bpo-36817: Add f-string debugging using '='. (GH-13123)

If a "=" is specified a the end of an f-string expression, the f-string will evaluate to the text of the expression, followed by '=', followed by the repr of the value of the expression.

1 files changed, 19 insertions, 6 deletions

diff --git a/Python/compile.c b/Python/compile.c
index 86f2a09..dd27ba8 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -3946,8 +3946,8 @@ compiler_formatted_value(struct compiler *c, expr_ty e)
     /* Our oparg encodes 2 pieces of information: the conversion
        character, and whether or not a format_spec was provided.
 
-       Convert the conversion char to 2 bits:
-       None: 000  0x0  FVC_NONE
+       Convert the conversion char to 3 bits:
+           : 000  0x0  FVC_NONE   The default if nothing specified.
        !s  : 001  0x1  FVC_STR
        !r  : 010  0x2  FVC_REPR
        !a  : 011  0x3  FVC_ASCII
@@ -3957,19 +3957,26 @@ compiler_formatted_value(struct compiler *c, expr_ty e)
        no  : 000  0x0
     */
 
+    int conversion = e->v.FormattedValue.conversion;
     int oparg;
 
-    /* Evaluate the expression to be formatted. */
+    if (e->v.FormattedValue.expr_text) {
+        /* Push the text of the expression (which already has the '=' in
+           it. */
+        ADDOP_LOAD_CONST(c, e->v.FormattedValue.expr_text);
+    }
+
+    /* The expression to be formatted. */
     VISIT(c, expr, e->v.FormattedValue.value);
 
-    switch (e->v.FormattedValue.conversion) {
+    switch (conversion) {
     case 's': oparg = FVC_STR;   break;
     case 'r': oparg = FVC_REPR;  break;
     case 'a': oparg = FVC_ASCII; break;
     case -1:  oparg = FVC_NONE;  break;
     default:
-        PyErr_SetString(PyExc_SystemError,
-                        "Unrecognized conversion character");
+        PyErr_Format(PyExc_SystemError,
+                     "Unrecognized conversion character %d", conversion);
         return 0;
     }
     if (e->v.FormattedValue.format_spec) {
@@ -3980,6 +3987,12 @@ compiler_formatted_value(struct compiler *c, expr_ty e)
 
     /* And push our opcode and oparg */
     ADDOP_I(c, FORMAT_VALUE, oparg);
+
+    /* If we have expr_text, join the 2 strings on the stack. */
+    if (e->v.FormattedValue.expr_text) {
+        ADDOP_I(c, BUILD_STRING, 2);
+    }
+
     return 1;
 }