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# Python byte[]'s - Help me port this crucial Java byte[] method

Posted on 2014-03-01
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Hi
The following code is crucial, working code that I have been using in RTS projects that, now that I'm a Python convert, I need assistance in getting it right. It isn't obvious.

I might be suffering from lack of experience issues, even formatting.

This is the working Java code, below, that I'd like to  convert to Python that converts byte arrays to integers, and the other way.
I cant yet do a simple porting, if this even is simple?
Does Python even do static methods, for easy use from anywhere, or must I re-paste this into other classes? Should I make bytearrays.py with all these methods?
Thanks

``````public static byte[] intToByteArray(int value) {
byte[] b = new byte[4];
for (int i = 0; i < 4; i++) {
int offset = (b.length - 1 - i) * 8;
b[i] = (byte) ((value >>> offset) & 0xFF);
}
return b;
}
``````

``````public static int byteArrayToInt(byte[] b, int offset) {  #make offset 0 for beginning
int value = 0;
for (int i = 0; i < 4; i++) {
int shift = (4 - 1 - i) * 8;
value += (b[i + offset] & 0x000000FF) << shift;
}
return value;
}
``````
0
Question by:beavoid
• 5
• 4

LVL 28

Expert Comment

ID: 39900197
Try the following, but they are written as functions, not as methons. No need for methods here:
``````def intToBytes(n):
b = bytearray([0, 0, 0, 0])   # init
b[3] = n & 0xFF
n >>= 8
b[2] = n & 0xFF
n >>= 8
b[1] = n & 0xFF
n >>= 8
b[0] = n & 0xFF

# Return the result or as bytearray or as bytes (commented out)
##return bytes(b)  # uncomment if you need
return b

def bytesToInt(b, offset):
n = (b[offset+0]<<24) + (b[offset+1]<<16) + (b[offset+2]<<8) + b[offset+3]
return n
``````
0

Author Comment

ID: 39900410
Thanks for those functions,

but, for some absurd reason, they are not visible from my run method, and I don't know why Python's visibility is non-intuitive?

In my code below, it complains that intToBytes is not visible, when it is the first thing defined!

What could I be doing wrong?
Thanks

``````import threading, socket

def intToBytes(n):
print ("_________intToBytes___________")
b = bytearray([0, 0, 0, 0])   # init
b[3] = n & 0xFF
n >>= 8
b[2] = n & 0xFF
n >>= 8
b[1] = n & 0xFF
n >>= 8
b[0] = n & 0xFF

# Return the result or as bytearray or as bytes (commented out)
##return bytes(b)  # uncomment if you need
return b

def bytesToInt(b, offset):
n = (b[offset+0]<<24) + (b[offset+1]<<16) + (b[offset+2]<<8) + b[offset+3]
return n

def __init__(self, numclients):
print('Server constructor: ')
self.numClients=numclients
print(self.numClients, " clients")

def run(self):

print('Server run() . . . client join:')

B=intToBytes(150)

print(B[0])
print(" . ")
print([1])
print(" . ")
print([2])
print(" . ")
print([3])
print(" . ")

clientJoinMode=True

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
port = 1024
bufferSize = 1024
s.bind(("", port))

while clientJoinMode :   # CLIENT JOIN LOOP

print ('JOIN waiting on port: ', port)

``````
0

LVL 28

Expert Comment

ID: 39900438
Or you place them as functions outside the class definition, or add self as the very first argument. In the later case, you also should prepend self. when calling the methods. It must be explicitly written in Python. This means that then your line 47 must look like B = self.intToBytes(150)
0

Author Comment

ID: 39900477
Thanks.

How would you make my run method look like to work? It isn't obvious. - Just a conversion, and a printing of the bytes.
Would you make an external .py file with all the byte[] conversion methods?
0

LVL 28

Accepted Solution

pepr earned 500 total points
ID: 39900514
Try
``````import threading, socket

def intToBytes(self, n):
print ("_________intToBytes___________")
b = bytearray([0, 0, 0, 0])   # init
b[3] = n & 0xFF
n >>= 8
b[2] = n & 0xFF
n >>= 8
b[1] = n & 0xFF
n >>= 8
b[0] = n & 0xFF

# Return the result or as bytearray
return b

def bytesToInt(self, b, offset):
n = (b[offset+0]<<24) + (b[offset+1]<<16) + (b[offset+2]<<8) + b[offset+3]
return n

def __init__(self, numclients):
print('Server constructor: ')
self.numClients=numclients
print(self.numClients, " clients")

def run(self):

print('Server run() . . . client join:')

B = self.intToBytes(150)

print(B[0])
print(" . ")
print(B[1])
print(" . ")
print(B[2])
print(" . ")
print(B[3])
print(" . ")

clientJoinMode=True

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
port = 1024
bufferSize = 1024
s.bind(("", port))

while clientJoinMode :   # CLIENT JOIN LOOP

print ('JOIN waiting on port: ', port)

``````
It should print your 150 as four values in the bytes. Notice also the added B's in the print commands.
0

Author Closing Comment

ID: 39900590
Thanks
Why did I get visibility errors? I'd like to avoid that mistake again!
"self" must be first in the arguments declarations? Makes sense
0

Author Comment

ID: 39900637
Thanks again
One last thing,

if I convert -256 to bytes, and try to print it back at the end, I don't get -256,
I get  4294967040
it prints 255 255 255 0 as the four bytes

``````       B = self.intToBytes(-256)

print(B[0])
print(" . ")
print(B[1])
print(" . ")
print(B[2])
print(" . ")
print(B[3])
print(" . ")
print(self.bytesToInt(B,0))
``````
0

LVL 28

Expert Comment

ID: 39901777
This is because of how the negative numbers are expressed. Think about a single byte that has 8 bits. One can express 2^8 = 256 states using 8 bits. But you can decide how the states will be used for encoding numbers. The most natural (in some sense) is to use one byte for expressing numbers from zero to 255 (that is 256 states). If you need to express also negative values, then the most natural is to use half of the interval for positive numbers (here from zero to 127), and approximately the other half for the negative values (from minus one to -128). For negative values, the most significant bit of the number is set to one, and the shift to right puts one's to that bit. This is the reason why the all ones appeared. There is the agreement that the bytes and bytearray types interpret the values as small positive integers. This way the byte with all bits set to one is displayed as 255.
0

LVL 28

Expert Comment

ID: 39901804
For the self, yes. It must be explicitly declared in Python as the first argument. It could be given a different identifier but it is highly recommended to keep that convention.
0

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