import glob
filenames = glob.glob('/home/ftp/pub/praktikum/20150317/dark*.fits.gz')

import pyfits

for filename in filenames:
   f = pyfits.open(filename)
   exptime = f[0].header['EXPTIME']
   filt = f[0].header['FILTER']
# aaaaa
   if abs(exptime - 2) < 1e-3:
#   if 2 - 1e3 < exptime and exptime < 2 + 1e-3:
#      print("{0}| {1}s {2}".format(filename[-30:],exptime,filt))
      print("{0}".format(filename))