python - Limited digits with str.format(), and then only when they matter

Question

If we're printing a dollar amount, we usually want to always display two decimal digits.

cost1, cost2 = 123.456890123456789, 357.000
print '{c1:.2f}  {c2:.2f}'.format(c1=cost1, c2=cost2)

shows

123.46  357.00

But on other occasions we'd like to print the fractions only if they matter. If the two numbers above were volume, for instance, we may prefer to display

123.45 gal. 357 gal.

Can this be obtained directly with format?

Answer 1

In String Formatting Operations Python Docs describe the %g format which truncates trailing zeros.

>>> print "%g gallons" % (123.45)
123.45 gallons

>>> print "%g gallons" % (357)
357 gallons

>>> print "%g gallons" % (357.0)
357 gallons

Or using Python 3 string formatting:

>>> print "{:g} gal {:g} gal".format(123.45, 357.0)
123.45 gal 357 gal

The g formatter is unintuitive but you can get some interesting results by setting the precision:

>>> print "{:g} gal {:.3g} gal {:.4g} gal {:.5g} gal {:.6g} gal {:.7g} gal".format(*([123.456789] * 6))
123.457 gal 123 gal 123.5 gal 123.46 gal 123.457 gal 123.4568 gal

Note that in this case setting precision to .5 achieves the desired result of 2 decimal places.

Of course you could combine this with f floating point formatter first to get whatever you wanted.