bpo-37050: Remove expr_text from FormattedValue ast node, use Constant node instead (GH-13597)

When using the "=" debug functionality of f-strings, use another Constant node (or a merged constant node) instead of adding expr_text to the FormattedValue node.

Author: ericvsmith

Include/Python-ast.h

@@ -1150,6 +1150,24 @@ def __repr__(self):
 
         self.assertRaises(SyntaxError, eval, "f'{C=]'")
 
+        # Make sure leading and following text works.
+        x = 'foo'
+        self.assertEqual(f'X{x=}Y', 'Xx='+repr(x)+'Y')
+
+        # Make sure whitespace around the = works.
+        self.assertEqual(f'X{x  =}Y', 'Xx  ='+repr(x)+'Y')
+        self.assertEqual(f'X{x=  }Y', 'Xx=  '+repr(x)+'Y')
+        self.assertEqual(f'X{x  =  }Y', 'Xx  =  '+repr(x)+'Y')
+
+        # These next lines contains tabs.  Backslash escapes don't
+        # work in f-strings.
+        # patchcheck doens't like these tabs.  So the only way to test
+        # this will be to dynamically created and exec the f-strings.  But
+        # that's such a hassle I'll save it for another day.  For now, convert
+        # the tabs to spaces just to shut up patchcheck.
+        #self.assertEqual(f'X{x =}Y', 'Xx\t='+repr(x)+'Y')
+        #self.assertEqual(f'X{x =       }Y', 'Xx\t=\t'+repr(x)+'Y')
+
     def test_walrus(self):
         x = 20
         # This isn't an assignment expression, it's 'x', with a format

Lib/test/test_fstring.py

@@ -270,12 +270,6 @@ def test_annotations(self):
         eq("f'{x}'")
         eq("f'{x!r}'")
         eq("f'{x!a}'")
-        eq("f'{x=!r}'")
-        eq("f'{x=:}'")
-        eq("f'{x=:.2f}'")
-        eq("f'{x=!r}'")
-        eq("f'{x=!a}'")
-        eq("f'{x=!s:*^20}'")
         eq('(yield from outside_of_generator)')
         eq('(yield)')
         eq('(yield a + b)')
@@ -290,6 +284,15 @@ def test_annotations(self):
         eq("(x:=10)")
         eq("f'{(x:=10):=10}'")
 
+        # f-strings with '=' don't round trip very well, so set the expected
+        # result explicitely.
+        self.assertAnnotationEqual("f'{x=!r}'", expected="f'x={x!r}'")
+        self.assertAnnotationEqual("f'{x=:}'", expected="f'x={x:}'")
+        self.assertAnnotationEqual("f'{x=:.2f}'", expected="f'x={x:.2f}'")
+        self.assertAnnotationEqual("f'{x=!r}'", expected="f'x={x!r}'")
+        self.assertAnnotationEqual("f'{x=!a}'", expected="f'x={x!a}'")
+        self.assertAnnotationEqual("f'{x=!s:*^20}'", expected="f'x={x!s:*^20}'")
+
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_future.py

@@ -0,0 +1,4 @@
+Improve the AST for "debug" f-strings, which use '=' to print out the source
+of the expression being evaluated.  Delete expr_text from the FormattedValue
+node, and instead use a Constant string node (possibly merged with adjacent
+constant expressions inside the f-string).

