Add option to only blacken if the project uses black

* only blacken on save if the project uses black

To make it easier to use blacken across multiple projects, some of which
are *not* using black, add the following changes to blacken.el:

1. `blacken.el` now searches for a `pyproject.toml` file in parent
   directories of the current buffer.

2. If it finds such a file, it checks whether there is a `[tool.black]`
   section in it, and it only blackens on save if there is.

This allows me to auto-blacken my Python code for projects that use
black, but to avoid code churn in projects that do not.

* make blacken-only-if-project-is-blackened an option

Author: tgamblin

blacken.el

@@ -74,6 +74,11 @@ If `fill', the `fill-column' variable value is used."
   :type 'boolean
   :safe 'booleanp)
 
+(defcustom blacken-only-if-project-is-blackened nil
+  "Only blacken if project has a pyproject.toml with a [tool.black] section."
+  :type 'boolean
+  :safe 'booleanp)
+
 (defun blacken-call-bin (input-buffer output-buffer error-buffer)
   "Call process black.
 
@@ -113,9 +118,21 @@ Return black process the exit code."
    (when blacken-skip-string-normalization
      (list "--skip-string-normalization"))
    (when (string-match "\.pyi$" (buffer-file-name (current-buffer)))
-     (list "--pyi"))   
+     (list "--pyi"))
    '("-")))
 
+(defun blacken-regex-in-file (regex file)
+  "Reads a file and returns a list of lines in that file"
+  (with-temp-buffer
+    (insert-file-contents file)
+    (re-search-forward regex nil t 1)))
+
+(defun blacken-project-is-blackened (&optional display)
+  "Whether the project has a pyproject.toml with [tool.black] in it."
+  (let ((parent (locate-dominating-file default-directory "pyproject.toml")))
+    (and parent
+         (blacken-regex-in-file "^\\[tool.black\\]$" (concat parent "pyproject.toml")))))
+
 ;;;###autoload
 (defun blacken-buffer (&optional display)
   "Try to blacken the current buffer.
@@ -150,7 +167,9 @@ Show black output, if black exit abnormally and DISPLAY is t."
   "Automatically run black before saving."
   :lighter " Black"
   (if blacken-mode
-      (add-hook 'before-save-hook 'blacken-buffer nil t)
+      (when (or (not blacken-only-if-project-is-blackened)
+                (blacken-project-is-blackened))
+        (add-hook 'before-save-hook 'blacken-buffer nil t))
     (remove-hook 'before-save-hook 'blacken-buffer t)))
 
 (provide 'blacken)