is there built-in custom numeric formating in python?

Question

in C#, I can do (#.####), which prints up to 4 significant digits after the decimal point. 1.2 -> 1.2 1.234 -> 1.234 1.23456789 -> 1.2345

Afaik, in python, there is only the c-style %.4f which will always print to 4 decimal points padded with 0s at the end if needed. I don't want those 0s.

Any suggestions for what is the cleanest way to achieve what I need?

One possible solution is to print it first and trim ending 0s myself, but hoping to find more clever ways.

Answer 1

It looks like you can use the "g" format specifier for the .format() string function (only available in python 2.6+):

>>> x = 1.2
>>> print "{0:0.4g}".format(x)
1.2
>>> x = 1.234565789
>>> print "{0:0.4g}".format(x)
1.235

This includes digits before and after the decimal point, which is why you get 1.235 (rounded 1.2345 to 4 digits). If you only want significant digits to the right, you'd have to do something funny like

>>> x = 1.23456789
>>> # length of the number before the decimal
>>> left_sigdigs = len(str(x).partition(".")[0])
>>> format_string = "{0:0." + str(4 + left_sigdigs) + "g}"
>>> print format_string.format(x)
1.2346

Note that it still rounds, unlike your example.

Answer 2

Large numbers would be formatted differently, but, for those you mention:

>>> for x in (1.2, 1.234, 1.23456789):
...   print '%.4g' % x
... 
1.2
1.234
1.235

This is the traditional equivalent (working in all versions of Python 2.whatever) of the more modern {0:0g}.format approach mentioned in @Daniel's answer (suitable only for Python 3.whatever and 2.6 or better). For example, if you're using Google App Engine (currently supporting Python 2.5, only), you need to use the %-operator approach to formatting (can't use the 2.6-or-better .format method).