Misc/NEWS.d/next/Core and Builtins/2019-05-27-14-46-24.bpo-37050.7MyZGg.rst

@@ -76,7 +76,7 @@ module Python
          -- x < 4 < 3 and (x < 4) < 3
          | Compare(expr left, cmpop* ops, expr* comparators)
          | Call(expr func, expr* args, keyword* keywords)
-         | FormattedValue(expr value, int? conversion, expr? format_spec, string? expr_text)
+         | FormattedValue(expr value, int? conversion, expr? format_spec)
          | JoinedStr(expr* values)
          | Constant(constant value, string? kind)
 

Parser/Python.asdl

@@ -5006,10 +5006,16 @@ fstring_parse(const char **str, const char *end, int raw, int recurse_lvl,
    closing brace doesn't match an opening paren, for example. It
    doesn't need to error on all invalid expressions, just correctly
    find the end of all valid ones. Any errors inside the expression
-   will be caught when we parse it later. */
+   will be caught when we parse it later.
+
+   *expression is set to the expression.  For an '=' "debug" expression,
+   *expr_text is set to the debug text (the original text of the expression,
+   *including the '=' and any whitespace around it, as a string object).  If
+   *not a debug expression, *expr_text set to NULL. */
 static int
 fstring_find_expr(const char **str, const char *end, int raw, int recurse_lvl,
-                  expr_ty *expression, struct compiling *c, const node *n)
+                  PyObject **expr_text, expr_ty *expression,
+                  struct compiling *c, const node *n)
 {
     /* Return -1 on error, else 0. */
 
@@ -5020,9 +5026,6 @@ 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_flag = 0; /* Are we using the = feature? */
-    PyObject *expr_text = NULL; /* The text of the expression, used for =. */
-    const char *expr_text_end;
 
     /* 0 if we're not in a string, else the quote char we're trying to
        match (single or double quote). */
@@ -5198,15 +5201,21 @@ fstring_find_expr(const char **str, const char *end, int raw, int recurse_lvl,
        expr_text. */
     if (**str == '=') {
         *str += 1;
-        equal_flag = 1;
 
         /* Skip over ASCII whitespace.  No need to test for end of string
            here, since we know there's at least a trailing quote somewhere
            ahead. */
         while (Py_ISSPACE(**str)) {
             *str += 1;
         }
-        expr_text_end = *str;
+
+        /* Set *expr_text to the text of the expression. */
+        *expr_text = PyUnicode_FromStringAndSize(expr_start, *str-expr_start);
+        if (!*expr_text) {
+            goto error;
+        }
+    } else {
+        *expr_text = NULL;
     }
 
     /* Check for a conversion char, if present. */
@@ -5227,17 +5236,6 @@ fstring_find_expr(const char **str, const char *end, int raw, int recurse_lvl,
         }
 
     }
-    if (equal_flag) {
-        Py_ssize_t len = expr_text_end - expr_start;
-        expr_text = PyUnicode_FromStringAndSize(expr_start, len);
-        if (!expr_text) {
-            goto error;
-        }
-        if (PyArena_AddPyObject(c->c_arena, expr_text) < 0) {
-            Py_DECREF(expr_text);
-            goto error;
-        }
-    }
 
     /* Check for the format spec, if present. */
     if (*str >= end)
@@ -5261,16 +5259,16 @@ fstring_find_expr(const char **str, const char *end, int raw, int recurse_lvl,
     assert(**str == '}');
     *str += 1;
 
-    /* If we're in = mode, and have no format spec and no explict conversion,
-       set the conversion to 'r'. */
-    if (equal_flag && format_spec == NULL && conversion == -1) {
+    /* If we're in = mode (detected by non-NULL expr_text), and have no format
+       spec and no explict conversion, set the conversion to 'r'. */
+    if (*expr_text && format_spec == NULL && conversion == -1) {
         conversion = 'r';
     }
 
     /* And now create the FormattedValue node that represents this
        entire expression with the conversion and format spec. */
     *expression = FormattedValue(simple_expression, conversion,
-                                 format_spec, expr_text, LINENO(n),
+                                 format_spec, LINENO(n),
                                  n->n_col_offset, n->n_end_lineno,
                                  n->n_end_col_offset, c->c_arena);
     if (!*expression)
@@ -5313,7 +5311,7 @@ fstring_find_expr(const char **str, const char *end, int raw, int recurse_lvl,
 static int
 fstring_find_literal_and_expr(const char **str, const char *end, int raw,
                               int recurse_lvl, PyObject **literal,
-                              expr_ty *expression,
+                              PyObject **expr_text, expr_ty *expression,
                               struct compiling *c, const node *n)
 {
     int result;
@@ -5341,7 +5339,8 @@ fstring_find_literal_and_expr(const char **str, const char *end, int raw,
     /* We must now be the start of an expression, on a '{'. */
     assert(**str == '{');
 
