Python 3 Building an array of bytes

Question

I need to build a tcp frame with raw binary data, but all examples and tutorials I've found talking about bytes always involve conversion from a string, and that's not what I need.

In short, I need to build just an array of bytes:

0xA2 0x01 0x02 0x03 0x04

Please note that I come from C/C++ world.

I've tried this:

frame = b""
frame += bytes( int('0xA2',16) )
frame += bytes( int('0x01',16) )
frame += bytes( int('0x02',16) )
frame += bytes( int('0x03',16) )
frame += bytes( int('0x04',16) )

Then, throw this frame variable to send method of socket, but not working as expected as frame doesn't contain what I want...

I know this is a very basic question about Python, so if you could point me in the right direction...

Solution 1

Use a bytearray:

>>> frame = bytearray()
>>> frame.append(0xA2)
>>> frame.append(0x01)
>>> frame.append(0x02)
>>> frame.append(0x03)
>>> frame.append(0x04)
>>> frame
bytearray(b'\xa2\x01\x02\x03\x04')

or, using your code but fixing the errors:

frame = b""
frame += b'\xA2' 
frame += b'\x01' 
frame += b'\x02' 
frame += b'\x03'
frame += b'\x04'

Solution 2

what about simply constructing your frame from a standard list ?

frame = bytes([0xA2,0x01,0x02,0x03,0x04])

the bytes() constructor can build a byte frame from an iterable containing int values. an iterable is anything which implements the iterator protocol: an list, an iterator, an iterable object like what is returned by range()...

Solution 3

frame = b'\xa2\x01\x02\x03\x04'

wasn't mentionned till now...

Solution 4

agf's bytearray solution is workable, but if you find yourself needing to build up more complicated packets using datatypes other than bytes, you can try struct.pack(). http://docs.python.org/release/3.1.3/library/struct.html

View more solutions

Comments

[Ethan Furman]

The problem with the above code is that bytes behaves very differently if given an integer instead of a list. You should be using frame += bytes( [ int('0xa2',16) ] ).

[Robert Siemer]

I know that you want(ed) to do this dynamically, but what is completely unclear is this: where do the input bytes come from? You read hex notation from a text file? You write each byte down in the source code? You have them in an bytearray which you got from reading binary from stdin?

[Pherrymason]

Thank you, but I'm more interested in learning how to do it ;)

[Venec]

Well, scapy is just a nice tool to have for general network and packet projects. I think what youre looking for is something like this: > >>> from struct import * > >>>>>> packet = pack('Bbbbb',162,1,2,3,4,) > >>> packet > '\xa2\x01\x02\x03\x04'

[Venec]

the second comment is just python, pack the data packet = pack('Bbbbb',162,1,2,3,4,) then you have your frame. Python has no arrays, it only has Lists. for more info refer to the documentation about structs docs.python.org/library/struct.html

[agf]

@Venec Python 2.6+ does in fact have a bytearray type.

[Pherrymason]

but I need to build the frame dynamically

[Pherrymason]

you added the tag bytearray to the question when the answer may not use it as a solution.

[agf]

@clinisbut It's still related -- in the general sense, you're using a byte array (the concept) even if you're not using the bytearray type. Either of the two methods in my answer should solve your problem.

[Adrien Plisson]

this syntax for building an array of bytes works the same as for a string type: you can use + or other similar operators to build your byte array dynamically.

[Adrien Plisson]

you can replace bytearraywith bytes and get roughly the same result. a bytearray is a mutable type though, so it may be more useful than a bytes for constructing your frames.

[Lennart Regebro]

Scapy is brutal overkill for this.

[Adrien Plisson]

@agf: that's what you do. i was just telling clinisbut the difference between bytearrayand bytes. but now i may need to explain the difference between a mutable and an immutable type.