factor out hardcoded "sat/byte" and "sat/b" strings

Though note that the qml GUI has some more in qml/js context.

electrum/commands.py

@@ -1428,7 +1428,7 @@ command_options = {
     'nocheck':     (None, "Do not verify aliases"),
     'imax':        (None, "Maximum number of inputs"),
     'fee':         ("-f", "Transaction fee (absolute, in BTC)"),
-    'feerate':     (None, "Transaction fee rate (in sat/byte)"),
+    'feerate':     (None, f"Transaction fee rate (in {util.FEERATE_UI_NAME_SAT_PER_VBYTE})"),
     'from_addr':   ("-F", "Source address (must be a wallet address; use sweep to spend from non-wallet address)."),
     'from_coins':  (None, "Source coins (must be in wallet; use sweep to spend from non-wallet address)."),
     'change_addr': ("-c", "Change address. Default is a spare address, or the source address if it's not in the wallet"),
@@ -1461,7 +1461,7 @@ command_options = {
     'iknowwhatimdoing': (None, "Acknowledge that I understand the full implications of what I am about to do"),
     'gossip':      (None, "Apply command to gossip node instead of wallet"),
     'connection_string':      (None, "Lightning network node ID or network address"),
-    'new_fee_rate': (None, "The Updated/Increased Transaction fee rate (in sat/byte)"),
+    'new_fee_rate': (None, f"The Updated/Increased Transaction fee rate (in {util.FEERATE_UI_NAME_SAT_PER_VBYTE})"),
     'from_amount': (None, "Amount to convert (default: 1)"),
     'from_ccy':    (None, "Currency to convert from"),
     'to_ccy':      (None, "Currency to convert to"),

electrum/gui/qt/amountedit.py

@@ -10,7 +10,7 @@ from PyQt5.QtWidgets import (QLineEdit, QStyle, QStyleOptionFrame, QSizePolicy)
 from .util import char_width_in_lineedit, ColorScheme
 
 from electrum.util import (format_satoshis_plain, decimal_point_to_base_unit_name,
-                           FEERATE_PRECISION, quantize_feerate, DECIMAL_POINT)
+                           FEERATE_PRECISION, quantize_feerate, DECIMAL_POINT, FEERATE_UI_NAME_SAT_PER_VBYTE)
 from electrum.bitcoin import COIN, TOTAL_COIN_SUPPLY_LIMIT_IN_BTC
 
 _NOT_GIVEN = object()  # sentinel value
@@ -165,7 +165,7 @@ class FeerateEdit(BTCAmountEdit):
         self.extra_precision = FEERATE_PRECISION
 
     def _base_unit(self):
-        return 'sat/byte'
+        return FEERATE_UI_NAME_SAT_PER_VBYTE
 
     def _get_amount_from_text(self, text):
         sat_per_byte_amount = super()._get_amount_from_text(text)

electrum/simple_config.py

@@ -637,7 +637,7 @@ class SimpleConfig(Logger):
             fee_per_byte = None
         else:
             fee_per_byte = fee_per_kb/1000
-            rate_str = format_fee_satoshis(fee_per_byte) + ' sat/byte'
+            rate_str = format_fee_satoshis(fee_per_byte) + f" {util.FEERATE_UI_NAME_SAT_PER_VBYTE}"
 
         if dyn:
             if mempool:
@@ -875,7 +875,7 @@ class SimpleConfig(Logger):
 
     def format_fee_rate(self, fee_rate) -> str:
         """fee_rate is in sat/kvByte."""
-        return format_fee_satoshis(fee_rate/1000, num_zeros=self.num_zeros) + ' sat/byte'
+        return format_fee_satoshis(fee_rate/1000, num_zeros=self.num_zeros) + f" {util.FEERATE_UI_NAME_SAT_PER_VBYTE}"
 
     def get_base_unit(self):
         return decimal_point_to_base_unit_name(self.decimal_point)

electrum/util.py

@@ -792,6 +792,8 @@ def format_satoshis(
 
 FEERATE_PRECISION = 1  # num fractional decimal places for sat/byte fee rates
 _feerate_quanta = Decimal(10) ** (-FEERATE_PRECISION)
+FEERATE_UI_NAME_SAT_PER_VBYTE = "sat/byte"
+FEERATE_UI_NAME_SAT_PER_VBYTE_SHORT = "sat/b"
 
 
 def format_fee_satoshis(fee, *, num_zeros=0, precision=None):

electrum/wallet.py

@@ -1657,7 +1657,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
             if fee is not None:
                 size = tx.estimated_size()
                 fee_per_byte = fee / size
-                extra.append(format_fee_satoshis(fee_per_byte) + ' sat/b')
+                extra.append(format_fee_satoshis(fee_per_byte) + f" {util.FEERATE_UI_NAME_SAT_PER_VBYTE_SHORT}")
             if fee is not None and height in (TX_HEIGHT_UNCONF_PARENT, TX_HEIGHT_UNCONFIRMED) \
                and self.config.has_fee_mempool():
                 exp_n = self.config.fee_to_depth(fee_per_byte)
@@ -3184,7 +3184,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         elif feerate > FEERATE_WARNING_HIGH_FEE / 1000:
             long_warning = ' '.join([
                 _("The fee for this transaction seems unusually high."),
-                _("(feerate: {} sat/byte)").format(f'{feerate:.2f}')
+                _("(feerate: {})").format(self.config.format_fee_rate(1000 * feerate))
             ])
             short_warning = _("high fee rate") + "!"
         if long_warning is None: