Setting the length of a float or number

Question

I've been looking around to try and set the length of floats or decimals to 2 places, I'm doing this for a set of course work, I have tried getcontext but it does nothing.

from decimal import *
getcontext().prec = 2
price = ("22.5")
#I would like this to be 22.50, but as it comes form a list and I use float a bit, so I have to convert it to decimal (?)
price = Decimal(price)
print (price)

But the output is:

22.5

If anyone knows a better way to set the length of a decimal to 2 decimal places (using it in money) or where I'm going wrong, it would be helpful.

Answer 1

"float" is short for "floating point". Read about floating point on https://en.wikipedia.org/wiki/Floating-point_arithmetic and then never ever ever use it to represent money.

You're on the right track with Decimal. You just need to watch out for the distinction between the precision of the representation and the display.

The prec attribute of the context controls the precision of the representation of values that result from different operations. It does not control the precision of explicitly constructed values. And it does not control the precision of the display.

Consider:

>>> getcontext().prec = 2
>>> Decimal("1") / Decimal("3")
Decimal('0.33')
>>> 

vs

>>> getcontext().prec = 2
>>> Decimal("1") / Decimal("2")
Decimal('0.5')
>>> 

vs

>>> getcontext().prec = 2
>>> Decimal("0.12345")
Decimal('0.12345')
>>> 

To specify the precision for display purposes of a Decimal, you just have to take more control over the display code. Don't rely on str(Decimal(...)).

One option is to normalize the decimal for display:

>>> getcontext().prec = 2
>>> Decimal("0.12345").normalize()
Decimal('0.12')

This respects the prec setting from the context.

Another option is to quantize it to a specific precision:

>>> Decimal("0.12345").quantize(Decimal("1.00"))
Decimal('0.12')

This is independent of the prec setting from the context.

Decimals can also be rounded:

>>> round(Decimal("123.4567"), 2)
123.46

Though be very careful with this as the result of rounding is a float.

You can also format a Decimal directly into a string:

>>> "{:.2f}".format(Decimal("1.234"))
'1.23'

Answer 2

Try this:

print "{:.2f}".format(price)

This way there will be no need for globally set the precision.