gh-143990: Preserve the size when creating a Font from a named font

tkinter.font.Font now copies the options of a named font (via "font
configure") instead of the options resolved by "font actual", which
would resolve a size specified in pixels (a negative size) to points.
A font description is still resolved, as it cannot be parsed otherwise.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_tkinter/test_font.py

@@ -73,6 +73,44 @@ def test_create(self):
         self.assertRaises(tkinter.TclError, font.Font, root=self.root,
                           name='testfont', font=('Times', 10))
 
+    def test_create_from_named_font(self):
+        # gh-143990: a font created from a named font copies its configured
+        # options, preserving a size specified in pixels (a negative size).
+        sizetype = int if self.wantobjects else str
+        named = font.Font(root=self.root, name='my named font',  # name with spaces
+                          family='Times', size=-20, weight='bold')
+        # The source is the name of a named font or a Font representing one.
+        for source in ['my named font', named]:
+            with self.subTest(source=source):
+                f = font.Font(root=self.root, font=source)
+                self.assertEqual(f.cget('size'), sizetype(-20))
+                self.assertEqual(f.actual('family'), named.actual('family'))
+                self.assertEqual(f.actual('weight'), 'bold')
+        # Explicit options still override the copied settings.
+        f = font.Font(root=self.root, font=named, size=30)
+        self.assertEqual(f.cget('size'), sizetype(30))
+
+    def test_create_from_description(self):
+        # gh-143990: a font created from a font description is resolved via
+        # "font actual", so a size in pixels (negative) becomes a size in points.
+        descriptions = [
+            ('Times', -20),                     # tuple
+            ('Times', -20, 'bold'),             # tuple with a style
+            'Times -20',                        # string
+            'Times -20 bold',                   # string with a style
+            '{Times New Roman} -20',            # string, family with spaces
+            # a Font wrapping a description, as a tuple and as a string
+            font.Font(root=self.root, font=('Times', -20), exists=True),
+            font.Font(root=self.root, font='Times -20', exists=True),
+        ]
+        for desc in descriptions:
+            with self.subTest(font=desc):
+                f = font.Font(root=self.root, font=desc)
+                # resolved as if the description were wrapped by exists=True
+                wrapped = font.Font(root=self.root, font=desc, exists=True)
+                self.assertEqual(f.actual(), wrapped.actual())
+                self.assertGreater(int(f.cget('size')), 0)  # pixels -> points
+
     def test_existing(self):
         sizetype = int if self.wantobjects else str
 

Lib/tkinter/font.py

@@ -83,8 +83,15 @@ def __init__(self, root=None, font=None, name=None, exists=False,
             self.name = font
         else:
             if font:
-                # start from the actual settings of the given font
-                font = tk.splitlist(tk.call("font", "actual", font))
+                # start from the settings of the given font
+                try:
+                    # a named font: copy its options, preserving the size,
+                    # which can be negative (specified in pixels)
+                    font = tk.splitlist(tk.call("font", "configure", font))
+                except tkinter.TclError:
+                    # a font description: resolve it ("font configure" only
+                    # accepts a font name); this loses a size in pixels
+                    font = tk.splitlist(tk.call("font", "actual", font))
                 if options:
                     # explicit options override the corresponding settings
                     settings = self._mkdict(font)

Misc/NEWS.d/next/Library/2026-07-07-17-50-54.gh-issue-143990.FoNtCf.rst

@@ -0,0 +1,3 @@
+A :class:`tkinter.font.Font` created from a named font
+now copies its configured options rather than the options resolved by Tcl's ``font actual``,
+preserving a size specified in pixels (a negative size).