How the write(), read() and getvalue() methods of Python io.BytesIO work?

The issue is that you are positioned at the end of the stream. Think of the position like a cursor. Once you have written b' world', your cursor is at the end of the stream. When you try to .read(), you are reading everything after the position of the cursor - which is nothing, so you get the empty bytestring.

To navigate around the stream you can use the .seek method:

>>> import io>>> in_memory = io.BytesIO(b'hello', )>>> in_memory.write(b' world')>>> in_memory.seek(0)  # go to the start of the stream>>> print(in_memory.read())b' world'

Note that, just like a filestream in write ('w') mode, the initial bytes b'hello' have been overwritten by your writing of b' world'.

.getvalue() just returns the entire contents of the stream regardless of current position.

this is a memory stream but still a stream. The position is stored, so like any other stream if you try to read after having written, you have to re-position:

import ioin_memory = io.BytesIO(b'hello')in_memory.seek(0,2)   # seek to end, else we overwritein_memory.write(b' world')in_memory.seek(0)    # seek to startprint( in_memory.read() )

prints:

b'hello world'

while in_memory.getvalue() doesn't need the final seek(0) as it returns the contents of the stream from position 0.

BytesIO does behave like a file, only one that you can both read and write. The confusing part, maybe, is that the reading and writing position is the same one. So first you do:

in_memory = io.BytesIO(b'hello')

This gives you a bytes buffer in in_memory with the contents b'hello' and with the read/write position at the beginning (before the first b'h'). When you do:

in_memory.write(b' world')

You are effectively overwriting b'hello' with b' world' (and actually getting one byte further), and now you have the position at the end (after the last b'd'). So when you do:

print( in_memory.read() )

You see nothing because there is nothing to read after the current position. You can, however, use seek to move the position, so if you do

import ioin_memory = io.BytesIO(b'hello')in_memory.write(b' world')in_memory.seek(0)print( in_memory.read() )

You get:

b' world'

Note that you do not see the initial b'hello' because it was overwritten. If you want to write after the initial content, you can first seek to the end:

import ioin_memory = io.BytesIO(b'hello')in_memory.seek(0, 2)in_memory.write(b' world')in_memory.seek(0)print( in_memory.read() )

Output:

b'hello world'

EDIT: About getvalue, as pointed out by other answers, it gives you the full internal buffer, independently of the current position. This operation is obviously not available for files.