reviewed coding style with pylint

gpsd-i2c.service

@@ -7,8 +7,8 @@ Type=simple
 Restart=on-failure
 RestartSec=1
 WorkingDirectory=/opt/gpsd-i2c
-ExecStart=/usr/bin/socat pty,rawer,link=/dev/gpsd0 EXEC:./gpsd-i2c.py,pty,rawer
+ExecStart=/usr/bin/socat pty,rawer,link=/dev/gpsd0 EXEC:./gpsd_i2c.py,pty,rawer
 
 [Install]
 RequiredBy=gpsd.service
-WantedBy=multi-user.target
+WantedBy=multi-user.target

gpsd_i2c.py

@@ -0,0 +1,88 @@
+#!/usr/bin/env python3
+"""
+A simple way to read raw NMEA 0183 data from a GPS module that has I²C output capability.
+"""
+
+import os
+import signal
+import sys
+import smbus2
+
+def parse_int(val, default):
+    "Parse integer from string, first as decimal then as hex"
+    try:
+        result = int(val)
+    except (TypeError, ValueError):
+        try:
+            result = int(val,16)
+        except (TypeError, ValueError):
+            result = default
+    return result
+
+# Default to device 0-0042; can be overriden via environment
+I2C_BUS = parse_int(os.environ.get("I2C_BUS", None), 0)
+I2C_ADDRESS = parse_int(os.environ.get("I2C_ADDRESS", None), 0x42)
+
+def handle_ctrl_c(sig, frame):
+    "Exit handler"
+    # pylint: disable=unused-argument
+    sys.exit(130)
+
+# This will capture exit when using [Ctrl]+[C] for a graceful exit
+signal.signal(signal.SIGINT, handle_ctrl_c)
+
+def parse_response(gps_line):
+    "Parse GPS line"
+    # Check #1 -- make sure line starts with $ and $ doesn't appear twice
+    if gps_line[0] != 36 or gps_line.count(36) != 1:
+        return
+
+    # Check #2 -- 83 is maximum NMEA sentence length
+    if len(gps_line) > 83:
+        return
+
+    # Check #3 -- make sure that only readable ASCII characters
+    # and carriage return are seen
+    for char in gps_line:
+        if (char < 32 or char > 122) and char != 13:
+            return
+
+    gps_chars = ''.join(chr(char) for char in gps_line)
+
+    # Check #4 -- skip txbuff allocation error
+    if 'txtbuf' in gps_chars:
+        return
+
+    # Check #5 -- only split twice to avoid unpack error
+    gps_str, chk_sum = gps_chars.split('*', 2)
+
+    # Remove the $ and do a manual checksum on the rest of the NMEA sentence
+    chk_val = 0
+    for char in gps_str[1:]:
+        chk_val ^= ord(char)
+
+    # Compare the calculated checksum with the one in the NMEA sentence
+    if chk_val != int(chk_sum, 16):
+        return
+
+    # All checks passed
+    print(gps_chars)
+
+def read_gps(bus):
+    "read bytes from I2C device"
+    response = []
+    try:
+        while True:
+            byte = bus.read_byte(I2C_ADDRESS)
+            if byte == 255:
+                return
+            if byte == ord('\n'):
+                break
+            response.append(byte)
+        parse_response(response)
+    except IOError:
+        bus = smbus2.SMBus(I2C_BUS)
+
+BUS = smbus2.SMBus(I2C_BUS)
+while True:
+    read_gps(BUS)