do not use names which conflict with builtin str type

src/calibre/ebooks/chm/metadata.py

@@ -21,15 +21,15 @@ def _clean(s):
 
 
 def _detag(tag):
-    str = u""
+    ans = u""
     if tag is None:
-        return str
+        return ans
     for elem in tag:
         if hasattr(elem, "contents"):
-            str += _detag(elem)
+            ans += _detag(elem)
         else:
-            str += _clean(elem)
-    return str
+            ans += _clean(elem)
+    return ans
 
 
 def _metadata_from_table(soup, searchfor):

src/calibre/ebooks/conversion/cli.py

@@ -124,9 +124,9 @@ def recipe_test(option, opt_str, value, parser):
     assert value is None
     value = []
 
-    def floatable(str):
+    def floatable(s):
         try:
-            float(str)
+            float(s)
             return True
         except ValueError:
             return False