keeping same formatting for floating point values

Question

I have a python program that reads floating point values using the following regular expression

 (-?\d+\.\d+)

once I extract the value using float(match.group(1)), I get the actual floating point number. However, I am not able to distinguish if the number was 1.2345678 or 1.234 or 1.2340000.

The problem I am facing is to print out the floating point value again, with the exact same formatting. An easy solution is to "split and count" the floating point value when still a string, eg splitting at the decimal point, and counting the integer part length and the fractional part length, then create the formatter as

print "%"+str(total_len)+"."+str(fractional_len)+"f" % value

but maybe you know a standard way to achieve the same result ?

Answer 1

If you want to keep a fixed precision, avoid using floats and use Decimal instead:

>>> from decimal import Decimal
>>> d = Decimal('-1.2345')
>>> str(d)
'-1.2345'
>>> float(d)
-1.2344999999999999

Answer 2

>>> from decimal import Decimal as d
>>> d('1.13200000')
Decimal('1.13200000')
>>> print d('1.13200000')
1.13200000

Answer 3

You method is basically correct. String formatting has a less often used * operator you can put for the formatting sizes, here's some code:

import re

def parse_float(str):
  re_float = re.compile(r'(-?)(\d+)\.(\d+)')
  grps = re_float.search(str)
  sign, decimal, fraction = grps.groups()
  float_val = float('%s%s.%s' % (sign, decimal, fraction))
  total_len = len(grps.group(0))
  print '%*.*f'  % (total_len, len(fraction), float_val)

parse_float('1.2345678')
parse_float('1.234')
parse_float('1.2340000')

and it outputs

1.2345678
1.234
1.2340000