-    if (fstring_find_expr(str, end, raw, recurse_lvl, expression, c, n) < 0)
+    if (fstring_find_expr(str, end, raw, recurse_lvl, expr_text,
+                          expression, c, n) < 0)
         goto error;
 
     return 0;
@@ -5604,39 +5603,42 @@ FstringParser_ConcatFstring(FstringParser *state, const char **str,
 
     /* Parse the f-string. */
     while (1) {
-        PyObject *literal = NULL;
+        PyObject *literal[2] = {NULL, NULL};
         expr_ty expression = NULL;
 
         /* If there's a zero length literal in front of the
            expression, literal will be NULL. If we're at the end of
            the f-string, expression will be NULL (unless result == 1,
            see below). */
         int result = fstring_find_literal_and_expr(str, end, raw, recurse_lvl,
-                                                   &literal, &expression,
-                                                   c, n);
+                                                   &literal[0], &literal[1],
+                                                   &expression, c, n);
         if (result < 0)
             return -1;
 
-        /* Add the literal, if any. */
-        if (!literal) {
-            /* Do nothing. Just leave last_str alone (and possibly
-               NULL). */
-        } else if (!state->last_str) {
-            /*  Note that the literal can be zero length, if the
-                input string is "\\\n" or "\\\r", among others. */
-            state->last_str = literal;
-            literal = NULL;
-        } else {
-            /* We have a literal, concatenate it. */
-            assert(PyUnicode_GET_LENGTH(literal) != 0);
-            if (FstringParser_ConcatAndDel(state, literal) < 0)
-                return -1;
-            literal = NULL;
+        /* Add the literals, if any. */
+        for (int i = 0; i < 2; i++) {
+            if (!literal[i]) {
+                /* Do nothing. Just leave last_str alone (and possibly
+                   NULL). */
+            } else if (!state->last_str) {
+                /*  Note that the literal can be zero length, if the
+                    input string is "\\\n" or "\\\r", among others. */
+                state->last_str = literal[i];
+                literal[i] = NULL;
+            } else {
+                /* We have a literal, concatenate it. */
+                assert(PyUnicode_GET_LENGTH(literal[i]) != 0);
+                if (FstringParser_ConcatAndDel(state, literal[i]) < 0)
+                    return -1;
+                literal[i] = NULL;
+            }
         }
 
-        /* We've dealt with the literal now. It can't be leaked on further
+        /* We've dealt with the literals now. They can't be leaked on further
            errors. */
-        assert(literal == NULL);
+        assert(literal[0] == NULL);
+        assert(literal[1] == NULL);
 
         /* See if we should just loop around to get the next literal
            and expression, while ignoring the expression this

Python/Python-ast.c

@@ -665,11 +665,6 @@ append_formattedvalue(_PyUnicodeWriter *writer, expr_ty e, bool is_format_spec)
     }
     Py_DECREF(temp_fv_str);
 
-    if (e->v.FormattedValue.expr_text) {
-        /* Use the = for debug text expansion. */
-        APPEND_STR("=");
-    }
-
     if (e->v.FormattedValue.conversion > 0) {
         switch (e->v.FormattedValue.conversion) {
         case 'a':

Python/ast.c

@@ -3963,12 +3963,6 @@ compiler_formatted_value(struct compiler *c, expr_ty e)
     int conversion = e->v.FormattedValue.conversion;
     int oparg;
 
-    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);
 
@@ -3991,11 +3985,6 @@ 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;
 }