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Diffstat (limited to 'Doc/library')
| -rw-r--r-- | Doc/library/decimal.rst | 40 |
1 files changed, 15 insertions, 25 deletions
diff --git a/Doc/library/decimal.rst b/Doc/library/decimal.rst index 7bf4494..37d2b35 100644 --- a/Doc/library/decimal.rst +++ b/Doc/library/decimal.rst @@ -1581,31 +1581,21 @@ Q. Is there a way to convert a regular float to a :class:`Decimal`? A. Yes, all binary floating point numbers can be exactly expressed as a Decimal. An exact conversion may take more precision than intuition would -suggest, so trapping :const:`Inexact` will signal a need for more precision:: - - def floatToDecimal(f): - "Convert a floating point number to a Decimal with no loss of information" - # Transform (exactly) a float to a mantissa (0.5 <= abs(m) < 1.0) and an - # exponent. Double the mantissa until it is an integer. Use the integer - # mantissa and exponent to compute an equivalent Decimal. If this cannot - # be done exactly, then retry with more precision. - - mantissa, exponent = math.frexp(f) - while mantissa != int(mantissa): - mantissa *= 2.0 - exponent -= 1 - mantissa = int(mantissa) - - oldcontext = getcontext() - setcontext(Context(traps=[Inexact])) - try: - while True: - try: - return mantissa * Decimal(2) ** exponent - except Inexact: - getcontext().prec += 1 - finally: - setcontext(oldcontext) +suggest, so we trap :const:`Inexact` to signal a need for more precision:: + + def float_to_decimal(f): + "Convert a floating point number to a Decimal with no loss of information" + n, d = f.as_integer_ratio() + with localcontext() as ctx: + ctx.traps[Inexact] = True + while True: + try: + return Decimal(n) / Decimal(d) + except Inexact: + ctx.prec += 1 + + >>> float_to_decimal(math.pi) + Decimal("3.141592653589793115997963468544185161590576171875") Q. Why isn't the :func:`floatToDecimal` routine included in the module? |