How to round a number to significant figures in Python

I need to round a float to be displayed in a UI. e.g, to one significant figure:

1234 -> 1000

0.12 -> 0.1

0.012 -> 0.01

0.062 -> 0.06

6253 -> 6000

1999 -> 2000

Is there a nice way to do this using the Python library, or do I have to write it myself?

You can use negative numbers to round integers:

>>> round(1234, -3)
1000.0

Thus if you need only most significant digit:

>>> from math import log10, floor
>>> def round_to_1(x):
...   return round(x, -int(floor(log10(abs(x)))))
... 
>>> round_to_1(0.0232)
0.02
>>> round_to_1(1234243)
1000000.0
>>> round_to_1(13)
10.0
>>> round_to_1(4)
4.0
>>> round_to_1(19)
20.0

You'll probably have to take care of turning float to integer if it's bigger than 1.

%g in string formatting will format a float rounded to some number of significant figures. It will sometimes use 'e' scientific notation, so convert the rounded string back to a float then through %s string formatting.

>>> '%s' % float('%.1g' % 1234)
'1000'
>>> '%s' % float('%.1g' % 0.12)
'0.1'
>>> '%s' % float('%.1g' % 0.012)
'0.01'
>>> '%s' % float('%.1g' % 0.062)
'0.06'
>>> '%s' % float('%.1g' % 6253)
'6000.0'
>>> '%s' % float('%.1g' % 1999)
'2000.0'

If you want to have other than 1 significant decimal (otherwise the same as Evgeny):

>>> from math import log10, floor
>>> def round_sig(x, sig=2):
...   return round(x, sig-int(floor(log10(abs(x))))-1)
... 
>>> round_sig(0.0232)
0.023
>>> round_sig(0.0232, 1)
0.02
>>> round_sig(1234243, 3)
1230000.0

f'{float(f"{i:.1g}"):g}'
# Or with Python <3.6,
'{:g}'.format(float('{:.1g}'.format(i)))

This solution is different from all of the others because:

1. it exactly solves the OP question
2. it does not need any extra package
3. it does not need any user-defined auxiliary function or mathematical operation

For an arbitrary number n of significant figures, you can use:

print('{:g}'.format(float('{:.{p}g}'.format(i, p=n))))

Test:

a = [1234, 0.12, 0.012, 0.062, 6253, 1999, -3.14, 0., -48.01, 0.75]
b = ['{:g}'.format(float('{:.1g}'.format(i))) for i in a]
# b == ['1000', '0.1', '0.01', '0.06', '6000', '2000', '-3', '0', '-50', '0.8']

Note: with this solution, it is not possible to adapt the number of significant figures dynamically from the input because there is no standard way to distinguish numbers with different numbers of trailing zeros (3.14 == 3.1400). If you need to do so, then non-standard functions like the ones provided in the to-precision package are needed.

I have created the package to-precision that does what you want. It allows you to give your numbers more or less significant figures.

It also outputs standard, scientific, and engineering notation with a specified number of significant figures.

In the accepted answer there is the line

>>> round_to_1(1234243)
1000000.0

That actually specifies 8 sig figs. For the number 1234243 my library only displays one significant figure:

>>> from to_precision import to_precision
>>> to_precision(1234243, 1, 'std')
'1000000'
>>> to_precision(1234243, 1, 'sci')
'1e6'
>>> to_precision(1234243, 1, 'eng')
'1e6'

It will also round the last significant figure and can automatically choose what notation to use if a notation isn't specified:

>>> to_precision(599, 2)
'600'
>>> to_precision(1164, 2)
'1.2e3'