Fix crash on docstrings ending with "\ " (#2142)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

CHANGES.md

@@ -4,6 +4,8 @@
 
 #### _Black_
 
+- Fix crash on docstrings ending with "\ ". (#2142)
+
 - Reflect the `--skip-magic-trailing-comma` and `--experimental-string-processing` flags
   in the name of the cache file. Without this fix, changes in these flags would not take
   effect if the cache had already been populated. (#2131)

src/black/__init__.py

@@ -2186,7 +2186,13 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
                 if docstring[0] == quote_char:
                     docstring = " " + docstring
                 if docstring[-1] == quote_char:
-                    docstring = docstring + " "
+                    docstring += " "
+                if docstring[-1] == "\\":
+                    backslash_count = len(docstring) - len(docstring.rstrip("\\"))
+                    if backslash_count % 2:
+                        # Odd number of tailing backslashes, add some padding to
+                        # avoid escaping the closing string quote.
+                        docstring += " "
             else:
                 # Add some padding if the docstring is empty.
                 docstring = " "

tests/data/docstring.py

@@ -158,6 +158,22 @@ def docstring_with_inline_tabs_and_tab_indentation():
 	"""
 	pass
 
+
+def backslash_space():
+    """\ """
+
+
+def multiline_backslash_1():
+  '''
+  hey\there\
+  \ '''
+
+
+def multiline_backslash_2():
+  '''
+  hey there \ '''
+
+
 # output
 
 class MyClass:
@@ -316,3 +332,18 @@ def docstring_with_inline_tabs_and_tab_indentation():
     line ends with some tabs
     """
     pass
+
+
+def backslash_space():
+    """\ """
+
+
+def multiline_backslash_1():
+    """
+  hey\there\
+  \ """
+
+
+def multiline_backslash_2():
+    """
+    hey there \ """