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Moler is library to help in building automated tests

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Table of Contents

  1. Changelog
  2. Moler info
  3. Moler usage examples
  4. API design reasoning
  5. Designed API


View our chronological list of user-facing changes, large and small, made to the Moler project.

moler logo


Moler (name origin) is Python library that provides "bricks" for building automated tests. All these "bricks" have clearly defined responsibilities, have similar API, follow same construction pattern (so new ones are easy to create).

Here they are:

  • Commands as self-reliant object
    • to allow for command triggering and parsing encapsulated in single object (lower maintenance cost)
  • Event observers & callbacks (alarms are events example)
    • to allow for online reaction (not offline postprocessing)
  • Run observers/commands in the background
    • to allow for test logic decomposition into multiple commands running in parallel
    • to allow for handling unexpected system behavior (reboots, alarms)
  • State machines -> automatic auto-connecting after dropped connection
    • to increase framework auto-recovery and help in troubleshooting "what went wrong"
  • Automatic logging of all connections towards devices used by tests
    • to decrease investigation time by having logs focused on different parts of system under test

Moler usage examples

Let's see Moler in action. Here is hypothetical use case: "find PIDs of all python processes":

    from moler.config import load_config
    from moler.device.device import DeviceFactory

    load_config(config='my_devices.yml')                    # description of available devices
    my_unix = DeviceFactory.get_device(name='MyMachine')    # take specific device out of available ones
    ps_cmd = my_unix.get_cmd(cmd_name="ps",                 # take command of that device
                             cmd_params={"options": "-ef"})

    processes_info = ps_cmd()                               # run the command, it returns result
    for proc_info in processes_info:
        if 'python' in proc_info['CMD']:
            print("PID: {info[PID]} CMD: {info[CMD]}".format(info=proc_info))
  • To have command we ask device "give me such command".
  • To run command we just call it as function (command object is callable)
  • What command returns is usually dict or list of dicts - easy to process

Above code displays:

    PID: 1817 CMD: /usr/bin/python /usr/share/system-config-printer/
    PID: 21825 CMD: /usr/bin/python /home/gl/moler/examples/command/

How does it know what 'MyMachine' means? Code loads definition from my_devices.yml configuration file:


        DEVICE_CLASS: moler.device.unixlocal.UnixLocal

        DEVICE_CLASS: moler.device.unixremote.UnixRemote
          UNIX_LOCAL:                # from state
            UNIX_REMOTE:             # to state
              execute_command: ssh   # via command
              command_params:        # with params
                expected_prompt: demo@
                login: demo
                password: password
                set_timeout: False   # remote doesn't support: export TMOUT

We have remote machine in our config. Let's check if there is 'readme.txt' file on that machine (and some info about the file):

    remote_unix = DeviceFactory.get_device(name='RebexTestMachine')  # it starts in local shell
    remote_unix.goto_state(state="UNIX_REMOTE")                      # make it go to remote shell

    ls_cmd = remote_unix.get_cmd(cmd_name="ls", cmd_params={"options": "-l"})

    remote_files = ls_cmd()

    if 'readme.txt' in remote_files['files']:
        print("readme.txt file:")
        readme_file_info = remote_files['files']['readme.txt']
        for attr in readme_file_info:
            print("  {:<18}: {}".format(attr, readme_file_info[attr]))

As you may noticed device is state machine. State transitions are defined inside configuration file under CONNECTION_HOPS. Please note, that it is only config file who knows "I need to use ssh to be on remote" - client code just says "go to remote". Thanks to that you can exchange "how to reach remote" without any change in main code.

Above code displays:

    readme.txt file:
      permissions       : -rw-------
      hard_links_count  : 1
      owner             : demo
      group             : users
      size_raw          : 403
      size_bytes        : 403
      date              : Apr 08  2014
      name              : readme.txt

How about doing multiple things in parallel. Let's ping google while asking about readme.txt file:

my_unix = DeviceFactory.get_device(name='MyMachine')
host = ''
ping_cmd = my_unix.get_cmd(cmd_name="ping", cmd_params={"destination": host, "options": "-w 6"})

remote_unix = DeviceFactory.get_device(name='RebexTestMachine')
ls_cmd = remote_unix.get_cmd(cmd_name="ls", cmd_params={"options": "-l"})

print("Start pinging {} ...".format(host))
ping_cmd.start()                                # run command in background
print("Let's check readme.txt at {} while pinging {} ...".format(, host))

remote_files = ls_cmd()                         # foreground "run in the meantime"
file_info = remote_files['files']['readme.txt']
print("readme.txt file: owner={fi[owner]}, size={fi[size_bytes]}".format(fi=file_info))

ping_stats = ping_cmd.await_done(timeout=6)     # await background command
print("ping {}: {}={}, {}={} [{}]".format(host,'packet_loss',
Start pinging ...
Let's check readme.txt at RebexTestMachine while pinging ...
readme.txt file: owner=demo, size=403
ping packet_loss=0, time_avg=35.251 [ms]

Besides being callable command-object works as "Future" (result promise). You can start it in background and later await till it is done to grab result.

