THIS SHOULD BE EASY! But i've been unable to find the answer to this question.
Using python, I want to read a binary file into memory, modify the first four bytes of the file, and then write the file back.
There has to be a simple way to edit four measly bytes! right?
Why read the entire file to change four bytes at the beginning? Shouldn't this work?
with open("filename.txt", "r+b") as f:
f.write(chr(10) + chr(20) + chr(30) + chr(40))
Even if you need to read these bytes from the file to calculate the new values, you could still do:
with open("filename.txt", "r+b") as f:
fourbytes = [ord(b) for b in f.read(4)]
fourbytes[0] = fourbytes[1] # whatever, manipulate your bytes here
f.seek(0)
f.write("".join(chr(b) for b in fourbytes))
"r+b" mode, which in my understanding stands for 'read' and 'binary'?
r means "read", + means "plus write," b means "binary". (basically "+" enables both reading and writing regardless of whether you specified "r", "w", or "a" mode.) the main difference then becomes whether the file is created if necessary ("w") and where the writes occur ("a" always writes at the end)with open(filename, 'r+b') as f:
bytes = f.read(4)
newbytes = 'demo'
f.seek(0)
f.write(newbytes)
C:\junk>copy con qwerty.txt
qwertyuiop
^Z
1 file(s) copied.
C:\junk>\python27\python
Python 2.7.1 (r271:86832, Nov 27 2010, 18:30:46) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> f = open('qwerty.txt', 'r+b')
>>> f.write('asdf')
>>> f.close()
>>> open('qwerty.txt', 'rb').read()
'asdftyuiop\r\n'
>>>
Simply, but memory inefficiently,
The Python 3 way:
def binaryedit(fn):
f=open(fn,mode='rb')
fc=f.read()
f.close()
return b'rawr'+fc[4:]
The Python 2 way:
def binaryedit(fn):
f=open(fn,mode='rb')
fc=f.read()
f.close()
return 'rawr'+fc[4:]
If the files are huge, you can memory map them and edit/write just the bytes that need to change. There's barely any difference until they get over a meg or so, though.
f.read(4) to take the first 4 bytes, then f.read() at the end for the rest of it.this should help. http://www.johnny-lin.com/cdat_tips/tips_fileio/bin_array.html
import Numeric as N import array
num_lon = 144 num_lat = 73 tmpfile = "tmp.bin"
fileobj = open(tmpfile, mode='rb') binvalues = array.array('f') binvalues.read(fileobj, num_lon * num_lat)
data = N.array(binvalues, typecode=N.Float)
data = N.reshape(data, (num_lat, num_lon))
fileobj.close()
This looks a lot like HW so I will not give exact code. but here is enough information
