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"Command-line interface I/O broker with sessions for asynchronous applications."

Project description

PyCliBroker

Command-line interface I/O broker with sessions for asynchronous applications.

CliBroker provides an asynchronous interface to synchronize stdio read/write commands by FIFO. Sessions allow grouping such commands together, suspending calls outside of such a session until the active session has terminated. These sessions can also be nested.

CliBroker processes each request one after another in a single separate thread. CliBroker will properly handle cancellation of requests.

Table of Contents

Installation

Simply install via pip install clibroker.

Usage

CliBroker exposes a familiar IO-like interface. A simple example usage is as follows:

import asyncio
import clibroker as cli

async def main():
    cli.writeline('Hello, world!')

    t1 = asyncio.create_task(async1())
    t2 = asyncio.create_task(async2())
    await t1; await t2
    # > Hello, world!
    # > Say something: <input:"test 123">
    # > Thanks for those 9 characters.
    # > Foo

async def async1():
    await asyncio.sleep(0.1)
    await cli.writeline('Foo')

async def async2():
    async with cli.session(autoflush=True) as sess:
        await sess.write('Say something: ')
        input = await sess.readline()
        if len(input) > 0:
            await sess.writeline(f'Thanks for those {len(input)} characters.')
        else:
            await sess.writeline('Okay, then not.')

if __name__ == '__main__':
    asyncio.run(main())

Sessions

As mentioned above, clibroker.session is probably the most useful feature of this library. As the output of the code above demonstrates, it allows "grouping" CLI commands together and to postpone any other intermittent call until this session is closed.

CliBroker uses an implicit "global session" to expose specific top-level functions for CLI commands without an associated session: read, readline, write, writeline, password, and session. Their default behavior in the global session is documented in their respective sections below.

async Session.read(n: int) -> str

Read at most n characters from stdin.

async Session.readline() -> str

Read an entire line from stdin (up until and including the '\n' character).

async Session.password(prompt: str = 'Password: ') -> str

Read a password from stdin similar to Unix-style applications with an optional prompt. User input will not be echoed to stdout.

Caveat: It is possible to rebind output and input streams that a session uses - however Session.password will always use sys.stdin and sys.stdout (for the prompt). On one hand, this is a limitation of the standard library getpass. On the other hand, support for other streams is typically not needed are likely not mandatorily linked to a console.

async Session.write(*data, sep: str = ' ', err: bool = False, flush: Optional[bool] = None)

Write all data stringified and joined by sep.

If err is false, data is written to stdout, else to stderr.

flush dictates whether to immediately flush the output.

  • If flush is true, output is immediately flushed.
  • If flush is false, obviously it is not flushed.
  • If flush is None, resorts to associated session's default autoflush behavior.

Global session's autoflush behavior is true.

async Session.writeline(*data, sep: str = ' ', err: bool = False, flush: Optional[bool] = None)

Same as Session.write, except appends a newline character ('\n') to the data.

async Session.flush(flush_stdout: bool = True, flush_stderr: bool = True) -> None

Explicitly flush stdout and/or stderr.

This method is not exposed as top-level function of the global session as the global session always automatically flushes.

async Session.standby() -> str

A special Session.readline which lurks to intercept user input while no other read command (Session.read or Session.readline) is pending. If a read command is issued or a session opened, standby will be postponed until all requests are completed first.

async Session.session(autoflush: Optional[bool] = None) -> Session

Creates a new subsession. All subsequent CLI commands on this session will be postponed until this subsession is concluded.

autoflush determines the new session's default autoflushing behavior as used by Session.write and Session.writeline methods:

  • True: Automatically flush.
  • False: Do not automatically flush.
  • None: Adopt parent session's current autoflush behavior.

The global session by default uses autoflush=False.

Technical Details

Beware of backpressure! CliBroker buffers every single request internally in order to achieve predictability. Backpressure may build up when quickly and regularly queuing requests without ever synchronizing, most noticably with read requests.

Background Thread

As mentioned before, CliBroker employs a single background thread to process its queue of requests. In order to avoid regular termination this background thread is terminated upon completion of all requests. However, as this may lead to a considerable thread creation overhead, CliBroker waits for new requests for at most 10ms before terminating - a time span short enough to be barely noticeable but long enough to avoid unnecessary thread creation for most common tasks.

Global Session

The global session is simply a Session whose methods (partly with altered default behavior) are exposed as top-level bound functions.

The global session can be accessed and changed via clibroker.clibroker._session to further alter its default behavior (such as changing I/O streams). This interface may be useful to the advanced user. It was developed for the purposes of unit testing and is not intended as part of the public interface.

Further Testing

CliBroker was developed as part of my hobby project. Currently, I am its sole developer and maintainer. Naturally, I do not have extensive time to invest into the development of this project. I have set up unit tests for various cases, but these by far do not provide sufficient coverage.

The following points probably need more testing:

  • Multithreaded use - theoretically supported but untested
  • Request cancellation
  • standby feature
  • Python version - I'm not entirely sure what the oldest supported Python version is (3.6?)

License

MIT License

Copyright (c) 2021 Kiruse

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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