From aa0fe4698134bd3bd402e21bb02d5c8059b6333b Mon Sep 17 00:00:00 2001
From: Thomas Grainger <tagrain@gmail.com>
Date: Mon, 5 Jun 2017 18:37:59 +0100
Subject: [PATCH] convert f strings to concat

---
 py_backwards/transformers/formatted_values.py | 22 ++++++++++++++-----
 tests/transformers/test_formatted_values.py   |  5 +++--
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/py_backwards/transformers/formatted_values.py b/py_backwards/transformers/formatted_values.py
index 8620b47..f8f1481 100644
--- a/py_backwards/transformers/formatted_values.py
+++ b/py_backwards/transformers/formatted_values.py
@@ -1,3 +1,5 @@
+import functools
+
 from typed_ast import ast3 as ast
 from ..const import TARGET_ALL
 from .base import BaseNodeTransformer
@@ -7,7 +9,7 @@ class FormattedValuesTransformer(BaseNodeTransformer):
     """Compiles:
         f"hello {x}"
     To
-        ''.join(['hello ', '{}'.format(x)])
+        'hello ' + '{}'.format(x)
     
     """
     target = TARGET_ALL
@@ -28,9 +30,17 @@ def visit_FormattedValue(self, node: ast.FormattedValue) -> ast.Call:
 
     def visit_JoinedStr(self, node: ast.JoinedStr) -> ast.Call:
         self._tree_changed = True
+        NONE = ast.Str(s='')
+
+        init = ast.BinOp(left=NONE, right=NONE, op=ast.Add())
+
+        def to_binop(acc, next):
+            left, right, op = acc.left, acc.right, acc.op
+            if left is NONE:
+                return ast.BinOp(left=right, right=next, op=op)
+            return ast.BinOp(left=acc, right=next, op=op)
+
+        value = functools.reduce(to_binop, node.values, init)
+        concat_expr = ast.Expr(value=value)
 
-        join_call = ast.Call(func=ast.Attribute(value=ast.Str(s=''),
-                                                attr='join'),
-                             args=[ast.List(elts=node.values)],
-                             keywords=[])
-        return self.generic_visit(join_call)  # type: ignore
+        return self.generic_visit(concat_expr)  # type: ignore
diff --git a/tests/transformers/test_formatted_values.py b/tests/transformers/test_formatted_values.py
index c4ea950..e025e52 100644
--- a/tests/transformers/test_formatted_values.py
+++ b/tests/transformers/test_formatted_values.py
@@ -4,9 +4,10 @@
 
 @pytest.mark.parametrize('before, after', [
     ("f'hi'", "'hi'"),
-    ("f'hi {x}'", "''.join(['hi ', '{}'.format(x)])"),
+    ("f'hi {x}'", "'hi ' + '{}'.format(x)"),
+    ("d = f'{a} {b}!'", "d = '{}'.format(a) + ' ' + '{}'.format(b) + '!'"),
     ("f'hi {x.upper()} {y:1}'",
-     "''.join(['hi ', '{}'.format(x.upper()), ' ', '{:1}'.format(y)])")])
+     "'hi ' + '{}'.format(x.upper()) + ' ' + '{:1}'.format(y)")])
 def test_transform(transform, ast, before, after):
     code = transform(FormattedValuesTransformer, before)
     assert ast(code) == ast(after)