Update to optimize imports for faster startup times

Don't import all schemas instead import the schemas we actually use.
This also makes the importing of services lazy so that only schemas are
imported which we actually want to use.

This fixes a performance issue where just importing commercetools
would import all the marshmallow schemas which is terribly slow.

Author: mvantellingen

codegen/generate_schema.py

@@ -65,13 +65,7 @@ def get_module_nodes(self):
         return result
 
     def generate_init_module(self, modules):
-        nodes = [
-            ast.ImportFrom(
-                module=module, names=[ast.alias(name="*  # noqa", asname=None)], level=1
-            )
-            for module in sorted(modules)
-        ]
-        return ast.Module(body=nodes)
+        return ast.Module(body=[])
 
     def add_type_definition(self, resource):
         """Create a class definition"""
@@ -277,7 +271,7 @@ def _create_schema_property(self, prop):
         node = self._get_property_field(prop)
 
         if prop.many:
-            if node.func.id == "marshmallow.fields.Nested":
+            if node.func.id == "helpers.LazyNestedField":
                 node.keywords.append(
                     ast.keyword(
                         arg="many", value=ast.NameConstant(value=True, kind=None)
@@ -462,8 +456,11 @@ def _create_nested_field(self, type_obj):
             )
 
         """
+        self.generator.add_import_statement(
+            self.resource.package_name, "commercetools", "helpers"
+        )
         return ast.Call(
-            func=ast.Name(id="marshmallow.fields.Nested"),
+            func=ast.Name(id="helpers.LazyNestedField"),
             args=[],
             keywords=[
                 ast.keyword(