python - Most Pythonic way to print *at most* some number of decimal places

21
votes

I want to format a list of floating-point numbers with at most, say, 2 decimal places. But, I don't want trailing zeros, and I don't want trailing decimal points.

So, for example, 4.001 => 4, 4.797 => 4.8, 8.992 => 8.99, 13.577 => 13.58.

The simple solution is ('%.2f' % f).rstrip('.0')('%.2f' % f).rstrip('0').rstrip('.'). But, that looks rather ugly and seems fragile. Any nicer solutions, maybe with some magical format flags?

3
pythonfloating-pointstring-formatting
Am I missing the point here, or what about the round() function: docs.python.org/2/library/functions.html#round - TerryA
Though, I have to say, I'm not a huge fan of solutions that involve round simply because I am nervous I'll get 1.1000000000009962 as an output someday. - nneonneo
Aarrgh, it's not a duplicate! I don't want trailing .0. The other question permits that. - nneonneo
@nneonneo I believe int() will get rid of the .0, but use in caution as it will get rid of any other decimal places (so perhaps an if/else statement?) - TerryA
@Haidro: ...now that's just getting icky. I was hoping I didn't need an if/else, or I would just condition on the '.%2f' % f or something. (Still weirdly difficult to have a nice solution...I'm really used to things being really easy in Python) - nneonneo

3 Answers

21
votes

You need to separate the 0 and the . stripping; that way you won't ever strip away the natural 0.

Alternatively, use the format() function, but that really comes down to the same thing:

format(f, '.2f').rstrip('0').rstrip('.')

Some tests:

>>> def formatted(f): return format(f, '.2f').rstrip('0').rstrip('.')
... 
>>> formatted(0.0)
'0'
>>> formatted(4.797)
'4.8'
>>> formatted(4.001)
'4'
>>> formatted(13.577)
'13.58'
>>> formatted(0.000000000000000000001)
'0'
>>> formatted(10000000000)
'10000000000'
20
votes

The g formatter limits the output to n significant digits, dropping trailing zeroes:

>>> "{:.3g}".format(1.234)
'1.23'
>>> "{:.3g}".format(1.2)
'1.2'
>>> "{:.3g}".format(1)
'1'
5
votes

In general working with String[s] can be slow. However, this is another solution:

>>> from decimal import Decimal
>>> precision = Decimal('.00')
>>> Decimal('4.001').quantize(precision).normalize()
Decimal('4')
>>> Decimal('4.797').quantize(precision).normalize()
Decimal('4.8')
>>> Decimal('8.992').quantize(precision).normalize()
Decimal('8.99')
>>> Decimal('13.577').quantize(precision).normalize()
Decimal('13.58')

You may find more info here: http://docs.python.org/2/library/decimal.html