CODEWITHSUNDEEP
pythonpython-2.7python-3.xdecimal
Joshua Burns
Joshua Burns

Reputation: 8572

Format Python Decimal object to a specified precision

I've spent countless hours researching, reading, testing, and ultimately confused and dismayed at Python's Decimal object's lack of the most fundamental concept: Formatting a Decimal's output to a string.

Let's assume we have some strings or Decimal objects with the following values:

   0.0008
  11.1111
 222.2222
3333.3333
1234.5678

The goal is to simply set the Decimal's precision to the second decimal place. Eg, 11.1111 would be formatted as 11.11, and 1234.5678 as 1234.57.

I envision code similar to the following:

import decimal
decimals = [
  decimal.Decimal('0.0008'),
  decimal.Decimal('11.1111'),
  decimal.Decimal('222.2222'),
  decimal.Decimal('3333.3333'),
  decimal.Decimal('1234.5678'),
]
for dec in decimals:
  print dec.as_string(precision=2, rounding=ROUND_HALF_UP)

The resulting output would be:

0.00
11.11
222.22
3333.33
1234.57

Obviously we cannot make use of the Decimal's context's precision, because this takes into consideration the TOTAL number of digits, not just decimal precision.

I'm also not interested in converting the Decimal to a float to output its value. The ENTIRE reason behind Decimal is to avoid storing and running calculations on floats.

What other solutions are there? I understand there are many other similar questions on stack overflow, but none of them have I found to resolve the underlying issue I am inquiring of.

Thanks much!

Upvotes: 17

Views: 27395

Answers (2)

Martijn Pieters
Martijn Pieters

Reputation: 1121494

Just use string formatting or the format() function:

>>> for dec in decimals:
...    print format(dec, '7.2f')
... 
   0.00
  11.11
 222.22
3333.33
1234.57

decimal.Decimal supports the same format specifications as floats do, so you can use exponent, fixed point, general, number or percentage formatting as needed.

This is the official and pythonic method of formatting decimals; the Decimal class implements the .__format__() method to handle such formatting efficiently.

Upvotes: 29

gseattle
gseattle

Reputation: 1022

def d(_in, decimal_places = 3):
    ''' Convert number to Decimal and do rounding, for doing calculations
        Examples:
          46.18271  to   46.183   rounded up
          46.18749  to   46.187   rounded down
          117.34999999999999 to 117.350
         _rescale is a private function, bad practice yet works for now.
    '''
    return Decimal(_in)._rescale(-decimal_places, 'ROUND_HALF_EVEN')

Edit: Again, _rescale() is not meant to be used by us regular bipeds, it works in Python 2.7, is not available in 3.4.

Upvotes: 2

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