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

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"),