Uppercase digits in hex literals (#530)

README.md

@@ -374,7 +374,8 @@ an adoption helper, avoid using this for new projects.
 
 ### Numeric literals
 
-*Black* standardizes most numeric literals to use lowercase letters: `0xab`
+*Black* standardizes most numeric literals to use lowercase letters for the
+syntactic parts and uppercase letters for the digits themselves: `0xAB`
 instead of `0XAB` and `1e10` instead of `1E10`. Python 2 long literals are
 styled as `2L` instead of `2l` to avoid confusion between `l` and `1`. In
 Python 3.6+, *Black* adds underscores to long numeric literals to aid

black.py

@@ -2542,9 +2542,13 @@ def normalize_numeric_literal(leaf: Leaf, allow_underscores: bool) -> None:
     in Python 2 long literals), and long number literals are split using underscores.
     """
     text = leaf.value.lower()
-    if text.startswith(("0o", "0x", "0b")):
-        # Leave octal, hex, and binary literals alone.
+    if text.startswith(("0o", "0b")):
+        # Leave octal and binary literals alone.
         pass
+    elif text.startswith("0x"):
+        # Change hex literals to upper case.
+        before, after = text[:2], text[2:]
+        text = f"{before}{after.upper()}"
     elif "e" in text:
         before, after = text.split("e")
         sign = ""

tests/data/numeric_literals.py

@@ -34,7 +34,7 @@
 x = 123_456_789e123_456_789
 x = 123_456_789j
 x = 123_456_789.123_456_789j
-x = 0xb1acc
+x = 0xB1ACC
 x = 0b1011
 x = 0o777
 x = 0.000_000_006

tests/data/numeric_literals_py2.py

@@ -3,6 +3,7 @@
 x = 123456789L
 x = 123456789l
 x = 123456789
+x = 0xb1acc
 
 # output
 
@@ -12,3 +13,4 @@
 x = 123456789L
 x = 123456789L
 x = 123456789
+x = 0xB1ACC