[python] Convert floating point number to a certain precision, and then copy to string

I have a floating point number, say 135.12345678910. I want to concatenate that value to a string, but only want 135.123456789. With print, I can easily do this by doing something like:

print "%.9f" % numvar

with numvar being my original number. Is there an easy way to do this?

Python 3.6

Just to make it clear, you can use f-string formatting. This has almost the same syntax as the format method, but make it a bit nicer.

Example:

print(f'{numvar:.9f}')

More reading about the new f string:

• What's new in Python 3.6 (same link as above)
• PEP official documentation
• Python official documentation
• Really good blog post - talks about performance too

Here is a diagram of the execution times of the various tested methods (from last link above):

Using round:

>>> numvar = 135.12345678910
>>> str(round(numvar, 9))
'135.123456789'

In case the precision is not known until runtime, this other formatting option is useful:

>>> n = 9
>>> '%.*f' % (n, numvar)
'135.123456789'

The str function has a bug. Please try the following. You will see '0,196553' but the right output is '0,196554'. Because the str function's default value is ROUND_HALF_UP.

>>> value=0.196553500000 
>>> str("%f" % value).replace(".", ",")

It's not print that does the formatting, It's a property of strings, so you can just use

newstring = "%.9f" % numvar

To set precision with 9 digits, get:

print "%.9f" % numvar

Return precision with 2 digits:

print "%.2f" % numvar 

Return precision with 2 digits and float converted value:

numvar = 4.2345
print float("%.2f" % numvar)