storage:util:upgrade: make yaml import optional (unless it isn't)

Importing `yaml` may fail (if the user doesn't have PyYAML installed),
but don't die until we need to use it.  This way users without the old
YAML formats on disk can run BE without installing PyYAML.

diff --git a/libbe/storage/util/upgrade.py b/libbe/storage/util/upgrade.py
index fdbc578461563d78498cc97092203a6ba45bcf79..98091f2c31261d256f6c986b1fd4effde7484806 100644 (file)
--- a/libbe/storage/util/upgrade.py
+++ b/libbe/storage/util/upgrade.py
@@ -27,7 +27,11 @@ import os, os.path
 import sys
 import types
 
-import yaml
+try:
+    import yaml
+except ImportError as e:
+    yaml = None
+    _yaml_import_error = e
 
 import libbe
 import libbe.bug
@@ -50,6 +54,8 @@ def generate_yaml_mapfile(map):
     >>> generate_yaml_mapfile({'q':u'hello'})
     'q: hello\\n\\n'
     """
+    if yaml is None:
+        raise _yaml_import_error
     keys = map.keys()
     keys.sort()
     for key in keys:
@@ -93,6 +99,8 @@ def parse_yaml_mapfile(contents):
     >>> dict['q']
     u'Fran\\xe7ais'
     """
+    if yaml is None:
+        raise _yaml_import_error
     c = yaml.safe_load(contents)
     if type(c) == types.StringType:
         raise mapfile.InvalidMapfileContents(