Keep trailing zeroes in python

问题:

I am writing a class to represent money, and one issue I've been running into is that "1.50" != str(1.50). str(1.50) equals 1.5, and alll of a sudden, POOF. 45 cents have vanished and the amount is now 1 dollar and 5 cents. not one dollar and 50 cents. Any way I could prevent str from doing this, or am I doing something wrong? This is Python 2 BTW.

回答1:

When working with money, always represent money using the Decimal class.

http://docs.python.org/2/library/decimal.html

回答2:

You can use the format method on strings to specify how many decimal places you want to represent:

>>> "{:.2f}".format(1.5)
'1.50'

But even better would be to use the decimal module for representing money, since representation issues with binary floats can give you slightly off results if you're doing arithmetic. The documentation for that module mentions some of those issues specifically - one of the most interesting ones for money applications is:

>>> 0.1+0.1+0.1-0.3
5.551115123125783e-17
>>> from decimal import Decimal
>>> Decimal('.1') + Decimal('.1') + Decimal('.1') - Decimal('.3')
Decimal('0.0')

回答3:

x = 1.500000

print '%.2f' % x
print '{:.3f}'.format(x)

result

1.50
1.500

回答4:

The proposed solutions do not work when the magnitude of the number is not known in advance, which is common in scientific applications rather than in money-related ones. I give an alternative solution for those, like me, coming to this question looking for the scientific case.

For example, if I want to print x = 1.500e-4 to three significant digits (common situation when dealing with measurements with a given uncertainty), the following command obviously does not give the correct result:

x = 1.500e-4
print(f"{x:.3f}")

----> 0.000

Here I used the modern Python 3.6+ f-strings for the formatting.

One may think of using the g format specifier, but this also does not give the desired result to three significant digits as the trailing zero is omitted:

x = 1.500e-4
print(f"{x:.3g}")

----> 0.00015

The correct answer can be obtained using the g format specifier together with a rather obscure option, the hash character #, of the format-specification mini language, in the following way:

x = 1.500e-4
print(f"{x:#.3g}")

----> 0.000150

This formatting also works unchanged in the simpler case of the original question:

x = 1.500
print(f"{x:#.3g}")

----> 1.50