gh-151678: Add tests for tkinter.filedialog (GH-151781)

Exercise the native dialogs (Open, SaveAs and Directory) through the
_test_callback seam without opening them, and test the pure-Python
FileDialog selection, filter and ok/cancel logic without entering its
modal loop.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Lib/test/test_tkinter/test_filedialog.py

@@ -0,0 +1,73 @@
+import os
+import unittest
+from tkinter import filedialog
+from tkinter.commondialog import Dialog
+from test.support import requires, swap_attr
+from test.test_tkinter.support import setUpModule  # noqa: F401
+from test.test_tkinter.support import AbstractTkTest
+
+requires('gui')
+
+
+class NativeDialogTest(AbstractTkTest, unittest.TestCase):
+    # Open, SaveAs and Directory wrap modal native dialogs.  The _test_callback
+    # seam is called by show() just before the dialog would open; replacing it
+    # with a function that raises exercises show() without blocking on the
+    # dialog.
+
+    def check(self, dialog_class, command):
+        self.assertEqual(dialog_class.command, command)
+        master = None
+        def test_callback(dialog, parent):
+            nonlocal master
+            master = parent
+            raise ZeroDivisionError
+        with swap_attr(Dialog, '_test_callback', test_callback):
+            d = dialog_class(self.root, title='Test')
+            self.assertRaises(ZeroDivisionError, d.show)
+        self.assertIs(master, self.root)
+
+    def test_open(self):
+        self.check(filedialog.Open, 'tk_getOpenFile')
+
+    def test_saveas(self):
+        self.check(filedialog.SaveAs, 'tk_getSaveFile')
+
+    def test_directory(self):
+        self.check(filedialog.Directory, 'tk_chooseDirectory')
+
+
+class FileDialogTest(AbstractTkTest, unittest.TestCase):
+    # The pure-Python FileDialog runs its own modal loop in go(); its logic is
+    # exercised here without entering the loop.
+
+    def test_selection(self):
+        d = filedialog.FileDialog(self.root)
+        d.directory = os.path.abspath(os.sep)
+        d.set_selection('spam.txt')
+        self.assertEqual(os.path.basename(d.get_selection()), 'spam.txt')
+
+    def test_filter(self):
+        d = filedialog.FileDialog(self.root)
+        d.set_filter(os.getcwd(), '*.py')
+        self.assertEqual(d.get_filter(), (os.getcwd(), '*.py'))
+
+    def test_ok_cancel(self):
+        d = filedialog.FileDialog(self.root)
+        d.directory = os.getcwd()
+        d.set_selection('spam.txt')
+        d.ok_command()  # Accepts the current selection.
+        self.assertEqual(os.path.basename(d.how), 'spam.txt')
+        d.cancel_command()  # Returns no selection.
+        self.assertIsNone(d.how)
+
+    def test_subclasses(self):
+        for cls in filedialog.LoadFileDialog, filedialog.SaveFileDialog:
+            with self.subTest(cls=cls.__name__):
+                d = cls(self.root)
+                self.assertIsInstance(d, filedialog.FileDialog)
+                self.assertEqual(d.top.title(), cls.title)
+
+
+if __name__ == "__main__":
+    unittest.main()