[3.10] bpo-44305: Improve syntax error for try blocks without except or finally (GH-26523) (GH-26524)

(cherry picked from commit b250f89bb7)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Doc/whatsnew/3.10.rst

@@ -274,6 +274,20 @@ have been incorporated. Some of the most notable ones:
 
     (Contributed by Pablo Galindo in :issue:`43823`)
 
+* ``try`` blocks without ``except`` or ``finally`` blocks:
+
+    .. code-block:: python
+
+        >>> try
+        ...     x = 2
+        ... something = 3
+          File "<stdin>", line 3
+            something  = 3
+            ^^^^^^^^^
+        SyntaxError: expected 'except' or 'finally' block
+
+    (Contributed by Pablo Galindo in :issue:`44305`)
+
 * Usage of ``=`` instead of ``==`` in comparisons:
 
     .. code-block:: python

Grammar/python.gram

@@ -964,6 +964,7 @@ invalid_with_stmt_indent:
 invalid_try_stmt:
     | a='try' ':' NEWLINE !INDENT {
         RAISE_INDENTATION_ERROR("expected an indented block after 'try' statement on line %d", a->lineno) }
+    | 'try' ':' block !('except' | 'finally') { RAISE_SYNTAX_ERROR("expected 'except' or 'finally' block") }
 invalid_except_stmt:
     | 'except' a=expression ',' expressions ['as' NAME ] ':' {
         RAISE_SYNTAX_ERROR_STARTING_FROM(a, "multiple exception types must be parenthesized") }

Lib/test/test_syntax.py

@@ -882,6 +882,14 @@ leading to spurious errors.
    Traceback (most recent call last):
    SyntaxError: cannot assign to attribute here. Maybe you meant '==' instead of '='?
 
+Custom error messages for try blocks that are not followed by except/finally
+
+   >>> try:
+   ...    x = 34
+   ...
+   Traceback (most recent call last):
+   SyntaxError: expected 'except' or 'finally' block
+
 Ensure that early = are not matched by the parser as invalid comparisons
    >>> f(2, 4, x=34); 1 $ 2
    Traceback (most recent call last):

Misc/NEWS.d/next/Core and Builtins/2021-06-03-22-51-50.bpo-44305.66dVDG.rst

@@ -0,0 +1,2 @@
+Improve error message for ``try`` blocks without ``except`` or ``finally``
+blocks. Patch by Pablo Galindo.