Qt tx dialog: fix file extension when exporting (on MacOS...)

closes #5954

We are now giving every(?) hint possible to the MacOS file dialog...
The extension is put in the filename as before (which turned out not to be enough).
It is also set using QFileDialog.setDefaultSuffix, which again, turns out not to be enough.
In desperation, the file extension filter-list now contains *.psbt and *.txn as separate filters,
and the one with the expected extension is pre-selected. This seems enough...

electrum/gui/qt/main_window.py

@@ -45,7 +45,7 @@ from PyQt5.QtWidgets import (QMessageBox, QComboBox, QSystemTrayIcon, QTabWidget
                              QVBoxLayout, QGridLayout, QLineEdit,
                              QHBoxLayout, QPushButton, QScrollArea, QTextEdit,
                              QShortcut, QMainWindow, QCompleter, QInputDialog,
-                             QWidget, QSizePolicy, QStatusBar, QToolTip)
+                             QWidget, QSizePolicy, QStatusBar, QToolTip, QDialog)
 
 import electrum
 from electrum import (keystore, ecc, constants, util, bitcoin, commands,
@@ -89,7 +89,7 @@ from .util import (read_QIcon, ColorScheme, text_dialog, icon_path, WaitingDialo
                    CloseButton, HelpButton, MessageBoxMixin, EnterButton,
                    import_meta_gui, export_meta_gui,
                    filename_field, address_field, char_width_in_lineedit, webopen,
-                   TRANSACTION_FILE_EXTENSION_FILTER, MONOSPACE_FONT)
+                   TRANSACTION_FILE_EXTENSION_FILTER_ANY, MONOSPACE_FONT)
 from .util import ButtonsTextEdit
 from .installwizard import WIF_HELP_TEXT
 from .history_list import HistoryList, HistoryModel
@@ -780,13 +780,27 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
             self.config.set_key('io_dir', os.path.dirname(fileName), True)
         return fileName
 
-    def getSaveFileName(self, title, filename, filter = ""):
+    def getSaveFileName(self, title, filename, filter="",
+                        *, default_extension: str = None,
+                        default_filter: str = None) -> Optional[str]:
         directory = self.config.get('io_dir', os.path.expanduser('~'))
-        path = os.path.join( directory, filename )
-        fileName, __ = QFileDialog.getSaveFileName(self, title, path, filter)
-        if fileName and directory != os.path.dirname(fileName):
-            self.config.set_key('io_dir', os.path.dirname(fileName), True)
-        return fileName
+        path = os.path.join(directory, filename)
+
+        file_dialog = QFileDialog(self, title, path, filter)
+        file_dialog.setAcceptMode(QFileDialog.AcceptSave)
+        if default_extension:
+            # note: on MacOS, the selected filter's first extension seems to have priority over this...
+            file_dialog.setDefaultSuffix(default_extension)
+        if default_filter:
+            assert default_filter in filter, f"default_filter={default_filter!r} does not appear in filter={filter!r}"
+            file_dialog.selectNameFilter(default_filter)
+        if file_dialog.exec() != QDialog.Accepted:
+            return None
+
+        selected_path = file_dialog.selectedFiles()[0]
+        if selected_path and directory != os.path.dirname(selected_path):
+            self.config.set_key('io_dir', os.path.dirname(selected_path), True)
+        return selected_path
 
     def timer_actions(self):
         self.request_list.refresh_status()
@@ -2509,7 +2523,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
 
     def read_tx_from_file(self) -> Optional[Transaction]:
         fileName = self.getOpenFileName(_("Select your transaction file"),
-                                        TRANSACTION_FILE_EXTENSION_FILTER)
+                                        TRANSACTION_FILE_EXTENSION_FILTER_ANY)
         if not fileName:
             return
         try:

electrum/gui/qt/transaction_dialog.py

@@ -50,7 +50,9 @@ from electrum.logging import get_logger
 
 from .util import (MessageBoxMixin, read_QIcon, Buttons, icon_path,
                    MONOSPACE_FONT, ColorScheme, ButtonsLineEdit, text_dialog,
-                   char_width_in_lineedit, TRANSACTION_FILE_EXTENSION_FILTER,
+                   char_width_in_lineedit, TRANSACTION_FILE_EXTENSION_FILTER_SEPARATE,
+                   TRANSACTION_FILE_EXTENSION_FILTER_ONLY_COMPLETE_TX,
+                   TRANSACTION_FILE_EXTENSION_FILTER_ONLY_PARTIAL_TX,
                    BlockingWaitingDialog)
 
 from .fee_slider import FeeSlider
@@ -323,12 +325,19 @@ class BaseTxDialog(QDialog, MessageBoxMixin):
         if isinstance(tx, PartialTransaction):
             tx.finalize_psbt()
         if tx.is_complete():
-            name = 'signed_%s.txn' % (tx.txid()[0:8])
+            name = 'signed_%s' % (tx.txid()[0:8])
+            extension = 'txn'
+            default_filter = TRANSACTION_FILE_EXTENSION_FILTER_ONLY_COMPLETE_TX
         else:
-            name = self.wallet.basename() + time.strftime('-%Y%m%d-%H%M.psbt')
+            name = self.wallet.basename() + time.strftime('-%Y%m%d-%H%M')
+            extension = 'psbt'
+            default_filter = TRANSACTION_FILE_EXTENSION_FILTER_ONLY_PARTIAL_TX
+        name = f'{name}.{extension}'
         fileName = self.main_window.getSaveFileName(_("Select where to save your transaction"),
                                                     name,
-                                                    TRANSACTION_FILE_EXTENSION_FILTER)
+                                                    TRANSACTION_FILE_EXTENSION_FILTER_SEPARATE,
+                                                    default_extension=extension,
+                                                    default_filter=default_filter)
         if not fileName:
             return
         if tx.is_complete():  # network tx hex

electrum/gui/qt/util.py

@@ -51,7 +51,12 @@ pr_icons = {
 
 
 # filter tx files in QFileDialog:
-TRANSACTION_FILE_EXTENSION_FILTER = "Transaction (*.txn *.psbt);;All files (*)"
+TRANSACTION_FILE_EXTENSION_FILTER_ANY = "Transaction (*.txn *.psbt);;All files (*)"
+TRANSACTION_FILE_EXTENSION_FILTER_ONLY_PARTIAL_TX = "Partial Transaction (*.psbt)"
+TRANSACTION_FILE_EXTENSION_FILTER_ONLY_COMPLETE_TX = "Complete Transaction (*.txn)"
+TRANSACTION_FILE_EXTENSION_FILTER_SEPARATE = (f"{TRANSACTION_FILE_EXTENSION_FILTER_ONLY_PARTIAL_TX};;"
+                                              f"{TRANSACTION_FILE_EXTENSION_FILTER_ONLY_COMPLETE_TX};;"
+                                              f"All files (*)")
 
 
 class EnterButton(QPushButton):