args, **kwargs):
+        module = self._load()
+        return module(*args, **kwargs)
+
+
 # SGL API Components
 from sglang.api import (
     Runtime,
@@ -24,11 +48,12 @@ from sglang.api import (
 from sglang.global_config import global_config
 
 # SGL Backends
-from sglang.lang.backend.anthropic import Anthropic
-from sglang.lang.backend.litellm import LiteLLM
-from sglang.lang.backend.openai import OpenAI
 from sglang.lang.backend.runtime_endpoint import RuntimeEndpoint
-from sglang.lang.backend.vertexai import VertexAI
+
+Anthropic = LazyImport("sglang.lang.backend.anthropic", "Anthropic")
+LiteLLM = LazyImport("sglang.lang.backend.litellm", "LiteLLM")
+OpenAI = LazyImport("sglang.lang.backend.openai", "OpenAI")
+VertexAI = LazyImport("sglang.lang.backend.vertexai", "VertexAI")
 
 from .version import __version__