On Sat, 23 Oct 2010 12:42:50 am Richard D. Moores wrote:
> So I wrote a function:
>
> def float2n_decimals(floatt, n):
> """
> Given a float (floatt), return floatt to n decimal places.
>
> E.g., with n = 2, 81.34567 -> 81.35
> """
> return ("%%.%sf" % n) % floatt
>
> which works fine,
float2n_decimals(x, 3)
is better written in place as:
"%.*f" % (3, x)
There's no need for a function for something so simple.
> but a question remains: n is an integer. Why the
> 's' in '%sf'?
Your function might be more clear if you split the formatting into two
lines:
template = "%%.%sf" % n
return template % floatt
The first line builds a template string from n. Now it becomes clear
what %s is for: it's to insert the n as a string into the template. %d
would express the intention of the function better.
By the way, the function docstring is seriously misleading. It describes
a completely different function:
> Given a float (floatt), return floatt to n decimal places.
>
> E.g., with n = 2, 81.34567 -> 81.35
It does nothing of the sort! What you are describing is the built-in
function round:
>>> print round(81.34567, 2)
81.35
What your function does is return a string, not a float. Besides, given
a float, where does n come from? A global variable?
There is a convention for giving examples that you should follow.
Putting the three of these together, your docstring should say
something like:
Given a float floatt and an integer n, returns floatt formatted
as a string to n decimal places.
>>> float2n_decimals(2, 81.34567)
'81.35'
(BTW, I really hate the name "floatt". It makes me think you're
stuttering.)
--
Steven D'Aprano
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