qt block explorer: allow custom URL

The QTextEdit expects values such as (one per line):
```
https://blockstream.info/testnet/
https://192.168.0.38:3021/
("https://blockstream.info/testnet/", {'tx': 'tx/', 'addr': 'address/'})
```

closes #4831

electrum/gui/qt/settings_dialog.py

@@ -23,13 +23,14 @@
 # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
+import ast
 from typing import Optional, TYPE_CHECKING
 
 from PyQt5.QtCore import Qt
 from PyQt5.QtWidgets import (QComboBox,  QTabWidget,
                              QSpinBox,  QFileDialog, QCheckBox, QLabel,
                              QVBoxLayout, QGridLayout, QLineEdit,
-                             QPushButton, QWidget)
+                             QPushButton, QWidget, QHBoxLayout)
 
 from electrum.i18n import _
 from electrum import util, coinchooser, paymentrequest
@@ -328,16 +329,45 @@ Use this if you want your local watchtower to keep running after you close your
         tx_widgets.append((outrounding_cb, None))
 
         block_explorers = sorted(util.block_explorer_info().keys())
+        BLOCK_EX_CUSTOM_ITEM = _("Custom URL")
+        if BLOCK_EX_CUSTOM_ITEM in block_explorers:  # malicious translation?
+            block_explorers.remove(BLOCK_EX_CUSTOM_ITEM)
+        block_explorers.append(BLOCK_EX_CUSTOM_ITEM)
         msg = _('Choose which online block explorer to use for functions that open a web browser')
         block_ex_label = HelpLabel(_('Online Block Explorer') + ':', msg)
         block_ex_combo = QComboBox()
+        block_ex_custom_e = QLineEdit(self.config.get('block_explorer_custom') or '')
         block_ex_combo.addItems(block_explorers)
-        block_ex_combo.setCurrentIndex(block_ex_combo.findText(util.block_explorer(self.config)))
-        def on_be(x):
-            be_result = block_explorers[block_ex_combo.currentIndex()]
-            self.config.set_key('block_explorer', be_result, True)
-        block_ex_combo.currentIndexChanged.connect(on_be)
-        tx_widgets.append((block_ex_label, block_ex_combo))
+        block_ex_combo.setCurrentIndex(
+            block_ex_combo.findText(util.block_explorer(self.config) or BLOCK_EX_CUSTOM_ITEM))
+        def showhide_block_ex_custom_e():
+            block_ex_custom_e.setVisible(block_ex_combo.currentText() == BLOCK_EX_CUSTOM_ITEM)
+        showhide_block_ex_custom_e()
+        def on_be_combo(x):
+            if block_ex_combo.currentText() == BLOCK_EX_CUSTOM_ITEM:
+                on_be_edit()
+            else:
+                be_result = block_explorers[block_ex_combo.currentIndex()]
+                self.config.set_key('block_explorer_custom', None, False)
+                self.config.set_key('block_explorer', be_result, True)
+            showhide_block_ex_custom_e()
+        block_ex_combo.currentIndexChanged.connect(on_be_combo)
+        def on_be_edit():
+            val = block_ex_custom_e.text()
+            try:
+                val = ast.literal_eval(val)  # to also accept tuples
+            except:
+                pass
+            self.config.set_key('block_explorer_custom', val)
+        block_ex_custom_e.editingFinished.connect(on_be_edit)
+        block_ex_hbox = QHBoxLayout()
+        block_ex_hbox.setContentsMargins(0, 0, 0, 0)
+        block_ex_hbox.setSpacing(0)
+        block_ex_hbox.addWidget(block_ex_combo)
+        block_ex_hbox.addWidget(block_ex_custom_e)
+        block_ex_hbox_w = QWidget()
+        block_ex_hbox_w.setLayout(block_ex_hbox)
+        tx_widgets.append((block_ex_label, block_ex_hbox_w))
 
         # Fiat Currency
         hist_checkbox = QCheckBox()

electrum/util.py

@@ -796,19 +796,43 @@ testnet_block_explorers = {
                        {'tx': 'tx/', 'addr': 'address/'}),
 }
 
+_block_explorer_default_api_loc = {'tx': 'tx/', 'addr': 'address/'}
+
+
 def block_explorer_info():
     from . import constants
     return mainnet_block_explorers if not constants.net.TESTNET else testnet_block_explorers
 
-def block_explorer(config: 'SimpleConfig') -> str:
-    from . import constants
+
+def block_explorer(config: 'SimpleConfig') -> Optional[str]:
+    """Returns name of selected block explorer,
+    or None if a custom one (not among hardcoded ones) is configured.
+    """
+    if config.get('block_explorer_custom') is not None:
+        return None
     default_ = 'Blockstream.info'
     be_key = config.get('block_explorer', default_)
-    be = block_explorer_info().get(be_key)
-    return be_key if be is not None else default_
+    be_tuple = block_explorer_info().get(be_key)
+    if be_tuple is None:
+        be_key = default_
+    assert isinstance(be_key, str), f"{be_key!r} should be str"
+    return be_key
+
 
 def block_explorer_tuple(config: 'SimpleConfig') -> Optional[Tuple[str, dict]]:
-    return block_explorer_info().get(block_explorer(config))
+    custom_be = config.get('block_explorer_custom')
+    if custom_be:
+        if isinstance(custom_be, str):
+            return custom_be, _block_explorer_default_api_loc
+        if isinstance(custom_be, (tuple, list)) and len(custom_be) == 2:
+            return tuple(custom_be)
+        _logger.warning(f"not using 'block_explorer_custom' from config. "
+                        f"expected a str or a pair but got {custom_be!r}")
+        return None
+    else:
+        # using one of the hardcoded block explorers
+        return block_explorer_info().get(block_explorer(config))
+
 
 def block_explorer_URL(config: 'SimpleConfig', kind: str, item: str) -> Optional[str]:
     be_tuple = block_explorer_tuple(config)
@@ -818,6 +842,8 @@ def block_explorer_URL(config: 'SimpleConfig', kind: str, item: str) -> Optional
     kind_str = explorer_dict.get(kind)
     if kind_str is None:
         return
+    if explorer_url[-1] != "/":
+        explorer_url += "/"
     url_parts = [explorer_url, kind_str, item]
     return ''.join(url_parts)