Do not emit a leading zero in floatToGoString exponents >= 10

floatToGoString reproduces Go's strconv.FormatFloat(f, 'g', -1, 64),
which pads a float's exponent to a minimum of two digits. The exponent
was formatted as 'e+0{dot - 1}', which hard-codes a single leading zero
and only produces the right width while dot - 1 is a single digit. Once
the exponent reaches two digits (values >= 1e10, whose repr is still
plain decimal) it over-pads, e.g. 1e10 became '1e+010' instead of Go's
'1e+10'. That affects any emitted sample value or le/quantile bucket
boundary at or above 1e10.

Use '{dot - 1:02d}' so the exponent is zero-padded to a minimum of two
digits and not beyond, matching Go.

Add tests/test_utils.py covering both the previously-correct single-digit
exponents and the two-digit exponents that regressed.

Signed-off-by: Sean Kim <skim8705@gmail.com>

Author: sean-kim05

prometheus_client/utils.py

@@ -21,7 +21,7 @@ def floatToGoString(d):
         # We only need to care about positive values for le/quantile.
         if d > 0 and dot > 6:
             mantissa = f'{s[0]}.{s[1:dot]}{s[dot + 1:]}'.rstrip('0.')
-            return f'{mantissa}e+0{dot - 1}'
+            return f'{mantissa}e+{dot - 1:02d}'
         return s
 
 

tests/test_utils.py

@@ -0,0 +1,18 @@
+import unittest
+
+from prometheus_client.utils import floatToGoString
+
+
+class TestFloatToGoString(unittest.TestCase):
+    def test_exponent_two_digits_has_no_leading_zero(self):
+        # floatToGoString mirrors Go's strconv.FormatFloat(f, 'g', -1, 64),
+        # which pads the exponent to a minimum of two digits. A two-digit
+        # exponent must not gain a spurious leading zero.
+        self.assertEqual('1e+10', floatToGoString(1e10))
+        self.assertEqual('1e+15', floatToGoString(1e15))
+        self.assertEqual('1.234567890123e+12', floatToGoString(1234567890123.0))
+
+    def test_exponent_one_digit_is_zero_padded(self):
+        # Single-digit exponents keep the two-digit zero padding.
+        self.assertEqual('1e+06', floatToGoString(1e6))
+        self.assertEqual('1.234567e+06', floatToGoString(1234567.0))