gh-99606: Make code generated for an empty f-string identical to that of a normal empty string (#112407)

iritkatriel · web-flow · commit fb202af4470d · 2023-11-26T17:13:57.000Z
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
@@ -8,6 +8,7 @@
 # Unicode identifiers in tests is allowed by PEP 3131.
 
 import ast
+import dis
 import os
 import re
 import types
@@ -1738,5 +1739,14 @@ def test_syntax_warning_infinite_recursion_in_file(self):
             self.assertIn(rb'\1', stdout)
             self.assertEqual(len(stderr.strip().splitlines()), 2)
 
+    def test_fstring_without_formatting_bytecode(self):
+        # f-string without any formatting should emit the same bytecode
+        # as a normal string. See gh-99606.
+        def get_code(s):
+            return [(i.opname, i.oparg) for i in dis.get_instructions(s)]
+
+        for s in ["", "some string"]:
+            self.assertEqual(get_code(f"'{s}'"), get_code(f"f'{s}'"))
+
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-11-25-20-36-38.gh-issue-99606.fDY5hK.rst b/Misc/NEWS.d/next/Core and Builtins/2023-11-25-20-36-38.gh-issue-99606.fDY5hK.rst
@@ -0,0 +1,2 @@
+Make code generated for an empty f-string identical to the code of an empty
+normal string.
diff --git a/Python/compile.c b/Python/compile.c
@@ -5042,8 +5042,12 @@ compiler_joined_str(struct compiler *c, expr_ty e)
     }
     else {
         VISIT_SEQ(c, expr, e->v.JoinedStr.values);
-        if (asdl_seq_LEN(e->v.JoinedStr.values) != 1) {
-            ADDOP_I(c, loc, BUILD_STRING, asdl_seq_LEN(e->v.JoinedStr.values));
+        if (value_count > 1) {
+            ADDOP_I(c, loc, BUILD_STRING, value_count);
+        }
+        else if (value_count == 0) {
+            _Py_DECLARE_STR(empty, "");
+            ADDOP_LOAD_CONST_NEW(c, loc, Py_NewRef(&_Py_STR(empty)));
         }
     }
     return SUCCESS;

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Make code generated for an empty f-string identical to the code of an empty`
	`2`	`+normal string.`
Original file line number	Diff line number	Diff line change
`@@ -5042,8 +5042,12 @@ compiler_joined_str(struct compiler *c, expr_ty e)`
`5042`	`5042`	`}`
`5043`	`5043`	`else {`
`5044`	`5044`	`VISIT_SEQ(c, expr, e->v.JoinedStr.values);`
`5045`		`- if (asdl_seq_LEN(e->v.JoinedStr.values) != 1) {`
`5046`		`- ADDOP_I(c, loc, BUILD_STRING, asdl_seq_LEN(e->v.JoinedStr.values));`
	`5045`	`+ if (value_count > 1) {`
	`5046`	`+ ADDOP_I(c, loc, BUILD_STRING, value_count);`
	`5047`	`+ }`
	`5048`	`+ else if (value_count == 0) {`
	`5049`	`+ _Py_DECLARE_STR(empty, "");`
	`5050`	`+ ADDOP_LOAD_CONST_NEW(c, loc, Py_NewRef(&_Py_STR(empty)));`
`5047`	`5051`	`}`
`5048`	`5052`	`}`
`5049`	`5053`	`return SUCCESS;`