diff --git a/mypy/build.py b/mypy/build.py
index d52b54e3c024d..ab25c6a0ad232 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -787,12 +787,25 @@ class ModuleType:
 
 
 class ImportContext:
+    """
+    Describes module import context
+
+    Do we already discovered implementation?
+    What kind of module we discovered?
+    """
     def __init__(self) -> None:
         self.has_py = False  # type: bool
         self.type = None  # type: Optional[int]
+        # Paths can contain only one ".py" path, but multiple stubs
         self.paths = []  # type: List[str]
 
     def maybe_add_path(self, path: str, type: int) -> None:
+        """
+        Add path to import context.
+        Modifies self.paths in case if arguments satisfy import context state
+        """
+        assert path.endswith(('/',) + tuple(PYTHON_EXTENSIONS))
+
         if self.type is not None and self.type != type:
             return None
 
@@ -817,6 +830,8 @@ def maybe_add_path(self, path: str, type: int) -> None:
         self.paths.append(path)
 
     def _verify_module(self, path: str) -> bool:
+        # At this point we already know that that it's valid python path
+        # We only need to check file existence
         if not is_file(path):
             return False
 
@@ -860,6 +875,10 @@ def find_module(self, id: str) -> Optional[str]:
             return None
 
     def find_modules_recursive(self, module: str) -> List[BuildSource]:
+        """
+        Discover module and all it's children
+        Remove duplicates from discovered paths
+        """
         hits = set()  # type: Set[str]
         result = []  # type: List[BuildSource]
         for src in self._find_modules_recursive(module):
@@ -903,6 +922,9 @@ def _collect_paths(self, paths: List[str], last_comp: str) -> List[str]:
         sepinit = '__init__'
         ctx = ImportContext()
 
+        # Detect modules in following order: package, module, namespace.
+        # First hit determines module type, consistency of paths to given type
+        # ensured in ImportContext
         for path_item in paths:
             if is_module_path(path_item):
                 continue