Fix two docstring crashes (#3451)

CHANGES.md

@@ -32,6 +32,7 @@
 - Long values in dict literals are now wrapped in parentheses; correspondingly
   unnecessary parentheses around short values in dict literals are now removed; long
   string lambda values are now wrapped in parentheses (#3440)
+- Fix two crashes in preview style involving edge cases with docstrings (#3451)
 - Exclude string type annotations from improved string processing; fix crash when the
   return type annotation is stringified and spans across multiple lines (#3462)
 

src/black/linegen.py

@@ -401,6 +401,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
             else:
                 docstring = docstring.strip()
 
+            has_trailing_backslash = False
             if docstring:
                 # Add some padding if the docstring starts / ends with a quote mark.
                 if docstring[0] == quote_char:
@@ -413,6 +414,7 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
                         # Odd number of tailing backslashes, add some padding to
                         # avoid escaping the closing string quote.
                         docstring += " "
+                        has_trailing_backslash = True
             elif not docstring_started_empty:
                 docstring = " "
 
@@ -435,6 +437,8 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
                 if (
                     len(lines) > 1
                     and last_line_length + quote_len > self.mode.line_length
+                    and len(indent) + quote_len <= self.mode.line_length
+                    and not has_trailing_backslash
                 ):
                     leaf.value = prefix + quote + docstring + "\n" + indent + quote
                 else:

tests/data/miscellaneous/linelength6.py

@@ -0,0 +1,5 @@
+# Regression test for #3427, which reproes only with line length <= 6
+def f():
+    """
+    x
+    """

tests/data/simple_cases/docstring.py

@@ -173,6 +173,11 @@ def multiline_backslash_2():
   '''
   hey there \ '''
 
+# Regression test for #3425
+def multiline_backslash_really_long_dont_crash():
+    """
+    hey there hello guten tag hi hoow are you ola zdravstvuyte ciao como estas ca va \ """
+
 
 def multiline_backslash_3():
   '''
@@ -391,6 +396,12 @@ def multiline_backslash_2():
     hey there \ """
 
 
+# Regression test for #3425
+def multiline_backslash_really_long_dont_crash():
+    """
+    hey there hello guten tag hi hoow are you ola zdravstvuyte ciao como estas ca va \ """
+
+
 def multiline_backslash_3():
     """
     already escaped \\"""

tests/test_format.py

@@ -146,6 +146,13 @@ def test_docstring_no_string_normalization() -> None:
     assert_format(source, expected, mode)
 
 
+def test_docstring_line_length_6() -> None:
+    """Like test_docstring but with line length set to 6."""
+    source, expected = read_data("miscellaneous", "linelength6")
+    mode = black.Mode(line_length=6)
+    assert_format(source, expected, mode)
+
+
 def test_preview_docstring_no_string_normalization() -> None:
     """
     Like test_docstring but with string normalization off *and* the preview style