test: add assert_scale assertion to test framework

Github-Pull: bitcoin#19089
Rebased-From: 067e5dc

test/functional/test_framework/util.py

@@ -180,6 +180,18 @@ def assert_array_result(object_array, to_match, expected, should_not_find=False)
     if num_matched > 0 and should_not_find:
         raise AssertionError("Objects were found %s" % (str(to_match)))
 
+def assert_scale(number, expected_scale=8):
+    """Assert number has expected scale, e.g. fractional digits; number of
+    digits after the decimal. The default of 8 corresponds to a Bitcoin amount."""
+    number = str(number)
+    mantissa = number.split('.')[-1].upper()
+    if mantissa[:3] == '0E-':
+        assert_equal(mantissa, '0E-{}'.format(expected_scale))  # exponent notation
+    elif mantissa == number:
+        assert_equal(0, expected_scale)  # no mantissa, ergo, expected scale must be 0
+    else:
+        assert_equal(len(mantissa), expected_scale)
+
 # Utility functions
 ###################