NMEA data parsing using python 2.7 - python-2.7

Question

i want readings of present GPS location, when i run the below code in raspberry pi, program prints 10-12 outputs and then it displays the error as below:
Traceback (most recent call last):
File "simplegpsparsing.py", line 24, in
get_present_gps()
File "simplegpsparsing.py", line 16, in get_present_gps
lat, _, lon= line.split(',')[2:5]
ValueError: need more than 0 values to unpack
i want present value of GPS( buffers should be updated with immediate GPS) so that present GPS values can be known.
my code goes as below :
import os
import serial
def get_present_gps:
ser= serial.Serial('/dev/ttyAMA0',9600)
ser.open()
while True :
f=open('/home/pi/Desktop/gps1','w')
data=ser.read(ser.inWaiting()) # read no.of bytes in waiting
f.write(data) #write data into file
f.flush() # flush(method) from buffer into os buffer
os.fsync(f.fileno()) #ensure to write from os buffer(internal buffers)into disk
f = open('/home/pi/Desktop/gps1','r') # fetch the required file
for line in f.read().split('\n') :
if line.startswith( '$GPGGA' ) :
lat, _, lon = line.strip().split(',')[2:5]
try :
lat = float( lat )
lon = float( lon )
print lat
print lon
except :
pass
# something wrong happens with your data, print some error messages
get_present_gps()
if the serial port is left open without closing, will it create any problem? will the above code meet my requirement i.e getting the instantaneous value?

Answer 1

Here is a thought: parse the data string into chunks that are newline terminated; leave "unfinished" strings in the buffer until the next time around the loop. Untested, this will look something like this:
keepThis = ''
while True :
data=keepThis + ser.read(ser.inWaiting()) # read no.of bytes in waiting
m = data.split("\n") # get the individual lines from input
if len(m[-1]==0): # true if only complete lines present (nothing after last newline)
processThis = m
keepThis = ''
else:
processThis = m[0:-1] # skip incomplete line
keepThis = m[-1] # fragment
# process the complete lines:
for line in processThis:
if line.startswith( '$GPGGA' ) :
try :
lat, _, lon = line.strip().split(',')[2:5] # move inside `try` just in case...
lat = float( lat )
lon = float( lon )
print lat
print lon
except :
pass