Added Python and grammar versions to a parser error message (#4378)

---------

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
Co-authored-by: Shantanu Jain <hauntsaninja@gmail.com>

CHANGES.md

@@ -47,6 +47,8 @@
 
 <!-- Changes to Black's terminal output and error messages -->
 
+- Added Python target version information on parse error (#4378)
+
 ### _Blackd_
 
 <!-- Changes to blackd -->

src/black/mode.py

@@ -26,6 +26,10 @@ class TargetVersion(Enum):
     PY312 = 12
     PY313 = 13
 
+    def pretty(self) -> str:
+        assert self.name[:2] == "PY"
+        return f"Python {self.name[2]}.{self.name[3:]}"
+
 
 class Feature(Enum):
     F_STRINGS = 2

src/black/parsing.py

@@ -5,7 +5,7 @@
 import ast
 import sys
 import warnings
-from typing import Iterable, Iterator, List, Set, Tuple
+from typing import Collection, Iterator, List, Set, Tuple
 
 from black.mode import VERSION_TO_FEATURES, Feature, TargetVersion, supports_feature
 from black.nodes import syms
@@ -52,12 +52,20 @@ def get_grammars(target_versions: Set[TargetVersion]) -> List[Grammar]:
     return grammars
 
 
-def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -> Node:
+def lib2to3_parse(
+    src_txt: str, target_versions: Collection[TargetVersion] = ()
+) -> Node:
     """Given a string with source, return the lib2to3 Node."""
     if not src_txt.endswith("\n"):
         src_txt += "\n"
 
     grammars = get_grammars(set(target_versions))
+    if target_versions:
+        max_tv = max(target_versions, key=lambda tv: tv.value)
+        tv_str = f" for target version {max_tv.pretty()}"
+    else:
+        tv_str = ""
+
     errors = {}
     for grammar in grammars:
         drv = driver.Driver(grammar)
@@ -73,14 +81,14 @@ def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -
             except IndexError:
                 faulty_line = "<line number missing in source>"
             errors[grammar.version] = InvalidInput(
-                f"Cannot parse: {lineno}:{column}: {faulty_line}"
+                f"Cannot parse{tv_str}: {lineno}:{column}: {faulty_line}"
             )
 
         except TokenError as te:
             # In edge cases these are raised; and typically don't have a "faulty_line".
             lineno, column = te.args[1]
             errors[grammar.version] = InvalidInput(
-                f"Cannot parse: {lineno}:{column}: {te.args[0]}"
+                f"Cannot parse{tv_str}: {lineno}:{column}: {te.args[0]}"
             )
 
     else:

tests/test_format.py

@@ -87,4 +87,6 @@ def test_patma_invalid() -> None:
     with pytest.raises(black.parsing.InvalidInput) as exc_info:
         assert_format(source, expected, mode, minimum_version=(3, 10))
 
-    exc_info.match("Cannot parse: 10:11")
+    exc_info.match(
+        "Cannot parse for target version Python 3.10: 10:11:     case a := b:"
+    )