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From: Laura Creighton <lac@openend.se>
Newsgroups: comp.lang.python
Subject: Re: Problems using struct pack/unpack in files, and reading them.
Date: Fri, 13 Nov 2015 21:15:23 +0100
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Comments: In-reply-to kent nyberg <kent@z-sverige.nu> message dated "Fri, 13 Nov 2015 14:20:45 -0500."
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In a message of Fri, 13 Nov 2015 14:20:45 -0500, kent nyberg writes:
>Hi there,
>Im deeply sorry for yet another question to this list.  I have come acros=
s a problem to which google seems not =

>to eager to supply the anwser.
>
>The problem is the following.
>First I do this:
>
>def setup_drive():
>    test =3D pack('>HH',  0b1000000000000000, 0b1000000100000001)
>    file =3D open('drive.bin', 'wb')
>    for x in range(640000):
>        file.write(test)
>    file.close()
>
>
>I have a pack, which I fill the file with. The data is not of interrest r=
ight now.
>
>This all works great.
>Then I do this:
>
>
>def LoadCommandAndReact(place_to_read):
>    global RegisterAX
>    =

>    tmp =3D place_to_read.read()[RegisterAX:calcsize('HH')]
>    klar =3D unpack('>HH', tmp)
>
>    if place_to_read.closed:
>       print("Drive error. Drive closed.")
>    else:
>       pass
>
>    if checkfirstbit(klar[RegisterAX]):
>       print("First bit is set. Move Cmd.")
>       if (klar[0] & 0b0111111111111111):
>          print("Cmd ERROR: Multiple commands is set.")
>          pass
>       else:
>          #Here something to do with mv command.
>          print("Adress to read to, from the offset of the command")
>          print(klar[RegisterAX+1])  #Variable is always Offset+1        =
  =

>    else:
>       print("First bit is not set.")
>    #Change RegisterAX offset+2bytes, =

>    RegisterAX=3D+2   #We read two bytes per cycle. Becaus command is fir=
st, and variables are second.    =

>
>
>
>
>This all works If I run the LoadCommand..  only once.
>But when I  run it for the second time,  it complains:
>
>Traceback (most recent call last):
>  File "core.py", line 98, in <module>
>    LoadCommandAndReact(readfile)
>  File "core.py", line 30, in LoadCommandAndReact
>    klar =3D unpack('>HH', tmp)
>struct.error: unpack requires a string argument of length 4
>
>
>
>Im having trouble understanding the error.   unpack requires a string arg=
ument of length 4.
>I have filled the file with data that should be readable, and is readable=
 by the function =

>when I run it for the first time. And since the file is filled with dupli=
cates; should it not be readable the second
>time aswell? I change the RegisterAX =3D+2. The amount I add should not m=
atter, Its just my "offset" of where to read in the file.
>The problem seems that the second time, unpack doesnt get the size of '>H=
H' to read. But surely =

>.read()[RegisterAX:calcsize('HH')]  should assure that its fed with corre=
ct data?
>
>I have filled the file with data of size '>HH' and first command reads th=
at.  So next read should read the next
>duplicate of that data that has been written to the file?
>
>
>I understand its hard to read, and a long email. So well, its just a cry =
out in the dark evening.  If some one can
>share some light.. I would be much happy about it.
>
>/Kent.
>-- =

>https://mail.python.org/mailman/listinfo/python-list

struct wants to know exactly how many of those things you have.
You only told it about one of them .... i.e.  If you want to unpack 999 =

elements from binary data

data =3D struct.unpack("999H", B)

Laura