[TD-6586]<fix>: fix binary/nchar convertion error in python connector (#7913)

src/connector/python/taos/field.py

@@ -165,12 +165,14 @@ def _crow_binary_to_python_block(data, num_of_rows, nbytes=None, precision=Field
     assert nbytes is not None
     res = []
     for i in range(abs(num_of_rows)):
-        try:
-            rbyte = ctypes.cast(data + nbytes * i, ctypes.POINTER(ctypes.c_short))[:1].pop()
-            tmpstr = ctypes.c_char_p(data + nbytes * i + 2)
-            res.append(tmpstr.value.decode()[0:rbyte])
-        except ValueError:
+        rbyte = ctypes.cast(data + nbytes * i, ctypes.POINTER(ctypes.c_short))[:1].pop()
+        chars = ctypes.cast(c_char_p(data + nbytes * i + 2), ctypes.POINTER(c_char * rbyte))
+        buffer = create_string_buffer(rbyte + 1)
+        buffer[:rbyte] = chars[0][:rbyte]
+        if rbyte == 1 and buffer[0] == b'\xff':
             res.append(None)
+        else:
+            res.append(cast(buffer, c_char_p).value.decode())
     return res
 
 
@@ -179,11 +181,14 @@ def _crow_nchar_to_python_block(data, num_of_rows, nbytes=None, precision=FieldT
     assert nbytes is not None
     res = []
     for i in range(abs(num_of_rows)):
-        try:
-            tmpstr = ctypes.c_char_p(data + nbytes * i + 2)
-            res.append(tmpstr.value.decode())
-        except ValueError:
+        rbyte = ctypes.cast(data + nbytes * i, ctypes.POINTER(ctypes.c_short))[:1].pop()
+        chars = ctypes.cast(c_char_p(data + nbytes * i + 2), ctypes.POINTER(c_char * rbyte))
+        buffer = create_string_buffer(rbyte + 1)
+        buffer[:rbyte] = chars[0][:rbyte]
+        if rbyte == 4 and buffer[:4] == b'\xff'*4:
             res.append(None)
+        else:
+            res.append(cast(buffer, c_char_p).value.decode())
     return res