* lisp/progmodes/python.el: Teach f-strings to `font-lock`

(python--f-string-p, python--font-lock-f-strings): New functions.
(python-font-lock-keywords-maximum-decoration): Use them.

1 files changed, 52 insertions, 3 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 76baa4469c7..d1871c93a78 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -501,6 +501,52 @@ The type returned can be `comment', `string' or `paren'."
         font-lock-string-face)
     font-lock-comment-face))
 
+(defun python--f-string-p (ppss)
+  "Return non-nil if the pos where PPSS was found is inside an f-string."
+  (and (nth 3 ppss)
+       (let ((spos (1- (nth 8 ppss))))
+         (and (memq (char-after spos) '(?f ?F))
+              (or (< (point-min) spos)
+                  (not (memq (char-syntax (char-before spos)) '(?w ?_))))))))
+
+(defun python--font-lock-f-strings (limit)
+  "Mark {...} holes as being code.
+Remove the (presumably `font-lock-string-face') `face' property from
+the {...} holes that appear within f-strings."
+  ;; FIXME: This will fail to properly highlight strings appearing
+  ;; within the {...} of an f-string.
+  ;; We could presumably fix it by running
+  ;; `font-lock-fontify-syntactically-region' (as is done in
+  ;; `sm-c--cpp-fontify-syntactically', for example) after removing
+  ;; the `face' property, but I'm not sure it's worth the effort and
+  ;; the risks.
+  (let ((ppss (syntax-ppss)))
+    (while
+        (progn
+          (while (and (not (python--f-string-p ppss))
+                      (re-search-forward "\\<f['\"]" limit 'move))
+            (setq ppss (syntax-ppss)))
+          (< (point) limit))
+      (cl-assert (python--f-string-p ppss))
+      (let ((send (save-excursion
+                   (goto-char (nth 8 ppss))
+                   (condition-case nil
+                       (progn (let ((forward-sexp-function nil))
+                                (forward-sexp 1))
+                              (min limit (1- (point))))
+                     (scan-error limit)))))
+        (while (re-search-forward "{" send t)
+          (if (eq ?\{ (char-after))
+              (forward-char 1)          ;Just skip over {{
+            (let ((beg (match-beginning 0))
+                  (end (condition-case nil
+                           (progn (up-list 1) (min send (point)))
+                         (scan-error send))))
+              (goto-char end)
+              (put-text-property beg end 'face nil))))
+        (goto-char (min limit (1+ send)))
+        (setq ppss (syntax-ppss))))))
+
 (defvar python-font-lock-keywords-level-1
   `((,(rx symbol-start "def" (1+ space) (group (1+ (or word ?_))))
      (1 font-lock-function-name-face))
@@ -567,7 +613,8 @@ This is the medium decoration level, including everything in
 builtins.")
 
 (defvar python-font-lock-keywords-maximum-decoration
-  `(,@python-font-lock-keywords-level-2
+  `((python--font-lock-f-strings)
+    ,@python-font-lock-keywords-level-2
     ;; Constants
     (,(rx symbol-start
           (or
@@ -575,7 +622,8 @@ builtins.")
            ;; copyright, license, credits, quit and exit are added by the site
            ;; module and they are not intended to be used in programs
            "copyright" "credits" "exit" "license" "quit")
-          symbol-end) . font-lock-constant-face)
+          symbol-end)
+     . font-lock-constant-face)
     ;; Decorators.
     (,(rx line-start (* (any " \t")) (group "@" (1+ (or word ?_))
                                             (0+ "." (1+ (or word ?_)))))
@@ -609,7 +657,8 @@ builtins.")
            ;; OS specific
            "VMSError" "WindowsError"
            )
-          symbol-end) . font-lock-type-face)
+          symbol-end)
+     . font-lock-type-face)
     ;; assignments
     ;; support for a = b = c = 5
     (,(lambda (limit)