From ca7a34db16d65afd1fbd43848bab475d9b88d233 Mon Sep 17 00:00:00 2001
From: Richard Si <63936253+ichard26@users.noreply.github.com>
Date: Sun, 30 May 2021 16:16:19 -0400
Subject: [PATCH] Regression fix: leave R prefixes capitalization alone

`black.strings.get_string_prefix` used to lowercase the extracted
prefix before returning it. This is wrong because 1) it ignores the
fact we should leave R prefixes alone because of MagicPython, and 2)
there is dedicated prefix casing handling code that fixes issue 1.
`.lower` is too naive.

This was originally fixed in 20.8b0, but was reintroduced since 21.4b0.
---
 CHANGES.md                    |  1 +
 src/black/strings.py          |  2 +-
 tests/data/string_prefixes.py | 21 +++++++++++++++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/CHANGES.md b/CHANGES.md
index 63c7a2cf30d..562b28d8039 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -11,6 +11,7 @@
 - Restored compatibility with Click 8.0 on Python 3.6 when LANG=C used (#2227)
 - Add extra uvloop install + import support if in python env (#2258)
 - Fix --experimental-string-processing crash when matching parens are not found (#2283)
+- Fix regression where `R` prefixes would be lowercased for docstrings (#2285)
 
 ### _Blackd_
 
diff --git a/src/black/strings.py b/src/black/strings.py
index 5b443ddebc9..80f588f5119 100644
--- a/src/black/strings.py
+++ b/src/black/strings.py
@@ -87,7 +87,7 @@ def get_string_prefix(string: str) -> str:
     prefix = ""
     prefix_idx = 0
     while string[prefix_idx] in STRING_PREFIX_CHARS:
-        prefix += string[prefix_idx].lower()
+        prefix += string[prefix_idx]
         prefix_idx += 1
 
     return prefix
diff --git a/tests/data/string_prefixes.py b/tests/data/string_prefixes.py
index 0ca3686a2b6..9ddc2b540fc 100644
--- a/tests/data/string_prefixes.py
+++ b/tests/data/string_prefixes.py
@@ -6,6 +6,17 @@
 r"hello"
 fR"hello"
 
+
+def docstring_singleline():
+    R"""2020 was one hell of a year. The good news is that we were able to"""
+
+
+def docstring_multiline():
+    R"""
+    clear out all of the issues opened in that time :p
+    """
+
+
 # output
 
 
@@ -16,3 +27,13 @@
 b"hello"
 r"hello"
 fR"hello"
+
+
+def docstring_singleline():
+    R"""2020 was one hell of a year. The good news is that we were able to"""
+
+
+def docstring_multiline():
+    R"""
+    clear out all of the issues opened in that time :p
+    """