If we enhance our configuration with logging related info:

      PATH: ./logs
      DATE_FORMAT: "%H:%M:%S"

then above code will automatically create Molers' main log (moler.log) which shows activity on all devices:

22:30:19.723 INFO       moler               |More logs in: ./logs
22:30:19.747 INFO       MyMachine           |Connection to: 'MyMachine' has been opened.
22:30:19.748 INFO       MyMachine           |Changed state from 'NOT_CONNECTED' into 'UNIX_LOCAL'
22:30:19.866 INFO       MyMachine           |Event '':'[re.compile('^moler_bash#')]' started.
22:30:19.901 INFO       RebexTestMachine    |Connection to: 'RebexTestMachine' has been opened.
22:30:19.901 INFO       RebexTestMachine    |Changed state from 'NOT_CONNECTED' into 'UNIX_LOCAL'
22:30:19.919 INFO       RebexTestMachine    |Event '':'[re.compile('demo@')]' started.
22:30:19.920 INFO       RebexTestMachine    |Event '':'[re.compile('^moler_bash#')]' started.
22:30:19.921 INFO       RebexTestMachine    |Command 'moler.cmd.unix.ssh.Ssh':'TERM=xterm-mono ssh -l demo' started.
22:30:19.921 INFO       RebexTestMachine    |TERM=xterm-mono ssh -l demo
22:30:20.763 INFO       RebexTestMachine    |*********
22:30:20.909 INFO       RebexTestMachine    |Changed state from 'UNIX_LOCAL' into 'UNIX_REMOTE'
22:30:20.917 INFO       RebexTestMachine    |Command 'moler.cmd.unix.ssh.Ssh' finished.
22:30:20.919 INFO       MyMachine           |Command '':'ping -w 6' started.
22:30:20.920 INFO       MyMachine           |ping -w 6
22:30:20.920 INFO       RebexTestMachine    |Command '':'ls -l' started.
22:30:20.922 INFO       RebexTestMachine    |ls -l
22:30:20.985 INFO       RebexTestMachine    |Command '' finished.
22:30:26.968 INFO       MyMachine           |Command '' finished.
22:30:26.992 INFO       RebexTestMachine    |Event '': '[re.compile('^moler_bash#')]' finished.
22:30:27.011 INFO       RebexTestMachine    |Event '': '[re.compile('demo@')]' finished.
22:30:27.032 INFO       MyMachine           |Event '': '[re.compile('^moler_bash#')]' finished.

As you may noticed main log shows code progress from high-level view - data on connections are not visible, just activity of commands running on devices.

If you want to see in details what has happened on each device - you have it in device logs. Moler creates log per each device moler.RebexTestMachine.log:

22:30:19.901  |Changed state from 'NOT_CONNECTED' into 'UNIX_LOCAL'
22:30:19.902 <|
22:30:19.919  |Event '':'[re.compile('demo@')]' started.
22:30:19.920  |Event '':'[re.compile('^moler_bash#')]' started.
22:30:19.921  |Command 'moler.cmd.unix.ssh.Ssh':'TERM=xterm-mono ssh -l demo' started.
22:30:19.921 >|TERM=xterm-mono ssh -l demo

22:30:19.924 <|TERM=xterm-mono ssh -l demo

22:30:20.762 <|Password:
22:30:20.763 >|*********
22:30:20.763 <|

22:30:20.908 <|Welcome to Rebex Virtual Shell!
              |For a list of supported commands, type 'help'.
22:30:20.909  |Changed state from 'UNIX_LOCAL' into 'UNIX_REMOTE'
22:30:20.917  |Command 'moler.cmd.unix.ssh.Ssh' finished.
22:30:20.920  |Command '':'ls -l' started.
22:30:20.922 >|ls -l

22:30:20.974 <|ls -l

22:30:20.978 <|drwx------ 2 demo users          0 Jul 26  2017 .

