using python variables inside quotes

[lewst]

Greetings,

This is a simple question, so please excuse, but I am slowly learning
Python by trying to rewrite all my Perl scripts.  Thus far I like it
much better than Perl, but a small matter has me stumped.

My question deals with using variables inside quotes.  Take this
sample program for example:

  #!/usr/bin/env python

  import os, random

  imgdir = "/home/lewst/images/"
  piclist = os.listdir(imgdir)

  chosenpic = imgdir + random.choice(piclist)
  os.system("xv %s &" % chosenpic)

This script displays a random image from a directory full of images
using the "xv" image manipulation program.  

Is there a simpler way to access the `chosenpic' variable within the
os.system() call?  At first I tried the following:

  os.system("xv chosenpic &")

This didn't work, because it is looking for the literal "chosenpic"
instead of the name of one of my images.

In Perl, I could use the following:

  system("xv $chosenpic &");

Perl recognizes the `$' as a variable.  Since in Python variables are
not declared with punctuation marks like this, it is not as easy to
distinguish them from regular words.

Have I missed something trivial?  

(Please CC: me on followups)

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