add support for read/write byte from bytearray with tests (#268)

Author: lautarodapin

snap7/util.py

@@ -135,6 +135,28 @@ def set_bool(bytearray_: bytearray, byte_index: int, bool_index: int, value: boo
         bytearray_[byte_index] -= index_value
 
 
+def set_byte(bytearray_: bytearray, byte_index: int, _int: int) -> bytearray:
+    """
+    Set value in bytearray to byte
+    """
+    _int = int(_int)
+    _bytes = struct.pack('B', _int)
+    bytearray_[byte_index:byte_index + 1] = _bytes
+    return bytearray_
+
+
+def get_byte(bytearray_: bytearray, byte_index: int) -> int:
+    """
+    Get byte value from bytearray.
+    WORD 8bit 1bytes Decimal number unsigned B#(0) to B#(255) => 0 to 255
+    """
+    data = bytearray_[byte_index:byte_index + 1]
+    data[0] = data[0] & 0xff
+    packed = struct.pack('B', *data)
+    value = struct.unpack('B', packed)[0]
+    return value
+
+
 def set_word(bytearray_: bytearray, byte_index: int, _int: int):
     """
     Set value in bytearray to word

test/test_util.py

@@ -1,5 +1,6 @@
 import re
 import unittest
+import struct
 
 from snap7 import util, types
 
@@ -67,12 +68,30 @@
     32, 7, 18, 23, 50, 2, 133, 65,                 # these 8 values build the date and time 12 byte total
                                                    # data typ together, for details under this link
                                                    # https://support.industry.siemens.com/cs/document/36479/date_and_time-format-bei-s7-?dti=0&lc=de-DE
-    254, 254, 254, 254, 254, 127                   # test small int
+    254, 254, 254, 254, 254, 127,                  # test small int
+    128,                                           # test set byte
 ])
 
+_new_bytearray = bytearray(100)
+_new_bytearray[41:41 + 1] = struct.pack("B", 128)       # byte_index=41, value=128, bytes=1
+_new_bytearray[42:42 + 1] = struct.pack("B", 255)       # byte_index=41, value=255, bytes=1
+
 
 class TestS7util(unittest.TestCase):
 
+    def test_get_byte(self):
+        test_array = bytearray(_new_bytearray)
+        byte_ = util.get_byte(test_array, 41)
+        self.assertEqual(byte_, 128)
+        byte_ = util.get_byte(test_array, 42)
+        self.assertEqual(byte_, 255)
+
+    def test_set_byte(self):
+        test_array = bytearray(_new_bytearray)
+        util.set_byte(test_array, 41, 127)
+        byte_ = util.get_byte(test_array, 41)
+        self.assertEqual(byte_, 127)
+
     def test_get_s5time(self):
         """
         S5TIME extraction from bytearray