Getting number of bytes from BytesIO

Question

I want to get the exact number of bytes from a BytesIO object, the same bytes that read() returns.

The closest way to do it I found is:

readable = io.BytesIO(b'\xff\xd8')
print(sys.getsizeof(readable))

However, instead of 2 got 48 as a result.

Answer 1

io.BytesIO is a stream. Consume it to get the content's size:

>>> sys.getsizeof(io.BytesIO(b'\xff\xd8').getvalue())
35

and use len() instead of sys.getsizeof if you want to get its length:

>>> len(io.BytesIO(b'\xff\xd8').getvalue())
2

or read a specific number of bytes manually, so you'll know the size beforehand:

>>> io.BytesIO(b'\xff\xd8').read(2)
b'\xff\xd8'
>>> sys.getsizeof(io.BytesIO(b'\xff\xd8').read(2))
35
>>> len(io.BytesIO(b'\xff\xd8').read(2))
2

Since this is a stream, size can't be known before consuming the stream.

The reason you don't get 2 with your code is because you're asking the size of the BytesIO instance instead of the length of the stream's value:

>>> readable = io.BytesIO(b'\xff\xd8')
>>> readable
<_io.BytesIO object at 0x0000024A8CFC3BD0>
>>> sys.getsizeof(readable)
115
>>> len(readable.getvalue())
2