Use lists of words instead of byte strings

Lists of ints of received/transmitted words will be more useful in the
majority of applications than the raw byte strings.

This format also makes adding support for 16-bit and 32-bit transfers
easier to implement in the future.

Author: tomstokes

README.md

@@ -27,8 +27,8 @@ A pure Python SPI interface using the Linux spidev device
     spi.mode = SPI.MODE_0
     spi.bits_per_word = 8
     spi.speed = 500000
-    received = spi.transfer("\xAA\xBB\xCC")
-    spi.write("\x00\x11\x22")
+    received = spi.transfer([0x11, 0x22, 0xFF])
+    spi.write([0x12, 0x34, 0xAB, 0xCD])
     received = spi.read(10)
 
 

spi.py

@@ -366,27 +366,28 @@ def write(self, data, speed=0, bits_per_word=0, delay=0):
         """Perform half-duplex SPI write.
 
         Args:
-            data: Binary string of data to write
+            data: List of words to write
             speed: Optional temporary bitrate override in Hz. 0 (default)
                 uses existing spidev speed setting.
             bits_per_word: Optional temporary bits_per_word override. 0 (
                 default) is equivalent to 8 bits per word.
             delay: Optional delay in usecs between sending the last bit and
                 deselecting the chip select line. 0 (default) for no delay.
         """
+        data = array.array('B', data).tostring()
         length = len(data)
-        transmit_buffer = ctypes.create_string_buffer(str(data))
+        transmit_buffer = ctypes.create_string_buffer(data)
         spi_ioc_transfer = struct.pack(SPI._IOC_TRANSFER_FORMAT,
                                        ctypes.addressof(transmit_buffer), 0,
                                        length, speed, delay, bits_per_word, 0,
                                        0, 0, 0)
         fcntl.ioctl(self.handle, SPI._IOC_MESSAGE, spi_ioc_transfer)
 
     def read(self, length, speed=0, bits_per_word=0, delay=0):
-        """Perform half-duplex SPI read
+        """Perform half-duplex SPI read as a binary string
 
         Args:
-            length: Integer count of bytes to read
+            length: Integer count of words to read
             speed: Optional temporary bitrate override in Hz. 0 (default)
                 uses existing spidev speed setting.
             bits_per_word: Optional temporary bits_per_word override. 0 (
@@ -395,21 +396,21 @@ def read(self, length, speed=0, bits_per_word=0, delay=0):
                 deselecting the chip select line. 0 (default) for no delay.
 
         Returns:
-            Binary string of data read from device
+            List of words read from device
         """
         receive_buffer = ctypes.create_string_buffer(length)
         spi_ioc_transfer = struct.pack(SPI._IOC_TRANSFER_FORMAT, 0,
                                        ctypes.addressof(receive_buffer),
                                        length, speed, delay, bits_per_word, 0,
                                        0, 0, 0)
         fcntl.ioctl(self.handle, SPI._IOC_MESSAGE, spi_ioc_transfer)
-        return ctypes.string_at(receive_buffer, length)
+        return [ord(byte) for byte in ctypes.string_at(receive_buffer, length)]
 
     def transfer(self, data, speed=0, bits_per_word=0, delay=0):
         """Perform full-duplex SPI transfer
 
         Args:
-            data: Binary string to transmit
+            data: List of words to transmit
             speed: Optional temporary bitrate override in Hz. 0 (default)
                 uses existing spidev speed setting.
             bits_per_word: Optional temporary bits_per_word override. 0 (
@@ -418,7 +419,7 @@ def transfer(self, data, speed=0, bits_per_word=0, delay=0):
                 deselecting the chip select line. 0 (default) for no delay.
 
         Returns:
-            Binary string of bytes read from SPI bus during transfer
+            List of words read from SPI bus during transfer
         """
         length = len(data)
         transmit_buffer = ctypes.create_string_buffer(str(data))
@@ -429,4 +430,4 @@ def transfer(self, data, speed=0, bits_per_word=0, delay=0):
                                        length, speed, delay, bits_per_word, 0,
                                        0, 0, 0)
         fcntl.ioctl(self.handle, SPI._IOC_MESSAGE, spi_ioc_transfer)
-        return ctypes.string_at(receive_buffer, length)
+        return [ord(byte) for byte in ctypes.string_at(receive_buffer, length)]