diff --git a/contrib/import-checker.py b/contrib/import-checker.py
--- a/contrib/import-checker.py
+++ b/contrib/import-checker.py
@@ -88,9 +88,8 @@ def fromlocalfunc(modulename, localmods)
     `modulename` is an `dotted_name_of_path()`-ed source file path,
     which may have `.__init__` at the end of it, of the target source.
 
-    `localmods` is a dict (or set), of which key is an absolute
-    `dotted_name_of_path()`-ed source file path of locally defined (=
-    Mercurial specific) modules.
+    `localmods` is a set of absolute `dotted_name_of_path()`-ed source file
+    paths of locally defined (= Mercurial specific) modules.
 
     This function assumes that module names not existing in
     `localmods` are from the Python standard library.
@@ -114,9 +113,9 @@ def fromlocalfunc(modulename, localmods)
     convenient, even though this is also equivalent to "absname !=
     dottednpath")
 
-    >>> localmods = {'foo.__init__': True, 'foo.foo1': True,
-    ...              'foo.bar.__init__': True, 'foo.bar.bar1': True,
-    ...              'baz.__init__': True, 'baz.baz1': True }
+    >>> localmods = {'foo.__init__', 'foo.foo1',
+    ...              'foo.bar.__init__', 'foo.bar.bar1',
+    ...              'baz.__init__', 'baz.baz1'}
     >>> fromlocal = fromlocalfunc('foo.xxx', localmods)
     >>> # relative
     >>> fromlocal('foo1')
@@ -257,7 +256,7 @@ def imported_modules(source, modulename,
     Args:
       source: The python source to examine as a string.
       modulename: of specified python source (may have `__init__`)
-      localmods: dict of locally defined module names (may have `__init__`)
+      localmods: set of locally defined module names (may have `__init__`)
       ignore_nested: If true, import statements that do not start in
                      column zero will be ignored.
 
@@ -696,13 +695,14 @@ def main(argv):
     if argv[1] == '-':
         argv = argv[:1]
         argv.extend(l.rstrip() for l in sys.stdin.readlines())
-    localmods = {}
+    localmodpaths = {}
     used_imports = {}
     any_errors = False
     for source_path in argv[1:]:
         modname = dotted_name_of_path(source_path)
-        localmods[modname] = source_path
-    for localmodname, source_path in sorted(localmods.items()):
+        localmodpaths[modname] = source_path
+    localmods = set(localmodpaths)
+    for localmodname, source_path in sorted(localmodpaths.items()):
         for src, modname, name, line in sources(source_path, localmodname):
             try:
                 used_imports[modname] = sorted(