Python: Convert floating point numbers to decimal keeping N digits of precision.

floats.py

import math
from decimal import Decimal

def magnitude(value):
    """Decimal magnitude of abs(value) as int, or float('-inf') if value == 0.
    Example:
    
        >>> [magnitude(x) for x in [1.314234123412345678e-9, 124.355, -1000, 0]]
        [-9, 2, 3, -inf]
    """
    return int(math.floor(math.log10(abs(value)))) if value else float('-inf')

def approx(value, digits=4):
    """Convert value to Decimal keeping the given number of digits after the
    leading number.
    
        >>> [approx(x) for x in [1.314234123412345678e-9, 124.355, -1000, 0]]
        [Decimal('1.3142E-9'), Decimal('124.36'), Decimal('-1000.0'), Decimal('0')]
    """
    if not isinstance(value, Decimal): value = Decimal(value)
    return round(value, digits - magnitude(value)) if value != 0 else value

def decstr(x):
    """Convert (float) number x to string with sufficient precision
    for converting it back to float but without using the engineering
    notation. Example:
        
        >>> [decstr(x) for x in [1.314234123412345678e-9, 124.355, -1000, 0]]
        ['0.0000000013142341234123457', '124.355', '-1000', '0']
    """
    return format(Decimal(str(x)), 'f')

# To print values with reasonable precision and without scientific notation,
# use the 'f' format code:
if __name__ = '__main__':
    for x in [1.31423412341e-9, 124.355, -1000, 0]:
        print(format(approx(x), 'f'))
        # =>
        # 0.0000000013142
        # 124.36
        # -1000.0
        # 0