Let the default encodings search function lookup aliases before trying the codec…

Let the default encodings search function lookup aliases before trying the codec import. This allows applications to install codecs which override (non-special-cased) builtin codecs.

Lib/encodings/__init__.py

@@ -27,7 +27,7 @@ Written by Marc-Andre Lemburg (mal@lemburg.com).
 
 """#"
 
-import codecs, exceptions, types
+import codecs, exceptions, types, aliases
 
 _cache = {}
 _unknown = '--unknown--'
@@ -38,6 +38,7 @@ _norm_encoding_map = ('                                              . '
                       '                                                '
                       '                                                '
                       '                ')
+_aliases = aliases.aliases
 
 class CodecRegistryError(exceptions.LookupError,
                          exceptions.SystemError):
@@ -74,23 +75,31 @@ def search_function(encoding):
 
     # Import the module:
     #
-    # First look in the encodings package, then try to lookup the
-    # encoding in the aliases mapping and retry the import using the
-    # default import module lookup scheme with the alias name.
+    # First try to find an alias for the normalized encoding
+    # name and lookup the module using the aliased name, then try to
+    # lookup the module using the standard import scheme, i.e. first
+    # try in the encodings package, then at top-level.
     #
-    modname = normalize_encoding(encoding)
-    try:
-        mod = __import__('encodings.' + modname,
-                         globals(), locals(), _import_tail)
-    except ImportError:
-        import aliases
-        modname = (aliases.aliases.get(modname) or
-                   aliases.aliases.get(modname.replace('.', '_')) or
-                   modname)
+    norm_encoding = normalize_encoding(encoding)
+    aliased_encoding = _aliases.get(norm_encoding) or \
+                       _aliases.get(norm_encoding.replace('.', '_'))
+    if aliased_encoding is not None:
+        modnames = [aliased_encoding,
+                    norm_encoding]
+    else:
+        modnames = [norm_encoding]
+    for modname in modnames:
+        if not modname:
+            continue
         try:
-            mod = __import__(modname, globals(), locals(), _import_tail)
+            mod = __import__(modname,
+                             globals(), locals(), _import_tail)
         except ImportError:
-            mod = None
+            pass
+        else:
+            break
+    else:
+        mod = None
 
     try:
         getregentry = mod.getregentry
@@ -125,10 +134,9 @@ def search_function(encoding):
     except AttributeError:
         pass
     else:
-        import aliases
         for alias in codecaliases:
-            if not aliases.aliases.has_key(alias):
-                aliases.aliases[alias] = modname
+            if not _aliases.has_key(alias):
+                _aliases[alias] = modname
 
     # Return the registry entry
     return entry