4
\$\begingroup\$

I recently needed to run some command line applications from Python. Whilst this is fairly simple with subprocess.Popen. I wanted to be able to properly pipe the output.

When running the programs from Python I want three things:

  1. The option to have a low memory footprint.

    This is possible with Popen by passing some, not all, file objects to stdout or stderr.

  2. To be able to pass data into multiple outputs. If I want to pass data from Popen.stdout to sys.stdout and my_file I should be able to.

  3. To output live. When using Popen.communicate or stdout=None it is not possible to log the output and also display the output.

All of these are fairly easy to implement by using Popen.stdout with threads. Even though the core of the code is fairly simple. Having to manually create and manage threads each time I use Popen is not desirable. Further more, given that the code is a convenience wrapper, I decided to try and make the usage as simple as possible.

"""
Teetime - adding tee like functionality to Popen.

Conveniently allow Popen to pass data to any number of sinks.
Sinks can be shared between both stdout and std err and be handled correctly.

.. code-block::

    import sys
    import teetime

    with open('log.txt', 'wb') as f:
        teetime.popen_call(
            ['python', 'test.py'],
            stdout=(sys.stdout.buffered, f),
            stderr=(sys.stderr.buffered, f),
        )

The :code:`popen_call` function is a convenience over :code:`Sinks`.
If you need to interact with the process when it's live then you can
modify the following to suite your needs.

.. code-block::

    import sys
    import teetime

    with open('log.txt', 'wb') as f:
        sinks = Sinks(
            (sys.stdout.buffered, f),
            (sys.stderr.buffered, f),
        )
        process = sinks.popen(['python', 'test.py'])
        with self.make_threads(process) as threads:
            threads.join()
        self.reset_head()
"""

from __future__ import annotations

from typing import (
    Any, Callable, Iterator, List, Optional, Sequence, Tuple, Type
)
import collections
import io
import queue
import subprocess
import threading

QUEUE_SENTINEL = object()
FILE_SENTINEL = b''

TSink = Callable[[bytes], None]
_Threads = collections.namedtuple('Threads', 'out, err, both, queue')
_Sinks = collections.namedtuple('Sinks', 'out, err, both')


class Threads(_Threads):
    """
    Thread manager and interface.

    Because we need to listen to stdout and stderr at the same time we
    need to put the listeners in threads. This is so we can handle
    changes when they happen. This comes with two benifits:

    1.  We don't have to store all the print information in memory.
    2.  We can display changes as soon as they happen.

    To allow stdout and stderr to merge correctly we have to use an
    atomic queue. :code:`queue.Queue` is one such example. If we have
    no need of combining output as there are no both sinks then the
    queue and associated thread won't be created.

    This object holds the three threads and the message queue.
    """

    out: threading.Thread
    err: threading.Thread
    both: threading.Thread
    queue: queue.Queue

    def __new__(
        cls,
        process: subprocess.Popen,
        sinks: Sinks,
        flush: bool = True,
    ) -> Threads:
        """
        Create Threads from an active process and sinks.

        This creates the required threads to handle the output and sinks.
        These threads are created as daemons and started on creation.
        This also creates the message queue that merges both stdout and
        stderr when needed.

        :param process: Process to tee output from.
        :param sinks: Places to tee output to.
        :param flush: Whether to force flush flushable sinks.
        :return: A thread manager for the process and sinks.
        """
        o_thread = None
        e_thread = None
        b_thread = None
        _queue: Optional[queue.Queue] = None
        out, err, both = sinks.as_callbacks(flush=flush)

        if both:
            _queue = queue.Queue()
            b_thread = cls._make_thread(
                target=cls._handle_sinks,
                args=(iter(_queue.get, QUEUE_SENTINEL), both)
            )
            if out is None:
                raise ValueError('both is defined, but out is None')
            if err is None:
                raise ValueError('both is defined, but err is None')
            out += (_queue.put,)
            err += (_queue.put,)

        if out:
            o_thread = cls._make_thread(
                target=cls._handle_sinks,
                args=(iter(process.stdout.readline, FILE_SENTINEL), out)
            )

        if err:
            e_thread = cls._make_thread(
                target=cls._handle_sinks,
                args=(iter(process.stderr.readline, FILE_SENTINEL), err)
            )

        return _Threads.__new__(
            cls,
            o_thread,
            e_thread,
            b_thread,
            _queue
        )

    def __enter__(self) -> Threads:
        """Context manager pattern."""
        return self

    def __exit__(self, _1: Any, _2: Any, _3: Any) -> None:
        """Context manager pattern."""
        self.end_queue()

