# Buggy code
def launch_kernel(
    cmd,
    stdin=None,
    stdout=None,
    stderr=None,
    independent=False,
    cwd=None,
    ipython_kernel=True,
    **kw,
):
    """Launches a localhost kernel, binding to the specified ports.

    Parameters
    ----------
    cmd : Popen list,
        A string of Python code that imports and executes a kernel entry point.

    stdin, stdout, stderr : optional (default None)
        Standards streams, as defined in subprocess.Popen.

    independent : bool, optional (default False)
        If set, the kernel process is guaranteed to survive if this process
        dies. If not set, an effort is made to ensure that the kernel is killed
        when this process dies. Note that in this case it is still good practice
        to kill kernels manually before exiting.

    cwd : path, optional
        The working dir of the kernel process (default: cwd of this process).

    ipython_kernel : bool, optional
        Whether the kernel is an official IPython one,
        and should get a bit of special treatment.

    Returns
    -------

    Popen instance for the kernel subprocess
    """

    # Popen will fail (sometimes with a deadlock) if stdin, stdout, and stderr
    # are invalid. Unfortunately, there is in general no way to detect whether
    # they are valid.  The following two blocks redirect them to (temporary)
    # pipes in certain important cases.

    # If this process has been backgrounded, our stdin is invalid. Since there
    # is no compelling reason for the kernel to inherit our stdin anyway, we'll
    # place this one safe and always redirect.
    redirect_in = True
    _stdin = PIPE if stdin is None else stdin

    # If this process in running on pythonw, we know that stdin, stdout, and
    # stderr are all invalid.
    redirect_out = sys.executable.endswith("pythonw.exe")
    if redirect_out:
        _stdout = PIPE if stdout is None else stdout
        _stderr = PIPE if stderr is None else stderr
    else:
        _stdout, _stderr = stdout, stderr

    # Spawn a kernel.
    if sys.platform == "win32":
        from IPython.kernel.zmq.parentpoller import ParentPollerWindows

        # Create a Win32 event for interrupting the kernel.
        interrupt_event = ParentPollerWindows.create_interrupt_event()
        if ipython_kernel:
            cmd += ["--interrupt=%i" % interrupt_event]

            # If the kernel is running on pythonw and stdout/stderr are not been
            # re-directed, it will crash when more than 4KB of data is written to
            # stdout or stderr. This is a bug that has been with Python for a very
            # long time; see http://bugs.python.org/issue706263.
            # A cleaner solution to this problem would be to pass os.devnull to
            # Popen directly. Unfortunately, that does not work.
            if cmd[0].endswith("pythonw.exe"):
                if stdout is None:
                    cmd.append("--no-stdout")
                if stderr is None:
                    cmd.append("--no-stderr")

        # Launch the kernel process.
        if independent:
            proc = Popen(
                cmd,
                creationflags=512,  # CREATE_NEW_PROCESS_GROUP
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                env=os.environ,
            )
        else:
            if ipython_kernel:
                try:
                    from _winapi import (
                        DuplicateHandle,
                        GetCurrentProcess,
                        DUPLICATE_SAME_ACCESS,
                    )
                except:
                    from _subprocess import (
                        DuplicateHandle,
                        GetCurrentProcess,
                        DUPLICATE_SAME_ACCESS,
                    )
                pid = GetCurrentProcess()
                handle = DuplicateHandle(
                    pid,
                    pid,
                    pid,
                    0,
                    True,  # Inheritable by new processes.
                    DUPLICATE_SAME_ACCESS,
                )
                cmd += ["--parent=%i" % handle]

            proc = Popen(
                cmd,
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                cwd=cwd,
                env=os.environ,
            )

        # Attach the interrupt event to the Popen objet so it can be used later.
        proc.win32_interrupt_event = interrupt_event

    else:
        if independent:
            proc = Popen(
                cmd,
                preexec_fn=lambda: os.setsid(),
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                cwd=cwd,
                env=os.environ,
            )
        else:
            if ipython_kernel:
                cmd += ["--parent=1"]
            proc = Popen(
                cmd,
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                cwd=cwd,
                env=os.environ,
            )

