Address review findings.

Author: ericvsmith

Lib/test/test_fstring.py

@@ -1093,7 +1093,8 @@ def test_debug_conversion(self):
         # Make sure nested fstrings still work.
         self.assertEqual(f'{f"{3.1415=:.1f}":*^20}', '*****3.1415=3.1*****')
 
-        # Make sure text before and after !d works correctly.
+        # Make sure text before and after an expression with = works
+        # correctly.
         pi = 'π'
         self.assertEqual(f'alpha α {pi=} ω omega', "alpha α pi='π' ω omega")
 
@@ -1117,7 +1118,7 @@ def test_debug_conversion(self):
 
         x = 20
         # This isn't an assignment expression, it's 'x', with a format
-        # spec of '=10'.
+        # spec of '=10'.  See test_walrus: you need to use parens.
         self.assertEqual(f'{x:=10}', '        20')
 
         # Test named function parameters, to make sure '=' parsing works
@@ -1147,6 +1148,15 @@ def __repr__(self):
         self.assertEqual(f'{C()=:x}', 'C()=FORMAT-x')
         self.assertEqual(f'{C()=!r:*^20}', 'C()=********REPR********')
 
+    def test_walrus(self):
+        x = 20
+        # This isn't an assignment expression, it's 'x', with a format
+        # spec of '=10'.  See test_walrus: you need to use parens.
+        self.assertEqual(f'{x:=10}', '        20')
+
+        # This is an assignment expression, which requires parens.
+        self.assertEqual(f'{(x:=10)}', '10')
+        self.assertEqual(x, 10)
 
 
 if __name__ == '__main__':

Misc/NEWS.d/next/Core and Builtins/2019-05-02-11-48-08.bpo-36774.ZqbJ1J.rst

@@ -1,7 +1,7 @@
-Add a ``=`` conversion feature to f-strings. This can precede ``!s``,
+Add a ``=`` feature f-strings for debugging. This can precede ``!s``,
 ``!r``, or ``!a``. It produces the text of the expression, followed by
 an equal sign, followed by the repr of the value of the expression. So
 ``f'{3*9+15=}'`` would be equal to the string ``'3*9+15=42'``.  If
-``=`` is specified, the default conversion is set to ``!r``.  Added a
-``!f`` conversion to switch back to the previous format behavior, if
-needed.
+``=`` is specified, the default conversion is set to ``!r``, unless a
+format spec is given, in which case the formatting behavior is
+unchanged, and __format__ will be used.

Python/ast.c

@@ -5020,7 +5020,7 @@ fstring_find_expr(const char **str, const char *end, int raw, int recurse_lvl,
     int conversion = -1; /* The conversion char.  Use default if not
                             specified, or !r if using = and no format
                             spec. */
-    int equal_conversion = 0; /* Are we using the = conversion? */
+    int equal_conversion = 0; /* Are we using the = feature? */
     PyObject *expr_text = NULL; /* The text of the expression, used for =. */
     const char *expr_text_end;
 
@@ -5194,7 +5194,8 @@ fstring_find_expr(const char **str, const char *end, int raw, int recurse_lvl,
     if (!simple_expression)
         goto error;
 
-    /* Check for = conversion. */
+    /* Check for =, which puts the text value of the expression in
+       expr_text. */
     if (**str == '=') {
         *str += 1;
         equal_conversion = 1;

Python/ast_unparse.c

@@ -656,7 +656,7 @@ append_formattedvalue(_PyUnicodeWriter *writer, expr_ty e, bool is_format_spec)
     Py_DECREF(temp_fv_str);
 
     if (e->v.FormattedValue.expr_text) {
-        /* Use the = debug conversion. */
+        /* Use the = for debug text expansion. */
         APPEND_STR("=");
     }