    @staticmethod
    def _make_thread(*args: Any, **kwargs: Any) -> threading.Thread:
        """Make thread."""
        thread = threading.Thread(*args, **kwargs)
        thread.daemon = True
        thread.start()
        return thread

    @staticmethod
    def _handle_sinks(_input: Iterator[bytes], sinks: List[TSink]) -> None:
        """Threaded function to pass data between source and sink."""
        for segment in _input:
            for sink in sinks:
                sink(segment)

    def join(
        self,
        out: bool = True,
        err: bool = True,
        both: bool = False,
    ) -> None:
        """Conveniently join threads."""
        if out and self.out is not None:
            self.out.join()
        if err and self.err is not None:
            self.err.join()
        if both and self.both is not None:
            self.both.join()

    def end_queue(self) -> None:
        """Send exit signal to the both thread."""
        if self.queue is not None:
            self.queue.put(QUEUE_SENTINEL)


def _flushed_write(sink: Any) -> TSink:
    """Flush on write."""
    def write(value: bytes) -> None:
        sink.write(value)
        sink.flush()
    return write


class Sinks(_Sinks):
    """
    Ease creation and usage of sinks.

    This handles where the output should be copied to.
    It also contains a few convenience functions to ease usage.
    These convenience functions are purely optional and the raw form
    is still available to some extent.
    """

    out: Optional[Tuple[Any, ...]]
    err: Optional[Tuple[Any, ...]]
    both: Tuple[Any, ...]

    def __new__(cls, out: Sequence[Any], err: Sequence[Any]):
        """Create new sinks."""
        both: Tuple[Any, ...] = ()
        if out and err:
            _out = set(out)
            _err = set(err)
            _both = _out & _err
            _out -= _both
            _err -= _both
            out = tuple(_out)
            err = tuple(_err)
            both = tuple(_both)
        return _Sinks.__new__(cls, out, err, both)

    def run(
        self,
        *args,
        flush: bool = True,
        threads: Type[Threads] = Threads,
        **kwargs,
    ) -> subprocess.Popen:
        """Conveniently create and execute a process and threads."""
        process = self.popen(*args, **kwargs)
        self.run_threads(process, flush=flush, threads=threads)
        return process

    def popen(
        self,
        *args: Any,
        **kwargs: Any,
    ) -> subprocess.Popen:
        """Conveniently create a process with out and err defined."""
        return subprocess.Popen(  # type: ignore
            *args,
            stdout=self.popen_out,
            stderr=self.popen_err,
            **kwargs,
        )

    @property
    def popen_out(self) -> Optional[int]:
        """Conveniently get the Popen output argument."""
        return subprocess.PIPE if self.out is not None else None

    @property
    def popen_err(self) -> Optional[int]:
        """Conveniently get the Popen error argument."""
        return subprocess.PIPE if self.err is not None else None

    def run_threads(
        self,
        process: subprocess.Popen,
        flush: bool = True,
        threads: Type[Threads] = Threads,
    ) -> None:
        """Conveniently create the threads and execute process."""
        with self.make_threads(
            process,
            flush=flush,
            threads=threads,
        ) as _threads:
            _threads.join()
        self.flush()
        self.reset_head()

    def make_threads(
        self,
        process: subprocess.Popen,
        flush: bool = True,
        threads: Type[Threads] = Threads,
    ) -> Threads:
        """Conveniently create threads."""
        return threads(process, self, flush=flush)

    def flush(self) -> None:
        """Flush all sinks."""
        for sinks in self:
            if sinks is None:
                continue
            for sink in sinks:
                if hasattr(sink, 'flush'):
                    sink.flush()

    def reset_head(self) -> None:
        """Reset the head of all sinks."""
        for sinks in self:
            for sink in sinks or []:
                if hasattr(sink, 'seek'):
                    try:
                        sink.seek(0)
                    except io.UnsupportedOperation:
                        pass

    @staticmethod
    def _to_callback(
        sinks: Optional[List[Any]],
        flush: bool = True,
    ) -> Optional[Tuple[TSink, ...]]:
        """Convert sinks to a callback."""
        if sinks is None:
            return None
        callbacks: List[TSink] = []
        for sink in sinks:
            if isinstance(sink, queue.Queue):
                callbacks.append(sink.put)
            elif hasattr(sink, 'write'):
                if flush and hasattr(sink, 'flush'):
                    callbacks.append(_flushed_write(sink))
                else:
                    callbacks.append(sink.write)
            else:
                raise ValueError(f'Unknown sink type {type(sink)} for {sink}')
        return tuple(callbacks)

    def as_callbacks(
        self,
        flush: bool = True,
    ) -> Tuple[Optional[Tuple[TSink, ...]], ...]:
        """Convert all sinks to their callbacks."""
        return tuple(self._to_callback(sinks, flush=flush) for sinks in self)


def popen_call(*args, stdout=None, stderr=None, **kwargs) -> subprocess.Popen:
    """Initialize process and wait for IO to complete."""
    return Sinks(stdout, stderr).run(*args, **kwargs)

Given that big chuck of code, the usage is fairly simple. Take the following, which logs stdout, stderr and both of the outputs into three different files. Whilst outputting the output to stdout and stderr live.

if __name__ == '__main__':
    import sys
    import tempfile

    with tempfile.TemporaryFile() as fout,\
         tempfile.TemporaryFile() as ferr,\
         tempfile.TemporaryFile() as fboth:
        process = popen_call(
            ['python', 'test.py'],
            stdout=(sys.stdout.buffer, fout, fboth),
            stderr=(sys.stderr.buffer, ferr, fboth),
        )
        process.wait()
        print('\n\noriginal')
        print(fboth.read().decode('utf-8'))
        print('\nstd.out')
        print(fout.read().decode('utf-8'))
        print('\nstd.err')
        print(ferr.read().decode('utf-8'), end='')

test.py

import sys
import random
import time

random.seed(42401)

for _ in range(10):
    f = random.choice([sys.stdout, sys.stderr])
    word = random.sample('Hello World!', random.randrange(1, 12))
    f.write(''.join(word) + '\n')
    f.flush()
    time.sleep(random.randrange(3))

Code tested on Python 3.7.2 only.

\$\endgroup\$
  • \$\begingroup\$ sys.stdout.buffered gives AttributeError: '_io.TextIOWrapper' object has no attribute 'buffered'. Unintentional typo in docstring? \$\endgroup\$ – RomanPerekhrest Dec 14 '19 at 22:45
  • \$\begingroup\$ @RomanPerekhrest Ah yes, the two example snippets in the documentation are broken, they should be sys.stdout.buffer. All the code works except those - as I haven't added tests for my docstrings yet. \$\endgroup\$ – Peilonrayz Dec 15 '19 at 7:33
3
\$\begingroup\$

Reconsidering valid/feasible "sinks"

When reasoning about the crucial idea and recalling the origin Unix tee command the obvious conclusion that comes out is that Sinks item should actually be I/O stream (wheather binary, text or buffered stream or OS-level file object) or optionally, queue.Queue instance. All other types are in-actual/invalid in such context.
Thus, it's reasonable to validate the input sequences (out, err) if they implementing the main sub-classes of basic io.IOBAse interface or queue.Queue class at the very start on constructing Sinks instance.
That will allow to eliminate noisy repetitive checks like if hasattr(sink, 'flush'), hasattr(sink, 'seek'), hasattr(sink, 'write') - assuming that "sinks" items are instances derived from any of (io.RawIOBase, io.BufferedIOBase, io.TextIOBase) which already implement flush/seek/write behavior.
With that in mind, I'd add the respective static methods to Sinks class:

@staticmethod
def _validate_sinks(sinks: Sequence[Any]):
    for sink in sinks:
        if not isinstance(sink, (io.RawIOBase, io.BufferedIOBase, io.TextIOBase, queue.Queue)):
            raise TypeError(f'Type `{type(sink)}` is not valid sink type')

@staticmethod
def is_iostream(sink):
    return isinstance(sink, (io.RawIOBase, io.BufferedIOBase, io.TextIOBase))

Now, the Sinks.__new__ method would look as (also see how redundant set reassignment optimized):

def __new__(cls, out: Sequence[Any], err: Sequence[Any]):
    """Create new sinks."""
    # Validating I/O streams
    if out:
        Sinks._validate_sinks(out)
    if err:
        Sinks._validate_sinks(err)

    both: Tuple[Any, ...] = ()
    if out and err:
        _out = set(out)
        _err = set(err)
        out = tuple(_out - _err)
        err = tuple(_err - _out)
        both = tuple(_out & _err)
    return _Sinks.__new__(cls, out, err, both)

Before posting optimized flush, reset_head and _to_callback methods - here are some subtle issues:

  • reset_head method.
    When running your approach "as is" I got OSError: [Errno 29] Illegal seek at the end.
    Some raw binary stream may not be seeakable.

    If False, seek(), tell() and truncate() will raise OSError.

    Therefore, let's capture 2 exceptions there except (io.UnsupportedOperation, OSError) as ex: (see the restructured method version below)

  • _to_callback method.
    The method is simplified due to preliminary initial validation.


Considering the above issues and some other minor but redundant moments/conditions like if sinks is None: continue, for sink in sinks or []: the mentioned 3 methods would look as below:

def flush(self) -> None:
    """Flush all sinks."""
    for sinks in filter(Sinks.is_iostream, self):
        for sink in sinks:
            sink.flush()

def reset_head(self) -> None:
    """Reset the head of all sinks."""
    for sinks in filter(Sinks.is_iostream, self):
        for sink in sinks:
            try:
                sink.seek(0)
            except (io.UnsupportedOperation, OSError) as ex:
                print(sink, sink.seekable(), ex)
                pass

@staticmethod
def _to_callback(
        sinks: Optional[List[Any]],
        flush: bool = True,
) -> Optional[Tuple[TSink, ...]]:
    """Convert sinks to a callback."""
    if sinks is None:
        return None
    callbacks: List[TSink] = []
    for sink in sinks:
        if isinstance(sink, queue.Queue):
            callbacks.append(sink.put)
        elif Sinks.is_iostream(sink):
            callbacks.append(_flushed_write(sink) if flush else sink.write)
    return tuple(callbacks)

Sample running (output):

doWlloHer
HlWd
lHl o!lreo
!
oWd H
lHlWd
delWlHlor
 olrdl!He
HolWlloe
!l


original
doWlloHer
HlWd
lHl o!lreo
!
oWd H
lHlWd
delWlHlor
 olrdl!He
HolWlloe
!l


std.out
lHl o!lreo
!
oWd H
lHlWd
!l


std.err
doWlloHer
HlWd
delWlHlor
 olrdl!He
HolWlloe
\$\endgroup\$
  • \$\begingroup\$ Thank you for the review! I'm too tired to make any sense of it right now :( I'll read it again tomorrow and try make sense of it :) \$\endgroup\$ – Peilonrayz Dec 15 '19 at 20:44
  • \$\begingroup\$ @Peilonrayz, That's absolutely Ok, take a rest, it's Sunday yet :) \$\endgroup\$ – RomanPerekhrest Dec 15 '19 at 20:52
  • \$\begingroup\$ I'm a little confused by the sentance "Sinks item should actually be I/O stream". Do you mean a sink or the Sinks class? I'm assuming the former. Why do you think an atomic message queue shouldn't be valid input? Whilst this is tee inspired, I fail to see why removing the option to pass a message queue is a good design choice. Is it possible to interact with stdin from stdout or stderr without the use of an atomic queue? An example use case is passing y to pip uninstall .... \$\endgroup\$ – Peilonrayz Dec 16 '19 at 12:10
  • \$\begingroup\$ Even though I'm not convinced on your stance, of only file objects. I do appreciate the review. It's highlighted some changes that I should make. Thank you. \$\endgroup\$ – Peilonrayz Dec 16 '19 at 12:13
  • 1
    \$\begingroup\$ I agree with you there, it would be better to validate to a common object. Again thank you for the review. And thank you for talking through your thought process. \$\endgroup\$ – Peilonrayz Dec 16 '19 at 13:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.