util.format_satoshis: fix for amounts with higher than sat precision

Previously, msat precision was leaking through format_satoshis if the
user's base unit was set to "sat". This was a bug.
Some features of format_satoshis did not work well with such values, such
as the "whitespaces" param.

Old code:

>>> util.format_satoshis(Decimal('45831275.748'), decimal_point=2)
'458312.76'
>>> util.format_satoshis(Decimal('45831275.748'), decimal_point=0)
'45831275.748'

New code:

>>> util.format_satoshis(Decimal('45831275.748'), decimal_point=2)
'458312.76'
>>> util.format_satoshis(Decimal('45831275.748'), decimal_point=0)
'45831276.'

electrum/tests/test_util.py

@@ -23,6 +23,22 @@ class TestUtil(ElectrumTestCase):
     def test_format_satoshis_decimal(self):
         self.assertEqual("0.00001234", format_satoshis(Decimal(1234)))
 
+    def test_format_satoshis_msat_resolution(self):
+        self.assertEqual("45831276.",    format_satoshis(Decimal("45831276"), decimal_point=0))
+        self.assertEqual("45831276.",    format_satoshis(Decimal("45831275.748"), decimal_point=0))
+        self.assertEqual("45831275.75", format_satoshis(Decimal("45831275.748"), decimal_point=0, precision=2))
+        self.assertEqual("45831275.748", format_satoshis(Decimal("45831275.748"), decimal_point=0, precision=3))
+
+        self.assertEqual("458312.76",    format_satoshis(Decimal("45831276"), decimal_point=2))
+        self.assertEqual("458312.76",    format_satoshis(Decimal("45831275.748"), decimal_point=2))
+        self.assertEqual("458312.7575", format_satoshis(Decimal("45831275.748"), decimal_point=2, precision=2))
+        self.assertEqual("458312.75748", format_satoshis(Decimal("45831275.748"), decimal_point=2, precision=3))
+
+        self.assertEqual("458.31276", format_satoshis(Decimal("45831276"), decimal_point=5))
+        self.assertEqual("458.31276", format_satoshis(Decimal("45831275.748"), decimal_point=5))
+        self.assertEqual("458.3127575", format_satoshis(Decimal("45831275.748"), decimal_point=5, precision=2))
+        self.assertEqual("458.31275748", format_satoshis(Decimal("45831275.748"), decimal_point=5, precision=3))
+
     def test_format_fee_float(self):
         self.assertEqual("1.7", format_fee_satoshis(1700/1000))
 
@@ -48,11 +64,12 @@ class TestUtil(ElectrumTestCase):
                          format_satoshis(-1234, whitespaces=True))
 
     def test_format_satoshis_diff_positive(self):
-        self.assertEqual("+0.00001234",
-                         format_satoshis(1234, is_diff=True))
+        self.assertEqual("+0.00001234", format_satoshis(1234, is_diff=True))
+        self.assertEqual("+456789.00001234", format_satoshis(45678900001234, is_diff=True))
 
     def test_format_satoshis_diff_negative(self):
         self.assertEqual("-0.00001234", format_satoshis(-1234, is_diff=True))
+        self.assertEqual("-456789.00001234", format_satoshis(-45678900001234, is_diff=True))
 
     def test_format_satoshis_plain(self):
         self.assertEqual("0.00001234", format_satoshis_plain(1234))

electrum/util.py

@@ -647,37 +647,37 @@ DECIMAL_POINT = localeconv()['decimal_point']  # type: str
 
 
 def format_satoshis(
-        x,  # in satoshis
+        x: Union[int, float, Decimal, str, None],  # amount in satoshis
         *,
-        num_zeros=0,
-        decimal_point=8,
-        precision=None,
-        is_diff=False,
-        whitespaces=False,
+        num_zeros: int = 0,
+        decimal_point: int = 8,  # how much to shift decimal point to left (default: sat->BTC)
+        precision: int = 0,  # extra digits after satoshi precision
+        is_diff: bool = False,  # if True, enforce a leading sign (+/-)
+        whitespaces: bool = False,  # if True, add whitespaces, to align numbers in a column
 ) -> str:
     if x is None:
         return 'unknown'
     if x == '!':
         return 'max'
-    if precision is None:
-        precision = decimal_point
+    assert isinstance(x, (int, float, Decimal)), f"{x!r} should be a number"
+    # lose redundant precision
+    x = Decimal(x).quantize(Decimal(10) ** (-precision))
     # format string
-    decimal_format = "." + str(precision) if precision > 0 else ""
+    overall_precision = decimal_point + precision  # max digits after final decimal point
+    decimal_format = "." + str(overall_precision) if overall_precision > 0 else ""
     if is_diff:
         decimal_format = '+' + decimal_format
     # initial result
     scale_factor = pow(10, decimal_point)
-    if not isinstance(x, Decimal):
-        x = Decimal(x).quantize(Decimal('1E-8'))
     result = ("{:" + decimal_format + "f}").format(x / scale_factor)
     if "." not in result: result += "."
     result = result.rstrip('0')
-    # extra decimal places
+    # add extra decimal places (zeros)
     integer_part, fract_part = result.split(".")
     if len(fract_part) < num_zeros:
         fract_part += "0" * (num_zeros - len(fract_part))
     result = integer_part + DECIMAL_POINT + fract_part
-    # leading/trailing whitespaces
+    # add leading/trailing whitespaces
     if whitespaces:
         result += " " * (decimal_point - len(fract_part))
         result = " " * (15 - len(result)) + result