tokenizer: skip lines that are just slash and whitespace (#4343)

Author: tusharsadhwani

AUTHORS.md

@@ -181,6 +181,7 @@ Multiple contributions by:
 - [Tony Narlock](mailto:[email protected])
 - [Tsuyoshi Hombashi](mailto:[email protected])
 - [Tushar Chandra](mailto:[email protected])
+- [Tushar Sadhwani](mailto:[email protected])
 - [Tzu-ping Chung](mailto:[email protected])
 - [Utsav Shah](mailto:[email protected])
 - utsav-dbx

CHANGES.md

@@ -34,6 +34,8 @@
 - Fix regression where Black failed to parse a multiline f-string containing another
   multiline string (#4339)
 
+- Fix bug with Black incorrectly parsing empty lines with a backslash (#4343)
+
 ### Performance
 
 <!-- Changes that improve Black's performance. -->

src/blib2to3/pgen2/tokenize.py

@@ -608,6 +608,12 @@ def generate_tokens(
         except StopIteration:
             line = ""
         lnum += 1
+
+        # skip lines that are just indent characters ending with a slash
+        # to avoid storing that line's indent information.
+        if not contstr and line.rstrip("\n").strip(" \t\f") == "\\":
+            continue
+
         pos, max = 0, len(line)
 
         if contstr:  # continued string

tests/data/cases/backslash_before_indent.py

@@ -0,0 +1,24 @@
+# flags: --minimum-version=3.10
+class Plotter:
+\
+    pass
+
+class AnotherCase:
+  \
+    """Some
+    \
+    Docstring
+    """
+
+# output
+
+class Plotter:
+
+    pass
+
+
+class AnotherCase:
+    """Some
+    \
+    Docstring
+    """

tests/data/cases/comment_after_escaped_newline.py

@@ -14,5 +14,7 @@ def bob():  # pylint: disable=W9016
     pass
 
 
-def bobtwo():  # some comment here
+def bobtwo():
+
+    # some comment here
     pass

tests/data/cases/form_feeds.py

@@ -156,6 +156,7 @@ def something(self):
 
 #
 
+
 #
 pass