python pack 4 byte integer with bytes in bytearray struct.pack

Question

I am attempting to pack the contents of a python bytearray into a 4byte signed integer using struct.pack. Unfortunately, pack wants a string, so after some googling I figured I needed to decode my bytearray to a string. I figured ascii meant since because an ascii character is a byte long. Unfortunately, ascii did not want to support my values > 127, so I thought I would use replace...

but when I do this decode returns an object of type unicode and now each of my bytes is a 4 character string...

This just seems a little ridiculous, im missing something obvious (ps I have been using python for about two weeks)

here is what I am trying to do...

    val = long(-5) 
s = bytearray(pack("<i", val)) 

s.pop() # pop off msb

# write it out the way we want to then read it in the way the code does
fout = open("test.bat", "wb")
fout.write(s) 
fout.close()

fin = open("test.bat", "rb") 

inBytes = bytearray(fin.read(3))
# extend sign bit
if (inBytes[2] & 0x80):
    inBytes.append(0xff)
else:
    inBytes.append(0x00)

nb = inBytes.decode('ascii', 'replace')
# ERROR:root:after decode, len: 4 type: <type 'unicode'>
logging.error("after decode, len: {0} type: {1}".format(len(nb), type(nb)))

# struct.error: unpack requires a string argument of length 4 
inInt32 = unpack('<i', inBytes.decode('ascii', 'replace'))[0]

fin.close()

Santa · Accepted Answer · 2011-05-20 19:01:07Z

1

All you need is to cast inBytes back to str:

>>> inint = struct.unpack('<i', str(inBytes))
>>> inint
(-5,)

answered May 20, 2011 at 19:01

Santa

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1 Comment

John Machin Over a year ago

-1 For Python 2.x, best to stick to str; str -> bytearray -> str is confusing. Also there's no such concept as "cast" in Python.

John Machin · Accepted Answer · 2011-05-20 22:47:24Z

When you read from a file in binary mode, you get an object that can be used immediately with struct.unpack.

Creating the input data:

>>> import struct
>>> f = open('foo.bin', 'wb')
>>> f.write(struct.pack('<i', -5)[:3])
3
>>> f.close()

Python 2.x .. it's a str object.

>>> f = open('foo.bin', 'rb')
>>> raw = f.read()
>>> f.close()
>>> print "received", type(raw), repr(raw)
received <type 'str'> '\xfb\xff\xff'
>>> if raw[2] >= '\x80':
...     raw += '\xff'
... else:
...     raw += '\x00'
...
>>> print "extended", type(raw), repr(raw)
extended <type 'str'> '\xfb\xff\xff\xff'
>>> number = struct.unpack('<i', raw)[0]
>>> print "number", number
number -5
>>>

Python 3.x ... it's a bytes object.

>>> f = open('foo.bin', 'rb')
>>> raw = f.read()
>>> f.close()
>>> print("received", type(raw), repr(raw))
received <class 'bytes'> b'\xfb\xff\xff'
>>> if raw[2] & 0x80:
...     raw += b'\xff'
... else:
...     raw += b'\x00'
...
>>> print("extended", type(raw), repr(raw))
extended <class 'bytes'> b'\xfb\xff\xff\xff'
>>> number = struct.unpack('<i', raw)[0]
>>> print("number", number)
number -5
>>>

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