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

Closes #47

src/black/__init__.py

@@ -246,6 +246,14 @@ def validate_regex(
         " Currently disabled because it leads to some crashes."
     ),
 )
+@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,
@@ -399,6 +407,7 @@ def main(
     skip_string_normalization: bool,
     skip_magic_trailing_comma: bool,
     experimental_string_processing: bool,
+    use_tabs: bool,
     quiet: bool,
     verbose: bool,
     required_version: Optional[str],
@@ -469,6 +478,7 @@ def main(
         string_normalization=not skip_string_normalization,
         magic_trailing_comma=not skip_magic_trailing_comma,
         experimental_string_processing=experimental_string_processing,
+        use_tabs=use_tabs,
     )
 
     if code is not None:

src/black/linegen.py

@@ -317,7 +317,8 @@ 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

@@ -380,11 +380,15 @@ 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.use_tabs else "\t"
+        indent = indent_style * self.depth
         leaves = iter(self.leaves)
         first = next(leaves)
         res = f"{first.prefix}{indent}{first.value}"

src/black/mode.py

@@ -130,6 +130,7 @@ class Mode:
     is_ipynb: bool = False
     magic_trailing_comma: bool = True
     experimental_string_processing: bool = False
+    use_tabs: bool = False
 
     def get_cache_key(self) -> str:
         if self.target_versions:
@@ -147,5 +148,6 @@ def get_cache_key(self) -> str:
             str(int(self.is_ipynb)),
             str(int(self.magic_trailing_comma)),
             str(int(self.experimental_string_processing)),
+            str(int(self.use_tabs)),
         ]
         return ".".join(parts)