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# 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](pycdlib-api.html#PyCdlib-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](https://docs.python.org/3/library/io.html#binary-i-o) 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](example-creating-new-basic-iso.md), 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](pycdlib-api.html#PyCdlib-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.
---
<div style="width: 100%; display: table;">
<div style="display: table-row;">
<div style="width: 33%; display: table-cell; text-align: left;">
<a href="example-walking-iso-filesystem.html"><-- Example: Walking the ISO filesystem</a>
</div>
<div style="width: 33%; display: table-cell; text-align: center;">
<a href="https://clalancette.github.io/pycdlib/">Top</a>
</div>
<div style="width: 33%; display: table-cell; text-align: right;">
<a href="design.html">Design --></a>
</div>
</div>
|
