The documentation for `IOBase.readlines` states that the `hint` optional argument should be used like so (https://docs.python.org/3/library/io.html#io.IOBase.readlines):

> Read and return a list of lines from the stream. hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint.

So in the case where `hint` is null, I would expect `readlines` to return an empty list.

However, this is neither the case for `io.BytesIO` nor for `io.FileIO`:

```
import io

io.BytesIO(b"abc\ndef\nghi\n").readlines(0) 
# this evaluates to [b"abc\n", b"def\n", b"ghi\n"]
```

If this is the intended behaviour, the documentation should be updated, otherwise the code from the `io` module should be fixed.

Thanks for the report!

I agree docs could be improved. Note this method lives in `IOBase` so this behavior also applies to the file pointer returned from `open()` builtin.

I've put up the PR here: https://github.com/python/cpython/pull/27061/files

New changeset 3b5b99da4b256a31933112f4a2385386149c19e1 by andrei kulakov in branch 'main':
bpo-43126: Expand docs on io.IOBase.readlines() method (#27061)
https://github.com/python/cpython/commit/3b5b99da4b256a31933112f4a2385386149c19e1

New changeset 9b369c952cbefb064dda6cb781e66cc1b793fffa by Miss Islington (bot) in branch '3.10':
bpo-43126: Expand docs on io.IOBase.readlines() method (GH-27061) (GH-27113)
https://github.com/python/cpython/commit/9b369c952cbefb064dda6cb781e66cc1b793fffa

New changeset a3d20bfee38c71df88d69064d47fe98a1d59e624 by Miss Islington (bot) in branch '3.9':
bpo-43126: Expand docs on io.IOBase.readlines() method (GH-27061) (#27114)
https://github.com/python/cpython/commit/a3d20bfee38c71df88d69064d47fe98a1d59e624

Thanks, Andrei! ✨ 🍰 ✨