simple_config: implement complex default values

Besides a literal value, the default can now also be a callable,
which gets called with the config and evaluated as needed, lazily.

This potentially allows e.g. the default value of one configvar to
depend on the current value of another configvar.

electrum/simple_config.py

@@ -4,7 +4,7 @@ import time
 import os
 import stat
 from decimal import Decimal
-from typing import Union, Optional, Dict, Sequence, Tuple, Any, Set
+from typing import Union, Optional, Dict, Sequence, Tuple, Any, Set, Callable
 from numbers import Real
 from functools import cached_property
 
@@ -54,23 +54,34 @@ FINAL_CONFIG_VERSION = 3
 
 class ConfigVar(property):
 
-    def __init__(self, key: str, *, default, type_=None):
+    def __init__(
+        self,
+        key: str,
+        *,
+        default: Union[Any, Callable[['SimpleConfig'], Any]],  # typically a literal, but can also be a callable
+        type_=None,
+    ):
         self._key = key
         self._default = default
         self._type = type_
         property.__init__(self, self._get_config_value, self._set_config_value)
 
     def _get_config_value(self, config: 'SimpleConfig'):
-        value = config.get(self._key, default=self._default)
+        with config.lock:
-        if self._type is not None and value != self._default:
+            if config.is_set(self._key):
-            assert value is not None, f"got None for key={self._key!r}"
+                value = config.get(self._key)
-            try:
+                if self._type is not None:
-                value = self._type(value)
+                    assert value is not None, f"got None for key={self._key!r}"
-            except Exception as e:
+                    try:
-                raise ValueError(
+                        value = self._type(value)
-                    f"ConfigVar.get type-check and auto-conversion failed. "
+                    except Exception as e:
-                    f"key={self._key!r}. type={self._type}. value={value!r}") from e
+                        raise ValueError(
-        return value
+                            f"ConfigVar.get type-check and auto-conversion failed. "
+                            f"key={self._key!r}. type={self._type}. value={value!r}") from e
+            else:
+                d = self._default
+                value = d(config) if callable(d) else d
+            return value
 
     def _set_config_value(self, config: 'SimpleConfig', value, *, save=True):
         if self._type is not None and value is not None:
@@ -817,14 +828,14 @@ class SimpleConfig(Logger):
                     f"Either use config.cv.{name}.set() or assign to config.{name} instead.")
         return CVLookupHelper()
 
-    def get_swapserver_url(self):
+    def _default_swapserver_url(self) -> str:
         if constants.net == constants.BitcoinMainnet:
             default = 'https://swaps.electrum.org/api'
         elif constants.net == constants.BitcoinTestnet:
             default = 'https://swaps.electrum.org/testnet'
         else:
             default = 'http://localhost:5455'
-        return self.SWAPSERVER_URL or default
+        return default
 
     # config variables ----->
     NETWORK_AUTO_CONNECT = ConfigVar('auto_connect', default=True, type_=bool)
@@ -940,7 +951,7 @@ class SimpleConfig(Logger):
     CONFIG_FORGET_CHANGES = ConfigVar('forget_config', default=False, type_=bool)
 
     # submarine swap server
-    SWAPSERVER_URL = ConfigVar('swapserver_url', default='', type_=str)
+    SWAPSERVER_URL = ConfigVar('swapserver_url', default=_default_swapserver_url, type_=str)
     SWAPSERVER_PORT = ConfigVar('swapserver_port', default=5455, type_=int)
     TEST_SWAPSERVER_REFUND = ConfigVar('test_swapserver_refund', default=False, type_=bool)
 

electrum/submarine_swaps.py

@@ -206,7 +206,7 @@ class SwapManager(Logger):
             if swap.prepay_hash is not None:
                 self.prepayments[swap.prepay_hash] = bytes.fromhex(k)
         # api url
-        self.api_url = wallet.config.get_swapserver_url()
+        self.api_url = wallet.config.SWAPSERVER_URL
         # init default min & max
         self.init_min_max_values()
 

electrum/tests/test_simple_config.py

@@ -3,9 +3,10 @@ import sys
 import os
 import tempfile
 import shutil
-
 from io import StringIO
-from electrum.simple_config import (SimpleConfig, read_user_config)
+
+from electrum.simple_config import SimpleConfig, read_user_config
+from electrum import constants
 
 from . import ElectrumTestCase
 
@@ -147,6 +148,22 @@ class Test_SimpleConfig(ElectrumTestCase):
         config.NETWORK_MAX_INCOMING_MSG_SIZE = None
         self.assertEqual(MAX_MSG_SIZE_DEFAULT, config.NETWORK_MAX_INCOMING_MSG_SIZE)
 
+    def test_configvars_get_default_value_complex_fn(self):
+        config = SimpleConfig(self.options)
+        self.assertEqual("https://swaps.electrum.org/api", config.SWAPSERVER_URL)
+
+        config.SWAPSERVER_URL = "http://localhost:9999"
+        self.assertEqual("http://localhost:9999", config.SWAPSERVER_URL)
+
+        config.SWAPSERVER_URL = None
+        self.assertEqual("https://swaps.electrum.org/api", config.SWAPSERVER_URL)
+
+        constants.set_testnet()
+        try:
+            self.assertEqual("https://swaps.electrum.org/testnet", config.SWAPSERVER_URL)
+        finally:
+            constants.set_mainnet()
+
     def test_configvars_is_set(self):
         config = SimpleConfig(self.options)
         self.assertEqual(MAX_MSG_SIZE_DEFAULT, config.NETWORK_MAX_INCOMING_MSG_SIZE)