diff --git a/python/mozbuild/mozbuild/base.py b/python/mozbuild/mozbuild/base.py
index d1ebd9fd4d9a..c33ddeddaf2a 100644
--- a/python/mozbuild/mozbuild/base.py
+++ b/python/mozbuild/mozbuild/base.py
@@ -777,16 +777,22 @@ class MachCommandConditions(object):
         """Must have a mercurial source checkout."""
         if hasattr(cls, 'substs'):
             top_srcdir = cls.substs.get('top_srcdir')
-            return top_srcdir and os.path.isdir(os.path.join(top_srcdir, '.hg'))
-        return False
+        elif hasattr(cls, 'topsrcdir'):
+            top_srcdir = cls.topsrcdir
+        else:
+            return False
+        return top_srcdir and os.path.isdir(os.path.join(top_srcdir, '.hg'))
 
     @staticmethod
     def is_git(cls):
         """Must have a git source checkout."""
         if hasattr(cls, 'substs'):
             top_srcdir = cls.substs.get('top_srcdir')
-            return top_srcdir and os.path.exists(os.path.join(top_srcdir, '.git'))
-        return False
+        elif hasattr(cls, 'topsrcdir'):
+            top_srcdir = cls.topsrcdir
+        else:
+            return False
+        return top_srcdir and os.path.exists(os.path.join(top_srcdir, '.git'))
 
 
 class PathArgument(object):
diff --git a/python/mozbuild/mozbuild/mach_commands.py b/python/mozbuild/mozbuild/mach_commands.py
index da8eee35f5c1..08711bfd1fa0 100644
--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -695,9 +695,9 @@ class Clobber(MachCommandBase):
                 raise
 
         if 'python' in what:
-            if os.path.isdir(mozpath.join(self.topsrcdir, '.hg')):
+            if conditions.is_hg(self):
                 cmd = ['hg', 'purge', '--all', '-I', 'glob:**.py[co]']
-            elif os.path.isdir(mozpath.join(self.topsrcdir, '.git')):
+            elif conditions.is_git(self):
                 cmd = ['git', 'clean', '-f', '-x', '*.py[co]']
             else:
                 cmd = ['find', '.', '-type', 'f', '-name', '*.py[co]', '-delete']