Make string additions work for file path completion

With this most simple cases of file path completions should be working now, fixes #493

jedi/api/file_name.py

@@ -3,13 +3,15 @@ import os
 from jedi._compatibility import FileNotFoundError
 from jedi.evaluate.names import AbstractArbitraryName
 from jedi.api import classes
+from jedi.evaluate.helpers import get_str_or_none
 
 
-def file_name_completions(evaluator, string, like_name):
+def file_name_completions(evaluator, module_context, start_leaf, string, like_name):
     base_name = os.path.basename(string)
     like_name = base_name + like_name
     string = os.path.dirname(string)
 
+    string = _get_string_additions(module_context, start_leaf) + string
     base_path = os.path.join(evaluator.project._path, string)
     try:
         listed = os.listdir(base_path)
@@ -29,6 +31,35 @@ def file_name_completions(evaluator, string, like_name):
             )
 
 
+def _get_string_additions(module_context, start_leaf):
+    addition = start_leaf.get_previous_leaf()
+    if addition != '+':
+        return ''
+    node = addition.parent
+    string = ''
+    was_addition = True
+    for child_node in reversed(node.children[:node.children.index(addition)]):
+        if was_addition:
+            was_addition = False
+            context = module_context.create_context(node)
+            contexts = context.eval_node(child_node)
+            if len(contexts) != 1:
+                return string
+            c, = contexts
+            s = get_str_or_none(c)
+            if s is None:
+                return string
+            string = s + string
+            continue
+
+        if child_node != '+':
+            break
+        was_addition = True
+        child_node
+    module_context
+    return string
+
+
 class FileName(AbstractArbitraryName):
     api_type = u'path'
     is_context_name = False