Re: [gentoo-user] new machine : Python calculator

[Marc Joliet]

Am Thu, 20 Sep 2012 05:05:11 -0400
schrieb Philip Webb <purslow@××××××××.net>:

> 120919 Marc Joliet wrote:
> > 120918 Philip Webb <purslow@××××××××.net> wrote:
> >> With Python running as interpreter, I would get much more capability,
> >> but I would need to enter the special line to load the math functions :
> >> is it possible to do it with some capitalised variable in  .bashrc ,
> >> which might list parameters telling Python3 what to load when it starts ?
> >> one of the 'man' files seems to refer to something like that, but briefly.
> > 3.) Put the "import" line in its own file and put it in the variable
> > PYTHONSTARTUP, e.g. "export PYTHONSTARTUP=/path/to/my/script.py".
> > Python executes it's contents before presenting the prompt,
> > so you can put whatever imports you want in that script.
>
> Thanks, that's what I saw in my brief glance at the 'man'.
> It works out of the box: the only problem is precision,
> which at  16  decimal places is a bit more than I usually need (smile).
> I can search out how to limit it to something more useful to me,
> but might you have a quick answer ?  Thanks for the above.

Reading up the "format specification mini language"
(http://docs.python.org/library/string.html#formatspec, and the format syntax
explained above it), you could do as follows, to print as float rounded to four
decimal places:

  print('{0:.4f}'.format(2.4))

Or, leaving out the zero (you only need the indexes if you print things out of
order or multiple times):

  print('{:.4f}'.format(2.4))

Also, I re-remembered that there is an alternative formatting method  (I don't
print formatted output that often in python, I guess):

  print("%.4f" % 2.4)

will do the same as the above two examples. Either way, to make things easy,
you could define your own print function to do that for you, e.g.:

  def myprint(num, places=4, *args, **kargs):
      fmt_str = "{:." + str(places) + "f}"
      print(fmt_str.format(num), *args, **kargs)

Using it would look like (in IPython):

  In [13]: myprint(2.4)
  2.4000

  In [14]: myprint(2.4, 5)
  2.40000

You would put this in the startup script after the import line. Note that it
passes extra positional and keyword arguments to print(), so you can specify a
file to print to, for example. Also note that because of this, it won't work in
Python 2.

HTH
-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup