How to make Python format floats with certain amount of significant digits?

问题

I want my Python (2.4.3) output numbers to have a certain format. Specifically, if the number is a terminating decimal with <= 6 significant digits, show it all. However, if it has > 6 significant digits, then output only 6 significant digits.

"A" shows how Python is writing the floats. "B" shows how I want them written. How can I make Python format my numbers in that way?

A:
10188469102.605597
5.5657188485
3.539
22.1522612479
0
15.9638450858
0.284024
7.58096703786
24.3469152383

B:
1.01885e+10
5.56572
3.539
22.1523
0
15.9638
0.284024
7.58097
24.3469

回答1:

You'll want the g modifier for format that drops insignificant zeroes;

>>> "{0:.6g}".format(5.5657188485)
'5.56572'
>>> "{0:.6g}".format(3.539)
'3.539'

---

Sorry, my update also includes the fact that I am restricted to using Python 2.4.3, which does not have format() function.

The format specifiers work even without the .format() function:

>>> for i in a:
...    print '%.6g' % (i,)
...
1.01885e+10
5.56572
3.539
22.1523
0
15.9638
0.284024
7.58097
24.3469

回答2:

try this way

a=[10188469102.605597,5.5657188485,3.539,22.1522612479,0,15.9638450858,0.284024,7.58096703786,24.3469152383]

 for i in a:
    if i >100:
        print '{:.6e}'.format(i)
    else:
        print '{:.6f}'.format(i)

for lower version of python

for i in a:
    if i >100:
        print '%6e'%i
    else:
        print '%6f'%i

output

1.018847e+10
5.565719
3.539000
22.152261
0.000000
15.963845
0.284024
7.580967
24.346915

回答3:

There is a way to retain trailing zeros so that it consistently shows the number of significant digits. Not exactly what OP wanted, but probably useful to many.

a = [10188469102.605597,5.5657188485,3.539,22.1522612479,0,15.9638450858,0.284024,7.58096703786,24.3469152383]

for i in a:
    print("{:#.6g}".format(i))

Output

1.01885e+10
5.56572
3.53900
22.1523
0.00000
15.9638
0.284024
7.58097
24.3469

Note that this will only work with the format function and not with % operator.

Reference: an undocumented feature (as of 2019)

来源：https://stackoverflow.com/questions/25780022/how-to-make-python-format-floats-with-certain-amount-of-significant-digits