Example: Reading a large file in chunks
It may be useful in some applications to be able to read a file from an ISO a bit at a time and do some processing on it. PyCdlib provides the context manager open_file_from_iso API to allow opening a file and reading in parts of it. Here’s the complete code for this example:
try:
from cStringIO import StringIO as BytesIO
except ImportError:
from io import BytesIO
import pycdlib
iso = pycdlib.PyCdlib()
iso.new()
foostr = b'foofoo\n'
iso.add_fp(BytesIO(foostr), len(foostr), '/FOO.;1')
with iso.open_file_from_iso(iso_path='/FOO.;1') as infp:
all1 = infp.read()
infp.seek(0)
first = infp.read(3)
second = infp.read()
iso.close()
Let’s take a closer look at the code.
try:
from cStringIO import StringIO as BytesIO
except ImportError:
from io import BytesIO
import pycdlib
As we’ve seen before, import pycdlib. We also import the BytesIO module so we can use a python string as a file-like object.
iso = pycdlib.PyCdlib()
iso.new()
foostr = b'foofoo\n'
iso.add_fp(BytesIO(foostr), len(foostr), '/FOO.;1')
This code creates a new ISO, adds a single file to it, and writes it out. This is very similar to the code in Creating a new, basic ISO, so see that example for more information.
with iso.open_file_from_iso(iso_path='/FOO.;1') as infp:
Here we use the open_file_from_iso API to get a context manager to the file that we created; this will be used in the rest of the explanations below.
all = infp.read()
The first read call reads in all of the data in the file, so at the end of the call the “all” variable will contain foofoo\n.
infp.seek(0)
The ‘seek’ call then resets the file pointer back to the beginning of the file.
first = infp.read(3)
If the read API is passed a number, it will read and return that many bytes. In this case, the ‘first’ variable will end up containing foo.
second = infp.read()
And now we read the rest of the data, so the ‘second’ variable will end up containing foo\n.
iso.close()
As is the case in other examples, we close out the PyCdlib object.