Question

I have a file where some lines are metadata I can ignore and some lines are the printed results of struct.pack calls. Say that f.txt is:

key: 3175
\x00\x00\x00\x00\x00\x00\x00\x00
key: 3266
\x00\x00\x00\x00\x00\x00\x00\x00

In this case, the lines starting with “key” are the metadata and the byte strings are the values I want to extract. Also in this case, the two byte string lines were produced with struct.pack(‘d’, 0). The following code is what I would like to do:

import struct
for line in open('f.txt', 'r'):      
  # if not metadata, remove newline character and unpack
  if line[0:3] != 'key':
    val = struct.unpack('d', line[0:-1])
    appendToList(val) # do something else with val

With this, I get: “struct.error: unpack requires a string argument of length 8”.

If we modify the code slightly:

import struct
for line in open('f.txt', 'r'):      
  # if not metadata, remove newline character and unpack
  if line[0:3] != 'key': print line[:-1]

then the output is as expected:

\x00\x00\x00\x00\x00\x00\x00\x00
\x00\x00\x00\x00\x00\x00\x00\x00

If I put the byte string directly into the unpack call then, I have success:

import struct
# successful unpacking
struct.unpack('d', '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')

I have tried passing the following variations of line to unpack, all of which give the same result:

str(line)
repr(line)
b"%s" % line

Answer 1

for your string in txt file:

\x00\x00\x00\x00\x00\x00\x00\x00

which in python it acturally is:

\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00

so you should parse this string and convert it. for your sample, use following code can get what you want:

s = line.strip().split('\\x')
r = ''
for v in s:
    if len(v) > 0:
        print v
        r += struct.pack('b', int(v, 16))
val = struct.unpack('d', r)[0]
print val