Read bytes from string in Python

Question

I have a python string representing bytes read from network, I need to read successively several bytes from this string. For example I have 9 bytes in a string, I need to read 4 bytes as integer, 2 bytes as short, and 3 bytes of custom data type.

Is there a reader in python to do something like:

reader = reader(my_string) 
integer = int.from_bytes(reader.read(4), 'big')
short = int.from_bytes(reader.read(2), 'big')
custom = customType.Unpack(reader.read(3))

I thought use struct.unpack, but I don't know how to handle non-primitive types.

Any idea ?

Thanks.

• John Zwinck: Well, what is your custom, non-primitive type, exactly?
• user3442365: It's a representation of another datas, as primitive types, and only this custom type know how to unpack it. Maybe that in my example, the custom type is 1 byte and 1 short.
• John Zwinck: I don't understand. You're saying you have the code which knows how to unpack it, and you want us to tell you how to unpack it, without the code you have to unpack it?
• Antimony: The struct module is tailor made for this.
• user3442365: What I want is to extract datas from string representing bytes. I have the code to unpack everything but I need a reader to read N bytes for each types. If I send the reader to my custom class I want it to be able to read from the current position in the string. For example : I read 4 bytes, and give the reader to another class wich will read N bytes, and then give it to another class wich will read X more bytes etc ...

Answer 1

I suppose you want this:

import struct
integer, short = struct.unpack('>ih', my_string)
custom = customType.Unpack(my_string[6:9])

Or maybe this:

from StringIO import StringIO
reader = StringIO(my_string)
integer, short = struct.unpack('>ih', reader.read(6))
custom = customType.Unpack(reader.read(3))

• stanleyxu2005: Read the section struct and then import struct