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How to round Decimal numbers with Symmetric Arithmetic Rounding (as described in article “Rounding” on wikipedia.org?.

Rounding can be done with decimal.Decimal object’s quantize method. Use decimal.ROUND_HALF_UP flag to get a Symmetric Arithmetic Rounding.

For example:

>>> from decimal import Decimal, ROUND_HALF_UP
>>> Decimal("234.4535").quantize(Decimal("0.001"), ROUND_HALF_UP)
Decimal("234.454")
>>> Decimal("234.4534").quantize(Decimal("0.001"), ROUND_HALF_UP)
Decimal("234.453")
>>> Decimal("-234.4535").quantize(Decimal("0.001"), ROUND_HALF_UP)
Decimal("-234.454")
>>> Decimal("-234.4535").quantize(Decimal("0.001"), ROUND_HALF_UP)
Decimal("-234.454")

So, the “traditional” round function will look like this:

from decimal import Decimal, ROUND_HALF_UP

def round(d, digits=0):
    """
    Symmetric Arithmetic Rounding for decimal numbers

    d       - Decimal number to round
    digits  - number of digits after the point to leave

    For example:
    >>> round(Decimal("234.4536"), 3)
    Decimal("234.454")
    >>> round(Decimal("234.4535"), 3)
    Decimal("234.454")
    >>> round(Decimal("234.4534"), 3)
    Decimal("234.453")
    >>> round(Decimal("-234.4535"), 3)
    Decimal("-234.454")
    >>> round(Decimal("234.4535"), -1)
    Decimal("2.3E+2")
    """
    return d.quantize(Decimal("1") / (Decimal('10') ** digits), ROUND_HALF_UP) 

Tags: decimal, Python, round

This entry was posted on Sunday, October 28th, 2007 at 6:20 pm and is filed under Python. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.