Change get_previous_leaf to return None if there is no leaf anymore (at the start of the file).

jedi/parser/__init__.py

@@ -212,16 +212,14 @@ class Parser(object):
             if '\n' not in prefix:
                 # Basically if the last line doesn't end with a newline. we
                 # have to add the previous line's end_position.
-                try:
-                    last_end = endmarker.get_previous_leaf().end_pos[1]
-                except IndexError:
-                    pass
+                previous_leaf = endmarker.get_previous_leaf()
+                if previous_leaf is not None:
+                    last_end = previous_leaf.end_pos[1]
             last_line = re.sub('.*\n', '', prefix)
             endmarker.start_pos = endmarker.line - 1, last_end + len(last_line)
         else:
-            try:
-                newline = endmarker.get_previous_leaf()
-            except IndexError:
+            newline = endmarker.get_previous_leaf()
+            if newline is None:
                 return  # This means that the parser is empty.
 
             assert newline.value.endswith('\n')

jedi/parser/tree.py

@@ -232,7 +232,7 @@ class Base(object):
             if i == 0:
                 node = node.parent
                 if node.parent is None:
-                    raise IndexError('Cannot access the previous element of the first one.')
+                    return None
             else:
                 node = c[i - 1]
                 break
@@ -286,10 +286,10 @@ class Leaf(Base):
         self.indent = value[1]
 
     def get_start_pos_of_prefix(self):
-        try:
-            return self.get_previous_leaf().end_pos
-        except IndexError:
+        previous_leaf = self.get_previous_leaf()
+        if previous_leaf is None:
             return self.line - self.prefix.count('\n'), 0  # It's the first leaf.
+        return previous_leaf.end_pos
 
     @property
     def end_pos(self):