Python Format Default Rounding When Formatting Float Number

I'm trying to solve some floating-point problems in my code in Python 2.7.10. When testing I've encountered a strange behaviour with the format method: print '{}'.format(0.30000000

Solution 1:

Since you do not specify a format, the default type coercion to string is used. So this isn't really an issue with format. Python 2 truncates to a precision of 12 characters (excluding the leading zero, if any) for float.__str__, and after truncation in this case, all trailing zeros are cleaned up:

>>> str(0.3000000000004) # unlike str(0.3000000000014) -> '0.300000000001''0.3'

Adding the format_spec:f gives you the default precision of 6:

>>> '{:f}'.format(0.3000000000004)
'0.300000'

Specify the width and precision or use repr to get a good representation:

>>> '{!r}'.format(0.3000000000004)
'0.3000000000004'

The behavior is different in Python 3 though:

>>> str(0.3000000000004)
'0.3000000000004'

---

The formatting of floats is handled by just one function float_repr in Python 3 (it does not have a special float_str function):

(reprfunc)float_repr,                       /* tp_repr */
...
(reprfunc)float_repr,                        /* tp_str */

whereas Python2.7 defines a separate handler float_str and float_repr for __str__ and __repr__ respectively:

(reprfunc)float_repr,                       /* tp_repr */
...
(reprfunc)float_str,                        /* tp_str */

The deciding variable here which I think is the reason for the precision loss beyond 12d.p. is PyFloat_STR_PRECISION (defined in Python 2):

#define PyFloat_STR_PRECISION 12

It reverses to a truncation for floats taking more than 12 characters in the default conversion.

Solution 2:

This is just the way python (and many other languages) deal with float point inaccuracy. So internally this number is actually equals 0.3. There's a website dedicated to this problem in many languages: http://0.30000000000000004.com/

To this specific print, you can simply quote the number (make it a string), and the compiler will not try to round the number:

print"{}".format("0.3000000000004")

But then you also wouldn't need to use the format:

print"0.3000000000004"