Fix normalize_string_prefix not lowercasing uppercase T t-string prefix

CHANGES.md

@@ -32,6 +32,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Normalize the uppercase `T` prefix of t-strings to lowercase in preview style (e.g.
+  `T"..."` is now normalized to `t"..."`) (#5185)
 - Fix unnecessary parentheses around short RHS expressions in indexed assignments like
   `x[key] = expr` (#5095)
 - Parenthesize tuple expressions in `yield` statements for consistency with function

docs/the_black_code_style/future_style.md

@@ -43,6 +43,8 @@ Currently, the following features are included in the preview style:
   anyway; let the bracket explode instead. ([see below](labels/hug-comparator))
 - `parenthesize_tuple_in_yield`: Add parentheses around tuple expressions in `yield`
   statements.
+- `normalize_tstring_prefix`: Normalize the uppercase `T` prefix of t-strings to
+  lowercase (e.g. `T"..."` becomes `t"..."`).
 
 (labels/wrap-comprehension-in)=
 

src/black/linegen.py

@@ -473,7 +473,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
             # We're ignoring docstrings with backslash newline escapes because changing
             # indentation of those changes the AST representation of the code.
             if self.mode.string_normalization:
-                docstring = normalize_string_prefix(leaf.value)
+                docstring = normalize_string_prefix(leaf.value, self.mode)
                 # We handle string normalization at the end of this method, but since
                 # what we do right now acts differently depending on quote style (ex.
                 # see padding logic below), there's a possibility for unstable
@@ -550,7 +550,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
                 leaf.value = prefix + quote + docstring + quote
 
         if self.mode.string_normalization and leaf.type == token.STRING:
-            leaf.value = normalize_string_prefix(leaf.value)
+            leaf.value = normalize_string_prefix(leaf.value, self.mode)
             leaf.value = normalize_string_quotes(leaf.value)
         yield from self.visit_default(leaf)
 

src/black/mode.py

@@ -263,6 +263,7 @@ class Preview(Enum):
     pyi_blank_line_after_function_docstring = auto()
     hug_comparator = auto()
     parenthesize_tuple_in_yield = auto()
+    normalize_tstring_prefix = auto()
 
 
 UNSTABLE_FEATURES: set[Preview] = {

src/black/strings.py

@@ -6,11 +6,15 @@
 import sys
 from functools import lru_cache
 from re import Match, Pattern
-from typing import Final
+from typing import TYPE_CHECKING, Final
 
 from black._width_table import WIDTH_TABLE
 from blib2to3.pytree import Leaf
 
+if TYPE_CHECKING:
+    from black.mode import Mode
+
+
 STRING_PREFIX_CHARS: Final = "fturbFTURB"  # All possible string prefix characters.
 STRING_PREFIX_RE: Final = re.compile(
     r"^([" + STRING_PREFIX_CHARS + r"]*)(.*)$", re.DOTALL
@@ -142,7 +146,7 @@ def assert_is_leaf_string(string: str) -> None:
     ), f"{set(string[:quote_idx])} is NOT a subset of {set(STRING_PREFIX_CHARS)}."
 
 
-def normalize_string_prefix(s: str) -> str:
+def normalize_string_prefix(s: str, mode: "Mode | None" = None) -> str:
     """Make all string prefixes lowercase."""
     match = STRING_PREFIX_RE.match(s)
     assert match is not None, f"failed to match string {s!r}"
@@ -153,13 +157,21 @@ def normalize_string_prefix(s: str) -> str:
         .replace("U", "")
         .replace("u", "")
     )
+    if mode is None or _is_preview_tstring_normalization(mode):
+        new_prefix = new_prefix.replace("T", "t")
 
     # Python syntax guarantees max 2 prefixes and that one of them is "r"
     if len(new_prefix) == 2 and new_prefix[0].lower() != "r":
         new_prefix = new_prefix[::-1]
     return f"{new_prefix}{match.group(2)}"
 
 
+def _is_preview_tstring_normalization(mode: "Mode") -> bool:
+    from black.mode import Preview
+
+    return Preview.normalize_tstring_prefix in mode
+
+
 # Re(gex) does actually cache patterns internally but this still improves
 # performance on a long list literal of strings by 5-9% since lru_cache's
 # caching overhead is much lower.

tests/data/cases/preview_pep_750_normalize_tstring_prefix.py

@@ -0,0 +1,13 @@
+# flags: --preview --minimum-version=3.14
+
+# Uppercase T prefix is normalized to lowercase t in preview style
+x = T"bar {1 + 1}"
+x = T'bar {1 + 1}'
+rT'\{{\}}'
+
+# output
+
+# Uppercase T prefix is normalized to lowercase t in preview style
+x = t"bar {1 + 1}"
+x = t"bar {1 + 1}"
+rt"\{{\}}"