Python 3 Float Decimal Points/Precision

Question

I am reading a text file with floating point numbers, all with either 1 or 2 decimal points. I am using float() to convert a line into a float, and raising a ValueError if that fails. I am storing all floats in a list. When printing it out, I'd like to print it out as a 2 decimal places floating point.

Assume I have a text file with the numbers -3.65, 9.17, 1. I read each one, and once I convert them to float and append them to a list. Now in Python 2, calling float(-3.65) returns -3.65. In Python 3 however, float(-3.65) returns -3.6499999999999999 which loses its precision.

I want to print the list of floats, [-3.6499999999999999, 9.1699999999999999, 1.0] with 2 decimal points only. Doing something along the lines of '%.1f' % round(n, 1) would return a string. How can I return a list of all two decimal points of floats, and not strings? So far, I rounded it using [round(num, 2) for num in list] but would need to set the decimal points / precision instead of round().

In python 2, `float(-3.65)` is `-3.6499999999999999` too.. This is *normal*. `float` is by it's very nature not 100 precise. — Martijn Pieters, Jan 26 '13 at 18:32
Not in mine: `Python 2.7.2 (v2.7.2:8527427914a2, Jun 11 2011, 15:22:34) │ [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin │ Type "help", "copyright", "credits" or "license" for more information. │ >>> float(-3.65) │ -3.65` — darksky, Jan 26 '13 at 18:33
Also, let me point out I know that `float` is not precise and I know the reasoning behind this. I'm looking an answer that will show how to print it out in 2 decimal points in Python 3. Thanks — darksky, Jan 26 '13 at 18:34
Basically, you just can't really do this because of the way float point works. I suggest you consider using the `Decimal` class in the `decimal` module. — martineau, Jan 26 '13 at 18:35
That is just the representation of Python rounding it to at most 16 positions behind the decimal. — Martijn Pieters, Jan 26 '13 at 18:36

Andrew E · Answer 1 · 2019-11-13T14:37:39.460

The comments state the objective is to print to 2 decimal places.

There's a simple answer for Python 3:

>>> num=3.65
>>> "The number is {:.2f}".format(num)
'The number is 3.65'

or equivalently with f-strings (Python 3.6+):

>>> num = 3.65
>>> f"The number is {num:.2f}"
'The number is 3.65'

As always, the float value is an approximation:

>>> "{}".format(num)
'3.65'
>>> "{:.10f}".format(num)
'3.6500000000'
>>> "{:.20f}".format(num)
'3.64999999999999991118'

I think most use cases will want to work with floats and then only print to a specific precision.

Those that want the numbers themselves to be stored to exactly 2 decimal digits of precision, I suggest use the decimal type. More reading on floating point precision for those that are interested.

Using f-strings does not seem to be very readable to another user who may not be aware of the feature. On the other hand, the using "{}".format() is much more intuitive when reading. — MonsieurBeilto, Apr 15 '19 at 01:54

score 59 · Answer 2 · edited Jul 20 '16 at 11:55

59

The simple way to do this is by using the round buit-in.

round(2.6463636263,2) would be displayed as 2.65.

edited Jul 20 '16 at 11:55

MartyIX

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answered Oct 30 '13 at 00:47

slurry

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Though be aware of the note from the [documentation on `round()`](https://docs.python.org/3/library/functions.html#round) that `round()` can give surprising results when given float values. The example mentioned there: "`round(2.675, 2)` gives `2.67` instead of the expected `2.68`." – Gary Mar 23 '19 at 15:33

NPE · Accepted Answer · 2013-01-26T18:54:55.740

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In a word, you can't.

3.65 cannot be represented exactly as a float. The number that you're getting is the nearest number to 3.65 that has an exact float representation.

The difference between (older?) Python 2 and 3 is purely due to the default formatting.

I am seeing the following both in Python 2.7.3 and 3.3.0:

In [1]: 3.65
Out[1]: 3.65

In [2]: '%.20f' % 3.65
Out[2]: '3.64999999999999991118'

For an exact decimal datatype, see decimal.Decimal.

edited Jan 26 '13 at 18:54

answered Jan 26 '13 at 18:34

NPE

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So there is no way to limit the number of decimal points as an number type? To print 3.65, I would require to print it out as a string? – darksky Jan 26 '13 at 18:35
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@Darksky One can *only* print strings. Sometimes the conversion isn't as explicit: but it is always done. – Jan 26 '13 at 18:36
Yes, so how would I represent `3.64999999999999991118` as `3.65` or anything that's 2 decimal points, without it being a string? Any other type than float that I can use? – darksky Jan 26 '13 at 18:37
2

Use `Decimal` in the `decimal` module instead of float point. – martineau Jan 26 '13 at 18:38
I thought Python had this change awhile ago. A change to `float` formatting so that it tried to be the shortest string that would convert back into the exact `float` you were printing. Basically an attempt to make these kinds of rounding issues disappear most of the time. Did that change not make it into Python3? I thought I saw it in the changelog for 2.6 or 2.7. – Omnifarious Jan 26 '13 at 18:53
@Omnifarious: I've just tested, and I am seeing no difference between 2.7.3 and 3.3.0 – NPE Jan 26 '13 at 18:55
@NPE: I'm researching now. But my preliminary research leads me to believe that I'm remembering something about the `str` method and/or the `%g` format specifier. – Omnifarious Jan 26 '13 at 19:19
@NPE: Ahh, yes, it's `repr`. And it changed in Python2.7. Python3 also has it, so it's not different. http://docs.python.org/2.7/whatsnew/2.7.html (search for 7117) and http://bugs.python.org/issue7117 – Omnifarious Jan 26 '13 at 19:32

score 9 · Answer 4 · answered Apr 26 '18 at 08:13

Try this:

num = input("Please input your number: ")

num = float("%0.2f" % (num))

print(num)

I believe this is a lot simpler. For 1 decimal place use %0.1f. For 2 decimal places use %0.2f and so on.

Or, if you want to reduce it all to 2 lines:

num = float("%0.2f" % (float(input("Please input your number: "))))
print(num)

score 4 · Answer 5 · answered Oct 26 '17 at 01:15

4

Try to understand through this below function using python3

def floating_decimals(f_val, dec):
    prc = "{:."+str(dec)+"f}" #first cast decimal as str
    print(prc) #str format output is {:.3f}
    return prc.format(f_val)


print(floating_decimals(50.54187236456456564, 3))

Output is : 50.542

Hope this helps you!

answered Oct 26 '17 at 01:15

ArifMustafa

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The OP didn't need a variable to define precision - it was set at 2 places. But for variable precision, you don't need to use the arcane string addition `prc = "{:."+str(dec)+"f}"`. Instead, embed the precision: `print("The number is {:.{}f}".format(f_val, dec))`. The same but with f-strings: `print(f"The number is {f_val:.{dec}f}")`. – Andrew E Oct 27 '17 at 03:12

Python 3 Float Decimal Points/Precision

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