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

jedi/api/helpers.py

@@ -74,16 +74,14 @@ def _get_code_for_stack(code_lines, module_node, position):
             return u('')
 
         # If we're not on a comment simply get the previous leaf and proceed.
-        try:
         leaf = leaf.get_previous_leaf()
-        except IndexError:
+        if leaf is None:
             return u('')  # At the beginning of the file.
 
     is_after_newline = leaf.type == 'newline'
     while leaf.type == 'newline':
-        try:
         leaf = leaf.get_previous_leaf()
-        except IndexError:
+        if leaf is None:
             return u('')
 
     if leaf.type == 'error_leaf' or leaf.type == 'string':
@@ -244,6 +242,8 @@ def _get_call_signature_details_from_error_node(node, position):
             # until the parentheses is enough.
             children = node.children[index:]
             name = element.get_previous_leaf()
+            if name is None:
+                continue
             if name.type == 'name' or name.parent.type in ('trailer', 'atom'):
                 return CallSignatureDetails(
                     element,
@@ -255,9 +255,8 @@ def get_call_signature_details(module, position):
     leaf = module.get_leaf_for_position(position, include_prefixes=True)
     if leaf.start_pos >= position:
         # Whitespace / comments after the leaf count towards the previous leaf.
-        try:
         leaf = leaf.get_previous_leaf()
-        except IndexError:
+        if leaf is None:
             return None
 
     if leaf == ')':
@@ -281,6 +280,8 @@ def get_call_signature_details(module, position):
 
         if node.type == 'trailer' and node.children[0] == '(':
             leaf = node.get_previous_leaf()
+            if leaf is None:
+                return None
             return CallSignatureDetails(
                 node.children[0], *_get_index_and_key(node.children, position))
 

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:
+                previous_leaf = endmarker.get_previous_leaf()
-                    last_end = endmarker.get_previous_leaf().end_pos[1]
+                if previous_leaf is not None:
-                except IndexError:
+                    last_end = previous_leaf.end_pos[1]
-                    pass
             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:
+            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:
+        previous_leaf = self.get_previous_leaf()
-            return self.get_previous_leaf().end_pos
+        if previous_leaf is None:
-        except IndexError:
             return self.line - self.prefix.count('\n'), 0  # It's the first leaf.
+        return previous_leaf.end_pos
 
     @property
     def end_pos(self):