1 files changed, 12 insertions, 9 deletions
diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index bc76c90..fd63f97 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -88,7 +88,8 @@ def join(a, *p):
                           for s in (a, ) + p)
         if valid_types:
             # Must have a mixture of text and binary data
-            raise TypeError("Can't mix strings and bytes in path components.")
+            raise TypeError("Can't mix strings and bytes in path "
+                            "components.") from None
         raise
     return path
 
@@ -299,6 +300,7 @@ def expandvars(path):
         search = _varprogb.search
         start = b'{'
         end = b'}'
+        environ = getattr(os, 'environb', None)
     else:
         if '$' not in path:
             return path
@@ -308,6 +310,7 @@ def expandvars(path):
         search = _varprog.search
         start = '{'
         end = '}'
+        environ = os.environ
     i = 0
     while True:
         m = search(path, i)
@@ -317,18 +320,18 @@ def expandvars(path):
         name = m.group(1)
         if name.startswith(start) and name.endswith(end):
             name = name[1:-1]
-        if isinstance(name, bytes):
-            name = str(name, 'ASCII')
-        if name in os.environ:
+        try:
+            if environ is None:
+                value = os.fsencode(os.environ[os.fsdecode(name)])
+            else:
+                value = environ[name]
+        except KeyError:
+            i = j
+        else:
             tail = path[j:]
-            value = os.environ[name]
-            if isinstance(path, bytes):
-                value = value.encode('ASCII')
             path = path[:i] + value
             i = len(path)
             path += tail
-        else:
-            i = j
     return path