Fix metric() showing an extra significant digit on rounding carry

metric() derives the number of decimal places from the mantissa's
exponent, assuming the mantissa keeps its digit count. When rounding
carries it up a power of ten (9.999 -> 10.0, 99.99 -> 100) it gains an
integer digit and shows one significant figure too many, e.g.
metric(9999) returned '10.00 k' instead of '10.0 k'. The existing guard
only handled the mantissa reaching 1000 (a full SI-bucket crossing).

Detect the carry against the next power of ten and bump the exponent by
one, which recomputes the decimal places and, when the mantissa reaches
1000, still crosses into the next bucket exactly as before.

Author: vidigoat

src/humanize/number.py

@@ -548,8 +548,13 @@ def metric(value: float, unit: str = "", precision: int = 3) -> str:
     old_bucket = exponent // 3 * 3
     value /= 10**old_bucket
     digits = int(max(0, precision - exponent % 3 - 1))
-    if exponent < 30 and round(abs(value), digits) >= 1000:
-        exponent += 3 - exponent % 3
+    # Rounding to ``digits`` decimal places can carry the mantissa up to the
+    # next power of ten (e.g. 9.999 -> 10.0, 99.99 -> 100, 999.9 -> 1000),
+    # which adds an integer digit and would otherwise show one significant
+    # figure too many. Bump the exponent to absorb the carry -- crossing into
+    # the next SI bucket when the mantissa reaches 1000 -- and recompute.
+    if exponent < 30 and round(abs(value), digits) >= 10 ** (exponent % 3 + 1):
+        exponent += 1
         new_bucket = exponent // 3 * 3
         value /= 10 ** (new_bucket - old_bucket)
         digits = int(max(0, precision - exponent % 3 - 1))

tests/test_number.py

@@ -258,6 +258,15 @@ def test_clamp(test_args: list[typing.Any], expected: str) -> None:
         ([1234.56], "1.23 k"),
         ([12345, "", 6], "12.3450 k"),
         ([200_000], "200 k"),
+        # Rounding that carries the mantissa up a power of ten within a
+        # bucket must not add an extra significant figure (issue: metric
+        # showed one digit too many for e.g. 9999 -> "10.00 k").
+        ([9999], "10.0 k"),
+        ([99999], "100 k"),
+        ([9.99999], "10.0"),
+        ([-9999], "-10.0 k"),
+        ([9.999, "", 2], "10"),
+        ([999.4], "999"),
         ([999.9, "V"], "1.00 kV"),
         ([999.99, "V"], "1.00 kV"),
         ([999_999, "V"], "1.00 MV"),