In [21]:

    

filein = "testfile.txt"
fileout = "outfile.txt"
f = open(filein,'r')
filedata = f.read()
f.close()

water_level = "{0:.3f}".format(100)
newdata = filedata.replace("WATER_LEVEL","new data")
print "{0}".format(100.)
print(filedata.format( WATER_LEVEL=water_level ))
f = open(fileout,'w')
f.write(filedata.format(WATER_LEVEL="99.9"))
f.close()


    

    




100.0
Test file in.
# input water level: 100.000
! all done