Python - Number of Significant Digits in results of division

Question

Newbie here. I have the following code:

myADC = 128
maxVoltage = 5.0
maxADC = 255.0
VoltsPerADC = maxVoltage/maxADC
myVolts = myADC * VoltsPerADC
print "myADC = {0: >3}".format(myADC)
print "VoltsPerADC =  {0: >7}".format(VoltsPerADC)
print VoltsPerADC
print "myVolts = {0: >7}".format(myVolts)
print myVolts

This outputs the following:

myADC = 128
VoltsPerADC =  0.0196078
0.0196078431373
myVolts =  2.5098
2.50980392157

I have been searching for an explanation of how the number of significant digits is determined by default, but have had trouble locating an explanation that makes sense to me. This link link text suggests that by default the "print" statement prints numbers to 10 significant figures, but that does not seem to be the case in my results.

How are the number of significant digits/precision determined?

Can someone shed some light on this for me.

Thanks in advance for your time and patience.

Answer 1

The precision is determined by the hardware. Python uses hardware floats (actually doubles) for its floats. The implications are discussed in the tutorial: http://docs.python.org/tutorial/floatingpoint.html

If you want more control over precision and rounding, you should consider using the decimal module.

Answer 2

In general, if the documentation isn't at python.org, it is not authoritative. If you look at print it will tell you that floating point numbers are first converted to strings and float will tell you (as Thomas noted) that the machine native double type is how floats are stored and that the language makes no guarantees about significant digits.