    # Clean up pipes created to work around Popen bug.
    if redirect_in:
        if stdin is None:
            proc.stdin.close()
    if redirect_out:
        if stdout is None:
            proc.stdout.close()
        if stderr is None:
            proc.stderr.close()

    return proc


# Traceback (UnicodeEncodeError)
C:\python\ipydevel\notebooks\åäö> ipython notebook
[NotebookApp] Using existing profile dir: u'C:\\Users\\jstenar\\.ipython\\profile_default'
[NotebookApp] Serving notebooks from local directory: C:\python\ipydevel\notebooks\åäö
[NotebookApp] The IPython Notebook is running at: http://127.0.0.1:8888/
[NotebookApp] Use Control-C to stop this server and shut down all kernels.
[NotebookApp] Using MathJax from CDN: http://cdn.mathjax.org/mathjax/latest/MathJax.js
ERROR:root:Uncaught exception POST /kernels?notebook=b6abbe73-c225-4182-8b86-4c4094480d4d (127.0.0.1)
HTTPRequest(protocol='http', host='127.0.0.1:8888', method='POST', uri='/kernels?notebook=b6abbe73-c225-4182-8b86-4c4094480d4d', version='HTTP/1.1', remote_ip='127.0.0.1', body='', headers={'Origin': 'http://127.0.0.1:8888', 'Content-Length': '0', 'Accept-Language': 'sv-SE,sv;q=0.8,en-US;q=0.6,en;q=0.4', 'Accept-Encoding': 'gzip,deflate,sdch', 'Host': '127.0.0.1:8888', 'Accept': 'application/json, text/javascript, */*; q=0.01', 'User-Agent': 'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.22 (KHTML, like Gecko) Chrome/25.0.1364.172 Safari/537.22', 'Accept-Charset': 'ISO-8859-1,utf-8;q=0.7,*;q=0.3', 'Connection': 'keep-alive', 'X-Requested-With': 'XMLHttpRequest', 'Referer': 'http://127.0.0.1:8888/b6abbe73-c225-4182-8b86-4c4094480d4d'})
Traceback (most recent call last):
File "C:\python27\lib\site-packages\tornado\web.py", line 1021, in _execute
getattr(self, self.request.method.lower())(*args, **kwargs)
File "C:\python27\lib\site-packages\tornado\web.py", line 1794, in wrapper
return method(self, *args, **kwargs)
File "C:\python27\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\frontend\html\notebook\handlers.py", line 352, in post
kernel_id = km.start_kernel(notebook_id, cwd=nbm.notebook_dir)
File "C:\python27\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\frontend\html\notebook\kernelmanager.py", line 85, in start_kernel
kernel_id = super(MappingKernelManager, self).start_kernel(**kwargs)
File "C:\python27\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\kernel\multikernelmanager.py", line 98, in start_kernel
km.start_kernel(**kwargs)
File "C:\python27\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\kernel\kernelmanager.py", line 950, in start_kernel
**kw)
File "C:\python27\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\kernel\kernelmanager.py", line 919, in _launch_kernel
return launch_kernel(kernel_cmd, **kw)
File "C:\python27\lib\site-packages\ipython-1.0.dev-py2.7.egg\IPython\kernel\launcher.py", line 227, in launch_kernel
stdin=_stdin, stdout=_stdout, stderr=_stderr, cwd=cwd, env=os.environ)
File "C:\python27\lib\subprocess.py", line 679, in __init__
errread, errwrite)
File "C:\python27\lib\subprocess.py", line 893, in _execute_child
startupinfo)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 29-31: ordinal not in range(128)
ERROR:root:500 POST /kernels?notebook=b6abbe73-c225-4182-8b86-4c4094480d4d (127.0.0.1) 13.00ms

# Fixed code
def launch_kernel(
    cmd,
    stdin=None,
    stdout=None,
    stderr=None,
    independent=False,
    cwd=None,
    ipython_kernel=True,
    **kw,
):
    """Launches a localhost kernel, binding to the specified ports.

