Better context completions for finally/except/else/elif

Fixes #837

jedi/api/completion.py

@@ -1,5 +1,7 @@
 from parso.python import token
 from parso.python import tree
+from parso.tree import search_ancestor, Leaf
+
 from jedi import debug
 from jedi import settings
 from jedi.api import classes
@@ -137,6 +139,38 @@ class Completion:
         allowed_keywords, allowed_tokens = \
             helpers.get_possible_completion_types(grammar._pgen_grammar, self.stack)
 
+        if 'if' in allowed_keywords:
+            leaf = self._module_node.get_leaf_for_position(self._position, include_prefixes=True)
+            previous_leaf = leaf.get_previous_leaf()
+
+            indent = self._position[1]
+            if not (leaf.start_pos <= self._position <= leaf.end_pos):
+                indent = leaf.start_pos[1]
+
+            if previous_leaf is not None:
+                stmt = previous_leaf
+                while True:
+                    stmt = search_ancestor(
+                        stmt, 'if_stmt', 'for_stmt', 'while_stmt', 'try_stmt',
+                        'error_node',
+                    )
+                    if stmt is None:
+                        break
+
+                    type_ = stmt.type
+                    if type_ == 'error_node':
+                        first = stmt.children[0]
+                        if isinstance(first, Leaf):
+                            type_ = first.value + '_stmt'
+                    # Compare indents
+                    if stmt.start_pos[1] == indent:
+                        if type_ == 'if_stmt':
+                            allowed_keywords += ['elif', 'else']
+                        elif type_ == 'try_stmt':
+                            allowed_keywords += ['except', 'finally', 'else']
+                        elif type_ == 'for_stmt':
+                            allowed_keywords.append('else')
+
         completion_names = list(self._get_keyword_completion_names(allowed_keywords))
 
         if token.NAME in allowed_tokens or token.INDENT in allowed_tokens:

test/completion/completion.py

@@ -24,3 +24,27 @@ class MyClass:
 tuple,
           ):
         return 1
+
+
+if x:
+    pass
+#? ['else']
+else
+
+try:
+    pass
+#? ['except', 'Exception']
+except
+
+try:
+    pass
+#? 6 ['except', 'Exception']
+except AttributeError:
+    pass
+#? ['finally']
+finally
+
+for x in y:
+    pass
+#? ['else']
+else

test/test_api/test_completion.py

@@ -38,3 +38,12 @@ def test_indent_context():
     code = 'if 1:\nisinstanc'
     comp, = Script(code).completions()
     assert comp.name == 'isinstance'
+
+
+def test_keyword_context():
+    def get_names(*args, **kwargs):
+        return [d.name for d in Script(*args, **kwargs).completions()]
+
+    names = get_names('if 1:\n pass\n')
+    assert 'if' in names
+    assert 'elif' in names