You can do this the same way that (effectively) memmove
works: seek back and forth between the source range and the destination range:
count = (size+chunksize-1) // chunk size
for chunk in range(count):
f.seek(start + chunk * chunksize + deleted_line_size, 0)
buf = f.read(chunksize)
f.seek(start + chunk * chunksize, 0)
f.write(buf)
Using a temporary file and shutil
makes it a lot simpler—and, despite what you're expect, it may actually be faster. (There's twice as much writing, but a whole lot less seeking, and mostly block-aligned writing.) For example:
with tempfile.TemporaryFile('w') as ftemp:
shutil.copyfileobj(ftemp, f)
ftemp.seek(0, 0)
f.seek(start, 0)
shutil.copyfileobj(f, ftemp)
f.truncate()
However, if your files are big enough to fit in your virtual memory space (which they probably are in 64-bit land, but may not be in 32-bit land), it may be simpler to just mmap
the file and let the OS/libc take care of the work:
m = mmap.mmap(f.fileno(), access=mmap.ACCESS_WRITE)
m[start:end-deleted_line_size] = m[start+deleted_line_size:end]
m.close()
f.seek(end-deleted_line_size)
f.truncate()