Make sure device exists before opening

Without this check, the SPI interface would create the file if it did
not already exist (assuming correct permissions) but the ensuing ioctl
operations would fail.

Author: tomstokes

src/spi/spi.py

@@ -28,6 +28,7 @@
 import struct
 import fcntl
 import array
+import os.path
 
 
 def _ioc(direction, number, structure):
@@ -129,6 +130,10 @@ def __init__(self, device, speed=None, bits_per_word=None, phase=None,
         if isinstance(device, tuple):
             (bus, dev) = device
             device = "/dev/spidev{:d}.{:d}".format(bus, dev)
+
+        if not os.path.exists(device):
+            raise IOError("{} does not exist".format(device))
+
         self.handle = open(device, "w+")
 
         if speed is not None: