try this

from io import BytesIO

Opening a file with open and mode read-binary already gives you a Binary I/O object.

Documentation:

The easiest way to create a binary stream is with open() with 'b' in the mode string:

f = open("myfile.jpg", "rb")

So in normal circumstances, you'd be fine just passing the file handle wherever you need to supply it. If you really want/need to get a BytesIO instance, just pass the bytes you've read from the file when creating your BytesIO instance like so:

from io import BytesIO

with open(filepath, "rb") as fh:
    buf = BytesIO(fh.read())

This has the disadvantage of loading the entire file into memory, which might be avoidable if the code you're passing the instance to is smart enough to stream the file without keeping it in memory. Note that the example uses open as a context manager that will reliably close the file, even in case of errors.

For simplicity's sake, let's consider writing instead of reading for now.

So when you use open() like say:

with open("test.dat", "wb") as f:
    f.write(b"Hello World")
    f.write(b"Hello World")
    f.write(b"Hello World")

After executing that a file called test.dat will be created, containing 3x Hello World. The data wont be kept in memory after it's written to the file (unless being kept by a name).

Now when you consider io.BytesIO() instead:

with io.BytesIO() as f:
    f.write(b"Hello World")
    f.write(b"Hello World")
    f.write(b"Hello World")

Which instead of writing the contents to a file, it's written to an in memory buffer. In other words a chunk of RAM. Essentially writing the following would be the equivalent:

buffer = b""
buffer += b"Hello World"
buffer += b"Hello World"
buffer += b"Hello World"

In relation to the example with the with statement, then at the end there would also be a del buffer.

The key difference here is optimization and performance. io.BytesIO is able to do some optimizations that makes it faster than simply concatenating all the b"Hello World" one by one.

Just to prove it here's a small benchmark:

• Concat: 1.3529 seconds
• BytesIO: 0.0090 seconds

import io
import time

begin = time.time()
buffer = b""
for i in range(0, 50000):
    buffer += b"Hello World"
end = time.time()
seconds = end - begin
print("Concat:", seconds)

begin = time.time()
buffer = io.BytesIO()
for i in range(0, 50000):
    buffer.write(b"Hello World")
end = time.time()
seconds = end - begin
print("BytesIO:", seconds)

Besides the performance gain, using BytesIO instead of concatenating has the advantage that BytesIO can be used in place of a file object. So say you have a function that expects a file object to write to. Then you can give it that in-memory buffer instead of a file.

The difference is that open("myfile.jpg", "rb") simply loads and returns the contents of myfile.jpg; whereas, BytesIO again is just a buffer containing some data.

Since BytesIO is just a buffer - if you wanted to write the contents to a file later - you'd have to do:

buffer = io.BytesIO()
# ...
with open("test.dat", "wb") as f:
    f.write(buffer.getvalue())

Also, you didn't mention a version; I'm using Python 3. Related to the examples: I'm using the with statement instead of calling f.close()