Index: Lib/test/test_long.py
===================================================================
--- Lib/test/test_long.py	(revision 88061)
+++ Lib/test/test_long.py	(working copy)
@@ -5,6 +5,48 @@
 import random
 import math
 
+def long_to_float(n):
+    """Pure Python correctly-rounded integer-to-float conversion. """
+
+    # Constants, depending only on the floating-point format in use.
+    sig_bits = sys.float_info.mant_dig + 2
+    q_max, shift_max = 1 << sig_bits, sys.float_info.max_exp - sig_bits
+    correction = [0, -1, -2, 1, 0, -1, 2, 1]
+
+    # Reduce to case n positive.
+    if n == 0:
+        return 0.0
+    elif n < 0:
+        return -long_to_float(-n)
+
+    # q = top sig_bits significant bits of n, with 'sticky' lsb: that is, the
+    # lsb of the result is set iff it was set already, or any of the shifted
+    # out bits was set.
+    shift = n.bit_length() - sig_bits
+    q = n << -shift if shift < 0 else n >> shift | bool(n & (1 << shift) - 1)
+
+    # Round half to even.
+    q += correction[q & 7]
+
+    # Detect overflow.
+    if shift + (q == q_max) > shift_max:
+        raise OverflowError("integer too large to convert to float")
+
+    # At this point:
+    #
+    #   * The result of the correctly-rounded conversion is q * 2**shift, which
+    #     is exactly representable, and within the range of a float.
+    #
+    #   * q itself is an exactly representable integer in the range [q_max //
+    #     2, q_max].  q is a multiple of 4.
+    #
+    # Note that for the usual IEEE 754 format, q will fit into a 64-bit (signed
+    # or unsigned) integer at this point, so a low-level and efficient
+    # conversion of q to float(q) should be straightfoward to implement.
+    return math.ldexp(float(q), shift)
+
+
+
 # Used for lazy formatting of failure messages
 class Frm(object):
     def __init__(self, format, *args):
@@ -684,6 +726,9 @@
                          "test requires IEEE 754 doubles")
     def test_float_conversion(self):
         import sys
+        # Test 'long_to_float' instead of 'float' builtin.
+        float = long_to_float
+
         DBL_MAX = sys.float_info.max
         DBL_MAX_EXP = sys.float_info.max_exp
         DBL_MANT_DIG = sys.float_info.mant_dig