Python Decimals format

Question

What is a good way to format a python decimal like this way?

1.00 --> '1'
1.20 --> '1.2'
1.23 --> '1.23'
1.234 --> '1.23'
1.2345 --> '1.23'

Answer 1

If you have Python 2.6 or newer, use format:

'{0:.3g}'.format(num)

For Python 2.5 or older:

'%.3g'%(num)

Explanation:

{0}tells format to print the first argument -- in this case, num.

Everything after the colon (:) specifies the format_spec.

.3 sets the precision to 3.

g removes insignificant zeros. See http://en.wikipedia.org/wiki/Printf#fprintf

For example:

tests=[(1.00, '1'),
       (1.2, '1.2'),
       (1.23, '1.23'),
       (1.234, '1.23'),
       (1.2345, '1.23')]

for num, answer in tests:
    result = '{0:.3g}'.format(num)
    if result != answer:
        print('Error: {0} --> {1} != {2}'.format(num, result, answer))
        exit()
    else:
        print('{0} --> {1}'.format(num,result))

yields

1.0 --> 1
1.2 --> 1.2
1.23 --> 1.23
1.234 --> 1.23
1.2345 --> 1.23

---

Using Python 3.6 or newer, you could use f-strings:

In [40]: num = 1.234; f'{num:.3g}'
Out[40]: '1.23'

Answer 2

Only first part of Justin's answer is correct. Using "%.3g" will not work for all cases as .3 is not the precision, but total number of digits. Try it for numbers like 1000.123 and it breaks.

So, I would use what Justin is suggesting:

>>> ('%.4f' % 12340.123456).rstrip('0').rstrip('.')
'12340.1235'
>>> ('%.4f' % -400).rstrip('0').rstrip('.')
'-400'
>>> ('%.4f' % 0).rstrip('0').rstrip('.')
'0'
>>> ('%.4f' % .1).rstrip('0').rstrip('.')
'0.1'

Answer 3

Here's a function that will do the trick:

def myformat(x):
    return ('%.2f' % x).rstrip('0').rstrip('.')

And here are your examples:

>>> myformat(1.00)
'1'
>>> myformat(1.20)
'1.2'
>>> myformat(1.23)
'1.23'
>>> myformat(1.234)
'1.23'
>>> myformat(1.2345)
'1.23'

Edit:

From looking at other people's answers and experimenting, I found that g does all of the stripping stuff for you. So,

'%.3g' % x

works splendidly too and is slightly different from what other people are suggesting (using '{0:.3}'.format() stuff). I guess take your pick.

Answer 4

If using 3.6 or newer, just use f-Strings

print(f'{my_var:.1f}')

Answer 5

You can use "f-strings" (f for "formatted string literals"), the short format style from Python v3.6 on:

f'{1.234:.1f}'
Out: '1.2'

Or, as a test:

f'{1.234:.1f}' == '1.2'
Out: True

By the way, you can also use this with variables.

x = 1.234
f'{x:.1f} and {x:.2f} and {x}'
Out: '1.2 and 1.23 and 1.234'

If you need to use quotes in the text, embed the text with f'''...''' instead of f'...'.

Answer 6

Just use Python's standard string formatting methods:

>>> "{0:.2}".format(1.234232)
'1.2'
>>> "{0:.3}".format(1.234232)
'1.23'

If you are using a Python version under 2.6, use

>>> "%f" % 1.32423
'1.324230'
>>> "%.2f" % 1.32423
'1.32'
>>> "%d" % 1.32423
'1'