    Parameters
    ----------
    cmd : Popen list,
        A string of Python code that imports and executes a kernel entry point.

    stdin, stdout, stderr : optional (default None)
        Standards streams, as defined in subprocess.Popen.

    independent : bool, optional (default False)
        If set, the kernel process is guaranteed to survive if this process
        dies. If not set, an effort is made to ensure that the kernel is killed
        when this process dies. Note that in this case it is still good practice
        to kill kernels manually before exiting.

    cwd : path, optional
        The working dir of the kernel process (default: cwd of this process).

    ipython_kernel : bool, optional
        Whether the kernel is an official IPython one,
        and should get a bit of special treatment.

    Returns
    -------

    Popen instance for the kernel subprocess
    """

    # Popen will fail (sometimes with a deadlock) if stdin, stdout, and stderr
    # are invalid. Unfortunately, there is in general no way to detect whether
    # they are valid.  The following two blocks redirect them to (temporary)
    # pipes in certain important cases.

    # If this process has been backgrounded, our stdin is invalid. Since there
    # is no compelling reason for the kernel to inherit our stdin anyway, we'll
    # place this one safe and always redirect.
    redirect_in = True
    _stdin = PIPE if stdin is None else stdin

    # If this process in running on pythonw, we know that stdin, stdout, and
    # stderr are all invalid.
    redirect_out = sys.executable.endswith("pythonw.exe")
    if redirect_out:
        _stdout = PIPE if stdout is None else stdout
        _stderr = PIPE if stderr is None else stderr
    else:
        _stdout, _stderr = stdout, stderr

    # Spawn a kernel.
    if sys.platform == "win32":
        if cwd:
            # Popen on Python 2 on Windows cannot handle unicode cwd.
            cwd = cast_bytes_py2(cwd, sys.getfilesystemencoding() or "ascii")

        from IPython.kernel.zmq.parentpoller import ParentPollerWindows

        # Create a Win32 event for interrupting the kernel.
        interrupt_event = ParentPollerWindows.create_interrupt_event()
        if ipython_kernel:
            cmd += ["--interrupt=%i" % interrupt_event]

            # If the kernel is running on pythonw and stdout/stderr are not been
            # re-directed, it will crash when more than 4KB of data is written to
            # stdout or stderr. This is a bug that has been with Python for a very
            # long time; see http://bugs.python.org/issue706263.
            # A cleaner solution to this problem would be to pass os.devnull to
            # Popen directly. Unfortunately, that does not work.
            if cmd[0].endswith("pythonw.exe"):
                if stdout is None:
                    cmd.append("--no-stdout")
                if stderr is None:
                    cmd.append("--no-stderr")

        # Launch the kernel process.
        if independent:
            proc = Popen(
                cmd,
                creationflags=512,  # CREATE_NEW_PROCESS_GROUP
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                env=os.environ,
            )
        else:
            if ipython_kernel:
                try:
                    from _winapi import (
                        DuplicateHandle,
                        GetCurrentProcess,
                        DUPLICATE_SAME_ACCESS,
                    )
                except:
                    from _subprocess import (
                        DuplicateHandle,
                        GetCurrentProcess,
                        DUPLICATE_SAME_ACCESS,
                    )
                pid = GetCurrentProcess()
                handle = DuplicateHandle(
                    pid,
                    pid,
                    pid,
                    0,
                    True,  # Inheritable by new processes.
                    DUPLICATE_SAME_ACCESS,
                )
                cmd += ["--parent=%i" % handle]

            proc = Popen(
                cmd,
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                cwd=cwd,
                env=os.environ,
            )

        # Attach the interrupt event to the Popen objet so it can be used later.
        proc.win32_interrupt_event = interrupt_event

    else:
        if independent:
            proc = Popen(
                cmd,
                preexec_fn=lambda: os.setsid(),
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                cwd=cwd,
                env=os.environ,
            )
        else:
            if ipython_kernel:
                cmd += ["--parent=1"]
            proc = Popen(
                cmd,
                stdin=_stdin,
                stdout=_stdout,
                stderr=_stderr,
                cwd=cwd,
                env=os.environ,
            )

    # Clean up pipes created to work around Popen bug.
    if redirect_in:
        if stdin is None:
            proc.stdin.close()
    if redirect_out:
        if stdout is None:
            proc.stdout.close()
        if stderr is None:
            proc.stderr.close()

    return proc