Python struct.error: unpack requires a string argument of length 2 - python-2.7

Question

I have written some data using C++ in byte format. I am now trying to read that data again using Python, but I run into an error;
Traceback (most recent call last):
File "binary-reader.py", line 61, in <module>
interaction_types.append(struct.unpack('<H',fp.read(2))[0]);
struct.error: unpack requires a string argument of length 2
I don't really understand since it looks like I am giving a string of length 2, right? Furthermore, I do the same thing at line 32
There is another question like mine but it is without an answer is targeted for Python 3.
Here is my code
import sys
import struct
import os
print "Arguments : "
print str(sys.argv)
#N = #isects
# 2 2 3*4 2 3*4*N 4N 4N 3*4N 2N 2N
#imageX,imageY,throughput,#isects,isect_positions,primitive_ids,shape_ids,spectra,interaction_types,light_ids
file_path = str(sys.argv[1]);
byte_count = 0;
line_number = 1;
fp = open(file_path, "rb");
output = open('output.txt',"w");
file_size = os.path.getsize(file_path)
print "(input) file size = " + str(file_size);
while byte_count < file_size:
print "Line number = " + str(line_number)
print "Current byte count = " + str(byte_count)
# Do stuff with byte.
x = struct.unpack('<H', fp.read(2))[0]
y = struct.unpack('<H', fp.read(2))[0]
throughputOne = struct.unpack('<f', fp.read(4))[0]
throughputTwo = struct.unpack('<f', fp.read(4))[0]
throughputThree = struct.unpack('<f', fp.read(4))[0]
nrIsects = struct.unpack('<H',fp.read(2))[0]
# print "x = " + str(x)
# print "y = " + str(y)
# print "throughputOne = " + str(throughputOne)
# print "throughputTwo = " + str(throughputTwo)
# print "throughputThree = " + str(throughputThree)
print "nrIsects = " + str(nrIsects)
isect_positions = []
for i in range(nrIsects*3):
value = struct.unpack('<f',fp.read(4))[0]
isect_positions.append(value);
primitive_ids = []
for i in range(nrIsects):
value = struct.unpack('<I',fp.read(4))[0]
primitive_ids.append(value);
shape_ids = []
for i in range(nrIsects):
shape_ids.append(struct.unpack('<I',fp.read(4))[0]);
spectra = []
for i in range(nrIsects*3):
spectra.append(struct.unpack('<f',fp.read(4))[0]);
interaction_types = []
for i in range(nrIsects):
interaction_types.append(struct.unpack('<H',fp.read(2))[0]);
light_ids = []
for i in range(nrIsects):
light_ids.append(struct.unpack('<H',fp.read(2))[0]);
output_vars = [x,y,throughputOne,throughputTwo,throughputThree,nrIsects]
line_string = ""
for i in range(len(output_vars)):
output.write(str(output_vars[i]))
line_string += str(output_vars[i])
if i is not len(output_vars) - 1:
output.write(',')
line_string += ','
print line_string
#Update counters
byte_count += 18 + 36*nrIsects
line_number+=1
# raw_input('Press any key to continue.');
# print byte
And here is a link to a input file to use. You can run the code by passing a commandline argument specifying the path of the binary file. I have also written the code in ASCII, which reads
0,0,[0.127076,0.127076,0.127076],1,{[0.144978,-0.294863,2.991749]},{3917},{3916},{[1.375603,1.375603,1.375603]},{5},{0}
https://www.dropbox.com/s/tu1anqo5k0ygtd6/writetest.bin
EDIT: The layout of my file can be found as a comment in the code

Answer 1

50 bytes have already been read before the fp.read(2) that raises the error. Thus, fp.read(2) returns an empty string, and struct.unpack raises an exception:
In [83]: 2+2+4+4+4+2+12+4+4+12
Out[83]: 50
x = struct.unpack('<H', fp.read(2))[0] # 2 bytes read
y = struct.unpack('<H', fp.read(2))[0] # 2 bytes
throughputOne = struct.unpack('<f', fp.read(4))[0] # 4 bytes
throughputTwo = struct.unpack('<f', fp.read(4))[0] # 4 bytes
throughputThree = struct.unpack('<f', fp.read(4))[0] # 4 bytes
nrIsects = struct.unpack('<H',fp.read(2))[0] # 2 bytes
print "nrIsects = " + str(nrIsects)
isect_positions = []
for i in range(nrIsects*3):
value = struct.unpack('<f',fp.read(4))[0] # 12 bytes
isect_positions.append(value)
primitive_ids = []
for i in range(nrIsects):
value = struct.unpack('<I',fp.read(4))[0] # 4 bytes
primitive_ids.append(value)
shape_ids = []
for i in range(nrIsects):
shape_ids.append(struct.unpack('<I',fp.read(4))[0]) # 4 bytes
spectra = []
for i in range(nrIsects*3):
spectra.append(struct.unpack('<f',fp.read(4))[0]) # 12 bytes
interaction_types = []
for i in range(nrIsects):
interaction_types.append(struct.unpack('<H', fp.read(2))[0]) # error!