Create class in maintenance module to help the UIs

The class has methods commonly used by user interfaces, so that
they don't need to be implemented multiple times by different
UIs.

relational/maintenance.py

@@ -22,7 +22,9 @@
 import http.client
 import urllib.parse
 
-from relational import relation
+from relational.relation import relation
+from relational import parser
+from relational.rtypes import is_valid_relation_name
 
 
 def send_survey(data):
@@ -58,23 +60,24 @@ def check_latest_version():
     return s.decode().strip()
 
 
-class interface (object):
+class user_interface (object):
 
     '''It is used to provide services to the user interfaces, in order to
     reduce the amount of duplicated code present in different user interfaces.
     '''
 
     def __init__(self):
-        self.rels = {}
+        self.relations = {}
 
     def load(self, filename, name):
         '''Loads a relation from file, and gives it a name to
         be used in subsequent queries.'''
-        pass
+        rel = relation(filename)
+        self.set_relation(name, rel)
 
     def unload(self, name):
         '''Unloads an existing relation.'''
-        pass
+        del self.relations[name]
 
     def store(self, filename, name):
         '''Stores a relation to file.'''
@@ -82,14 +85,53 @@ class interface (object):
 
     def get_relation(self, name):
         '''Returns the relation corresponding to name.'''
-        pass
+        return self.relations[name]
 
     def set_relation(self, name, rel):
         '''Sets the relation corresponding to name.'''
-        pass
+        if not is_valid_relation_name(name):
+            raise Exception('Invalid name for destination relation')
+        self.relations[name] = rel
 
     def execute(self, query, relname='last_'):
         '''Executes a query, returns the result and if
         relname is not None, adds the result to the
         dictionary, with the name given in relname.'''
-        pass
+        if not is_valid_relation_name(relname):
+            raise Exception('Invalid name for destination relation')
+
+        expr = parser.parse(query)
+        result = eval(expr, self.relations)
+        self.relations[relname] = result
+        return result
+
+    def multi_execute(self, query):
+        '''Executes multiple queries, separated by \n
+
+        They can have a syntax of
+        [varname =] query
+        to assign the result to a new relation
+        '''
+        r = relation()
+        queries = query.split('\n')
+        for query in queries:
+            if query.strip() == '':
+                continue
+            parts = query.split('=', 1)
+            parts[0] = parts[0].strip()
+            if len(parts) > 1 and is_valid_relation_name(parts[0]):
+                relname, query = parts
+            else:
+                relname = 'last_'
+
+            try:
+                r = self.execute(query, relname)
+            except Exception as e:
+                raise Exception('Error in query: %s\n%s' % (
+                    query,
+                    str(e)
+                ))
+        return r
+
+
+