Add a --use-tabs argument to use tabs instead of spaces

Closes jleclanche#47, psf#513, psf#640, psf#2122, psf#2584, psf#2798

src/black/__init__.py

@@ -267,6 +267,14 @@ def validate_regex(
         " functionality in the next major release."
     ),
 )
+@click.option(
+    "--use-tabs",
+    is_flag=True,
+    help=(
+        "Use tabs instead of spaces for indentation. "
+        "Tabs are always equal to 4 spaces."
+    ),
+)
 @click.option(
     "--check",
     is_flag=True,
@@ -423,6 +431,7 @@ def main(  # noqa: C901
     skip_magic_trailing_comma: bool,
     experimental_string_processing: bool,
     preview: bool,
+    use_tabs: bool,
     quiet: bool,
     verbose: bool,
     required_version: Optional[str],
@@ -520,6 +529,7 @@ def main(  # noqa: C901
         experimental_string_processing=experimental_string_processing,
         preview=preview,
         python_cell_magics=set(python_cell_magics),
+        use_tabs=use_tabs,
     )
 
     if code is not None:

src/black/linegen.py

@@ -277,8 +277,9 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
             docstring_started_empty = not docstring
 
             if is_multiline_string(leaf):
-                indent = " " * 4 * self.current_line.depth
-                docstring = fix_docstring(docstring, indent)
+                indent_style = " " * 4 if not self.mode.use_tabs else "\t"
+                indent = indent_style * self.current_line.depth
+                docstring = fix_docstring(docstring, indent, not self.mode.use_tabs)
             else:
                 docstring = docstring.strip()
 
@@ -352,7 +353,8 @@ def transform_line(
         yield line
         return
 
-    line_str = line_to_string(line)
+    # Force spaces to ensure len(line) is correct
+    line_str = line.render(force_spaces=True).strip("\n")
 
     ll = mode.line_length
     sn = mode.string_normalization

src/black/lines.py

@@ -396,11 +396,15 @@ def clone(self) -> "Line":
         )
 
     def __str__(self) -> str:
+        return self.render()
+
+    def render(self, force_spaces: bool = False) -> str:
         """Render the line."""
         if not self:
             return "\n"
 
-        indent = "    " * self.depth
+        indent_style = "    " if force_spaces or not self.mode.use_tabs else "\t"
+        indent = indent_style * self.depth
         leaves = iter(self.leaves)
         first = next(leaves)
         res = f"{first.prefix}{indent}{first.value}"
@@ -612,7 +616,9 @@ def is_line_short_enough(line: Line, *, line_length: int, line_str: str = "") ->
     Uses the provided `line_str` rendering, if any, otherwise computes a new one.
     """
     if not line_str:
-        line_str = line_to_string(line)
+        line_str = line_to_string(line, force_spaces=True)
+    else:
+        line_str = line_str.expandtabs(tabsize=4)
     return (
         len(line_str) <= line_length
         and "\n" not in line_str  # multiline strings
@@ -765,9 +771,9 @@ def _can_omit_closing_paren(line: Line, *, last: Leaf, line_length: int) -> bool
     return False
 
 
-def line_to_string(line: Line) -> str:
+def line_to_string(line: Line, force_spaces: bool = False) -> str:
     """Returns the string representation of @line.
 
     WARNING: This is known to be computationally expensive.
     """
-    return str(line).strip("\n")
+    return line.render(force_spaces=force_spaces).strip("\n")

src/black/mode.py

@@ -146,6 +146,7 @@ class Mode:
     experimental_string_processing: bool = False
     python_cell_magics: Set[str] = field(default_factory=set)
     preview: bool = False
+    use_tabs: bool = False
 
     def __post_init__(self) -> None:
         if self.experimental_string_processing:
@@ -184,5 +185,6 @@ def get_cache_key(self) -> str:
             str(int(self.experimental_string_processing)),
             str(int(self.preview)),
             sha256((",".join(sorted(self.python_cell_magics))).encode()).hexdigest(),
+            str(int(self.use_tabs)),
         ]
         return ".".join(parts)

src/black/strings.py

@@ -60,11 +60,14 @@ def lines_with_leading_tabs_expanded(s: str) -> List[str]:
     return lines
 
 
-def fix_docstring(docstring: str, prefix: str) -> str:
+def fix_docstring(docstring: str, prefix: str, expand_leading_tabs: bool) -> str:
     # https://www.python.org/dev/peps/pep-0257/#handling-docstring-indentation
     if not docstring:
         return ""
-    lines = lines_with_leading_tabs_expanded(docstring)
+    if expand_leading_tabs:
+        lines = lines_with_leading_tabs_expanded(docstring)
+    else:
+        lines = docstring.splitlines()
     # Determine minimum indentation (first line doesn't count):
     indent = sys.maxsize
     for line in lines[1:]: