IPython/kernel/launcher.py

"""Utilities for launching kernels
"""

# Copyright (c) IPython Development Team.
# Distributed under the terms of the Modified BSD License.

import os
import sys
from subprocess import Popen, PIPE

from IPython.utils.encoding import getdefaultencoding
from IPython.utils.py3compat import cast_bytes_py2


def swallow_argv(argv, aliases=None, flags=None):
    """strip frontend-specific aliases and flags from an argument list
    
    For use primarily in frontend apps that want to pass a subset of command-line
    arguments through to a subprocess, where frontend-specific flags and aliases
    should be removed from the list.
    
    Parameters
    ----------
    
    argv : list(str)
        The starting argv, to be filtered
    aliases : container of aliases (dict, list, set, etc.)
        The frontend-specific aliases to be removed
    flags : container of flags (dict, list, set, etc.)
        The frontend-specific flags to be removed
    
    Returns
    -------
    
    argv : list(str)
        The argv list, excluding flags and aliases that have been stripped
    """
    
    if aliases is None:
        aliases = set()
    if flags is None:
        flags = set()
    
    stripped = list(argv) # copy
    
    swallow_next = False
    was_flag = False
    for a in argv:
        if a == '--':
            break
        if swallow_next:
            swallow_next = False
            # last arg was an alias, remove the next one
            # *unless* the last alias has a no-arg flag version, in which
            # case, don't swallow the next arg if it's also a flag:
            if not (was_flag and a.startswith('-')):
                stripped.remove(a)
                continue
        if a.startswith('-'):
            split = a.lstrip('-').split('=')
            name = split[0]
            # we use startswith because argparse accepts any arg to be specified
            # by any leading section, as long as it is unique,
            # so `--no-br` means `--no-browser` in the notebook, etc.
            if any(alias.startswith(name) for alias in aliases):
                stripped.remove(a)
                if len(split) == 1:
                    # alias passed with arg via space
                    swallow_next = True
                    # could have been a flag that matches an alias, e.g. `existing`
                    # in which case, we might not swallow the next arg
                    was_flag = name in flags
            elif len(split) == 1 and any(flag.startswith(name) for flag in flags):
                # strip flag, but don't swallow next, as flags don't take args
                stripped.remove(a)
    
    # return shortened list
    return stripped


def make_ipkernel_cmd(mod='IPython.kernel', executable=None, extra_arguments=[], **kw):
    """Build Popen command list for launching an IPython kernel.

    Parameters
    ----------
    mod : str, optional (default 'IPython.kernel')
        A string of an IPython module whose __main__ starts an IPython kernel

    executable : str, optional (default sys.executable)
        The Python executable to use for the kernel process.

    extra_arguments : list, optional
        A list of extra arguments to pass when executing the launch code.
    
    Returns
    -------
    
    A Popen command list
    """
    if executable is None:
        executable = sys.executable
    arguments = [ executable, '-m', mod, '-f', '{connection_file}' ]
    arguments.extend(extra_arguments)

    if sys.platform == 'win32':

        # 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 executable.endswith('pythonw.exe'):
            arguments.append('--no-stdout')
            arguments.append('--no-stderr')

    return arguments


def launch_kernel(cmd, stdin=None, stdout=None, stderr=None, env=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
    
    env = env if (env is not None) else os.environ.copy()

    encoding = getdefaultencoding(prefer_stream=False)
    
    # Spawn a kernel.
    if sys.platform == 'win32':
        # Popen on Python 2 on Windows cannot handle unicode args or cwd
        cmd = [ cast_bytes_py2(c, encoding) for c in cmd ]
        if 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()
        # Store this in an environment variable for third party kernels, but at
        # present, our own kernel expects this as a command line argument.
        env["IPY_INTERRUPT_EVENT"] = str(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=env)
        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=env)

        # 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=env)
        else:
            if ipython_kernel:
                cmd += ['--parent=1']
            proc = Popen(cmd,
                         stdin=_stdin, stdout=_stdout, stderr=_stderr, cwd=cwd, env=env)

    # 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

__all__ = [
    'swallow_argv',
    'make_ipkernel_cmd',
    'launch_kernel',
]