PyShell.py's PyShell object has the readline method, at line 1080 in the most recent code. It's meant for use with and without a subprocess. (See also Issue14254)

The IDLEfork project long ago created the subprocess and the RPC plumbing for the subprocess interaction with the *existing* PyShell(OutputWindow) object as stdin/stdout/stderr. This RPC, found in rpc.py, is used in run.py and PyShell's ModifiedInterpreter. The start_subprocess method connects the stdin/stdout/stderr from the IDLE front-end to the subprocess in run.py. The stdout/stderr objects are the PyShell(OutputWindow) object wrapped by PseudoFile. Stdin is not wrapped, but it should be.

I know that this issue deals with stdin specifically, but I hope that the additional stdout/stderr information places the problem into a greater context.