gh-129534: Fix asksaveasfilename not appending file extension when none is provided

Lib/tkinter/filedialog.py

@@ -384,10 +384,41 @@ def askopenfilename(**options):
     return Open(**options).show()
 
 
+
+
 def asksaveasfilename(**options):
-    "Ask for a filename to save as"
+    """Ask for a filename to save as, handling missing extensions properly."""
+
+    # If no default extension is provided, set it to the first valid filetype extension
+    if "defaultextension" not in options:
+        filetypes = options.get("filetypes")
+        if filetypes:
+            first_ext = filetypes[0][1]  # Example: "*.txt" from ("Text files", "*.txt")
+
+            # Validate first_ext: Must start with '*' and contain only one '.'
+            if first_ext.startswith("*") and first_ext.count(".") == 1:
+                ext = first_ext[1:]  # Extract ".txt"
+
+                # Ensure the extracted extension is a valid one (not patterns like "*.a[0-9]")
+                if ext.isalnum() or (ext.startswith(".") and ext[1:].isalnum()):
+                    options["defaultextension"] = ext  # Set default extension
+
+    filename = SaveAs(**options).show()
+
+    # Append extension if missing, but allow extensionless filenames
+    if filename:
+        # Ensure filename is not a relative path ("../", "./", "/absolute/path")
+        if not os.path.isabs(filename) and not filename.startswith((".", "..")):
+            # Check if there's no explicit extension
+            if '.' not in os.path.basename(filename):
+                ext = options.get("defaultextension", "")
+                if ext and not filename.endswith(ext):
+                    filename += ext
+
+    return filename
+
+
 
-    return SaveAs(**options).show()
 
 
 def askopenfilenames(**options):