file io - python overwrite bytes from offset1 to offset2 -

i've seen multiple tutorials in python, played around bit python file write(), tell(), seek() functions , os.write(), lseek() functions.

but still don't how can following: have: in file know start_offset , end_offset bytes. , need replace bytes start_offset end_offset different set of bytes. how do this??

ftell() returns me start_offset , regex + ftell() returns me end offset have bytes overwrite original ones in file.

but write() takes string write. how overwrite start_pos end_pos??

appreciate pointers/suggestions

despite shouted denial, can use mmap here.

if @ mmap constructor in docs, takes parameters offset , length. on platforms, both have multiple of pagesize or similar value, that's not hard.

so:

try:     pagesize = mmap.pagesize except nameerror:     pagesize = mmap.allocation_granularity  def overwrite(fileobj, start, end, newbytes):     startoffset, startremainder = divmod(start, pagesize)     offset = startoffset * pagesize     endoffset, endremainder = divmod(end, pagesize)     length = (endoffset + 1) * pagesize - offset     map = mmap.mmap(fileobj.fileno(), offset=offset, length=length,                     access=mmap.access_write)     map[startremainder:startremainder+end-start] = newbytes 

this has nice advantage if len(newbytes) != end - start you'll nice exception mmap, instead of overwriting more or less of file intended , leaving things corrupted.

but it's simpler use seek, in martijn pieters's answer. here's same function seek:

def overwrite(fileobj, start, end, newbytes):     if len(newbytes) != end - start:         raise valueerror('overwrite cannot expand or contract file')     fileobj.seek(start)     fileobj.write(newbytes)     

still, it's worth knowing mmap can don't dismiss in useful cases in future.

(also, versions of python, on platforms, can have files big seek in. example, linux /proc/*/map sparse file size of 1<<64, on distros, python can't find fseeko , therefore can't seek farther 1<<63. so, knowing alternative ways this—os.lseek, mmap, etc.—may work around problem 1 day.)