Fix intcomma() returning scientific notation for large floats

src/humanize/number.py

@@ -196,6 +196,23 @@ def intcomma(value: NumberOrString, ndigits: int | None = None) -> str:
 
     if ndigits is not None:
         result = f"{value:,.{ndigits}f}"
+    elif isinstance(value, float):
+        # f"{value:,}" formats floats via repr(), which switches to scientific
+        # notation at 1e16 (and for tiny floats like 1e-5), so no thousands
+        # separators are added. Expand to fixed-point first (see #431).
+        import decimal
+
+        expanded = f"{decimal.Decimal(repr(value)):f}"
+        if "." in expanded:
+            integer_part, fraction_part = expanded.split(".")
+        else:
+            integer_part, fraction_part = expanded, "0"
+        if integer_part.startswith("-"):
+            sign, integer_part = "-", integer_part[1:]
+        else:
+            sign = ""
+        grouped = f"{int(integer_part):,}" if integer_part else "0"
+        result = f"{sign}{grouped}.{fraction_part}"
     else:
         result = f"{value:,}"
     if thousands_sep != "," or decimal_sep != ".":

tests/test_number.py

@@ -90,6 +90,27 @@ def test_intcomma_large_integer(sign: int) -> None:
     assert humanize.intcomma(value) == expected
 
 
+@pytest.mark.parametrize(
+    "test_args, expected",
+    [
+        ([1e15], "1,000,000,000,000,000.0"),
+        ([1e16], "10,000,000,000,000,000.0"),
+        ([1.5e20], "150,000,000,000,000,000,000.0"),
+        ([-1e16], "-10,000,000,000,000,000.0"),
+        ([0.00001], "0.00001"),
+        ([1e16, 0], "10,000,000,000,000,000"),
+    ],
+    ids=str,
+)
+def test_intcomma_large_float_no_scientific_notation(
+    test_args: list[typing.Any], expected: str
+) -> None:
+    # https://github.com/python-humanize/humanize/issues/431
+    # Floats at 1e16 and above (and tiny floats like 1e-5) were rendered in
+    # scientific notation with no thousands separators.
+    assert humanize.intcomma(*test_args) == expected
+
+
 def test_intword_powers() -> None:
     # make sure that powers & human_powers have the same number of items
     assert len(number.powers) == len(number.human_powers)