Add humanize.metric() for converting big/small numbers to SI units.

src/humanize/__init__.py

@@ -10,6 +10,7 @@
     intword,
     ordinal,
     scientific,
+    metric,
 )
 from humanize.time import (
     naturaldate,

src/humanize/number.py

@@ -436,3 +436,55 @@ def clamp(value, format="{:}", floor=None, ceil=None, floor_token="<", ceil_toke
             "Invalid format. Must be either a valid formatting string, or a function "
             "that accepts value and returns a string."
         )
+
+
+def metric(value, unit="", precision=3):
+    """Return a value with a metric SI unit-prefix appended.
+
+    Examples:
+        ```pycon
+        >>> metric(1500, "V")
+        '1.50kV'
+        >>> metric(2e8, "W")
+        '200MW'
+        >>> metric(220e-6, "F")
+        '220μF'
+        >>> metric(1e-14, precision=4)
+        '10.00f'
+
+        ```
+
+    The unit prefix is always chosen so that non-significant zero digits are required.
+    i.e. `123,000` will become `123k` instead of `0.123M` and `1,230,000` will become
+    `1.23M` instead of `1230K`. For numbers that are either too huge or too tiny to
+    represent without resorting to either leading or trailing zeroes, it falls back to
+    `scientific()`.
+    ```pycon
+    >>> metric(1e40)
+    '1.00 x 10⁴⁰'
+
+    ```
+
+    Args:
+        value (int, float): Input number.
+        unit (str): Optional base unit.
+        precision (int): The number of digits the output should contain.
+
+    Returns:
+        str:
+    """
+    exponent = int(math.floor(math.log10(abs(value))))
+
+    if exponent >= 27 or exponent < -24:
+        return scientific(value, precision - 1) + unit
+    else:
+        value /= (10 ** (exponent // 3 * 3))
+        if exponent >= 3:
+            ordinal = "kMGTPEZY"[exponent // 3 - 1]
+        elif exponent < 0:
+            ordinal = "mμnpfazy"[(-exponent - 1) // 3]
+        else:
+            ordinal = ""
+        value = format(value, ".%if" % (precision - (exponent % 3) - 1))
+
+    return f'{value}{ordinal}{unit}'

tests/test_number.py

@@ -176,3 +176,35 @@ def test_scientific(test_args, expected):
 )
 def test_clamp(test_args, expected):
     assert humanize.clamp(*test_args) == expected
+
+
+@pytest.mark.parametrize(
+    "test_args, expected",
+    [
+        ([1, "Hz"], "1.00Hz"),
+        ([1.0, "W"], "1.00W"),
+        ([3, "C"], "3.00C"),
+        ([3, "W", 5], "3.0000W"),
+        ([1.23456], "1.23"),
+        ([12.3456], "12.3"),
+        ([123.456], "123"),
+        ([1234.56], "1.23k"),
+        ([12345, "", 6], "12.3450k"),
+        ([200_000], "200k"),
+        ([1e25, "m"], "10.0Ym"),
+        ([1e26, "m"], "100Ym"),
+        ([1e27, "A"], "1.00 x 10²⁷A"),
+        ([1.234e28, "A"], "1.23 x 10²⁸A"),
+        ([-1500, "V"], "-1.50kV"),
+        ([.12], "120m"),
+        ([.012], "12.0m"),
+        ([.0012], "1.20m"),
+        ([.00012], "120μ"),
+        ([1e-23], "10.0y"),
+        ([1e-24], "1.00y"),
+        ([1e-25], "1.00 x 10⁻²⁵"),
+        ([1e-26], "1.00 x 10⁻²⁶"),
+    ], ids=str
+)
+def test_metric(test_args, expected):
+    assert humanize.metric(*test_args) == expected