How do I read from binary file to an array of int16 in python

Question

I'm trying to parse aa binary file. the file contains a few data packets each one starts with a timestamp then definition of an array (rows and columns each an int32)the the array itself. I've started with trying to parse a single packet but I have a problem with reading the array:

tsSize = 8
rowSize = 4
columnSize=4
thresholdVectorSize=4
targetsCandidatesVectorSize=4
centerOfMassVectorSize=4
bytesReadUpToNow=0
with open("C:\\outputs\\out.bin", mode='rb') as file: # b is important -> binary
    fileContent = file.read()   
    TimeS = struct.unpack("Q", fileContent[bytesReadUpToNow:bytesReadUpToNow+tsSize])[0]
    bytesReadUpToNow+=tsSize
    dt =datetime.datetime.fromtimestamp(TimeS/1000.0)
    rows, columns = struct.unpack("ii", fileContent[bytesReadUpToNow:bytesReadUpToNow+rowSize+columnSize])
    bytesReadUpToNow=bytesReadUpToNow+rowSize+columnSize
    data = struct.unpack("h" * (rows*columns), fileContent[bytesReadUpToNow:rows*columns*2+bytesReadUpToNow])[0]
    print(sys.getsizeof(data))
    print(type(data))

Is there a way to predefined size of an array in python?

Answer 1

You can use "bytearray":

ba=bytearray(1000)      # zero-filled                                                 

In:  sys.getsizeof(ba)                                                       
Out: 1057

In:  ba[0:10]                                                                
Out: bytearray(b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')

In:  ba[5]=255                                                              

In:  ba[0:10]                                                               
Out: bytearray(b'\x00\x00\x00\x00\x00\xff\x00\x00\x00\x00')

Edit:

with open("data","wb") as ff: 
     ff.write(int.to_bytes(100,1,"big")) 
     ff.write(int.to_bytes(300,2,"big")) 
     ff.write(int.to_bytes(5001,4,"big")) 

ba=bytearray(7)
ba
Out: bytearray(b'\x00\x00\x00\x00\x00\x00\x00')

with open("data","rb") as ff: 
     ba[0:7]= ff.read(7) 

ba                                          
Out: bytearray(b'd\x01,\x00\x00\x13\x89')

int.from_bytes(ba[0:1],"big")
Out: 100

int.from_bytes(ba[1:3],"big")
Out: 300

int.from_bytes(ba[3:],"big") 
Out: 5001