22:30:20.979 <|drwx------ 2 demo users          0 Jul 26  2017 ..
              |drwx------ 2 demo users          0 Dec 03  2015 aspnet_client
              |drwx------ 2 demo users          0 Oct 27  2015 pub
              |-rw------- 1 demo users        403 Apr 08  2014 readme.txt
22:30:20.985  |Command '' finished.
22:30:26.992  |Event '': '[re.compile('^moler_bash#')]' finished.
22:30:27.011  |Event '': '[re.compile('demo@')]' finished.

and moler.MyMachine.log:

22:30:19.748  |Changed state from 'NOT_CONNECTED' into 'UNIX_LOCAL'
22:30:19.748 <|
22:30:19.866  |Event '':'[re.compile('^moler_bash#')]' started.
22:30:20.919  |Command '':'ping -w 6' started.
22:30:20.920 >|ping -w 6

22:30:20.921 <|ping -w 6

22:30:20.959 <|PING ( 56(84) bytes of data.
22:30:20.960 <|

22:30:21.000 <|64 bytes from ( icmp_seq=1 ttl=51 time=40.1 ms
22:30:21.001 <|

22:30:21.992 <|64 bytes from ( icmp_seq=2 ttl=51 time=31.0 ms

22:30:22.999 <|64 bytes from ( icmp_seq=3 ttl=51 time=36.5 ms

22:30:23.996 <|64 bytes from ( icmp_seq=4 ttl=51 time=31.4 ms

22:30:24.996 <|64 bytes from ( icmp_seq=5 ttl=51 time=29.8 ms

22:30:26.010 <|64 bytes from ( icmp_seq=6 ttl=51 time=42.4 ms

22:30:26.960 <|
              |--- ping statistics ---
              |6 packets transmitted, 6 received, 0% packet loss, time 5007ms
              |rtt min/avg/max/mdev = 29.888/35.251/42.405/4.786 ms
22:30:26.968  |Command '' finished.
22:30:27.032  |Event '': '[re.compile('^moler_bash#')]' finished.

If the log files are too large you can split files.

The log files can be split by size. For example let's assume we want split log files by 5 MB (5242880 bytes) and we want to keep maximum 999 files:

      PATH: ./logs
      DATE_FORMAT: "%H:%M:%S"
        KIND: size
        INTERVAL: 5242880
        BACKUP_COUNT: 999  # Default value

The log files can be split by time. For example let's assume we want split log files every 30 minutes (1800 seconds) and we want to keep maximum 999 files (default value):

      PATH: ./logs
      DATE_FORMAT: "%H:%M:%S"
        KIND: time
        INTERVAL: 1800
        BACKUP_COUNT: 999  # Default value

For space saving Moler can compress the logs after rotation. The external tool is used. Let's use the above examples to show how to compress logs:

      PATH: ./logs
      DATE_FORMAT: "%H:%M:%S"
        KIND: size
        INTERVAL: 5242880
        BACKUP_COUNT: 999  # Default value
        COMPRESS_AFTER_ROTATION: True  # Default is False
        COMPRESS_COMMAND: "zip -9mq {compressed} {log_input}"  # Default value
        COMPRESSED_FILE_EXTENSION: ".zip"  # Default value
      PATH: ./logs
      DATE_FORMAT: "%H:%M:%S"
        KIND: time
        INTERVAL: 1800
        BACKUP_COUNT: 999  # Default value
        COMPRESS_COMMAND: "zip -9mq {compressed} {log_input}"  # Default value      
        COMPRESSED_FILE_EXTENSION: ".zip"  # Default value

In a script we can also disable logging from device. Please use it very carefully. Investigation any issue may be impossible if we don't have full logs.

my_unix = DeviceFactory.get_device(name='MyMachine')

my_unix.disbale_logging()  # to disable logging on device

my_unix.enable_logging()  # to enable logging on device

In a script you can add suffix to all log files or only to files for specific devices. with disable logging from device.

from moler.config.loggers import change_logging_suffix
change_logging_suffix(".suffix1")  # all log files with suffix
change_logging_suffix(None)  # all log files without suffx

my_unix = DeviceFactory.get_device(name='MyMachine')

my_unix.set_logging_suffix("device_suffix")  # to add suffix to filename with logs

my_unix.set_suffix(None)  # to remove suffix from filename with logs

Previous examples ask device to create command. We can also create command ourselves giving it connection to operate on:

    import time
    from import Ping
    from moler.connection_factory import get_connection

    host = ''
    terminal = get_connection(io_type='terminal', variant='threaded')  # take connection
        ping_cmd = Ping(connection=terminal.moler_connection,
                        destination=host, options="-w 6")
        print("Start pinging {} ...".format(host))
        print("Doing other stuff while pinging {} ...".format(host))
        ping_stats = ping_cmd.await_done(timeout=4)
        print("ping {}: {}={}, {}={} [{}]".format(host,'packet_loss',

Please note also that connection is context manager doing open/close actions.

    Start pinging ...
    Doing other stuff while pinging ...
    ping packet_loss=0, time_avg=50.000 [ms]

Reuse freedom

Library gives you freedom which part you want to reuse. We are fan's of "take what you need only".

  • You may use configuration files or configure things by Python calls.

    load_config(config={'DEVICES': {'MyMachine': {'DEVICE_CLASS': 'moler.device.unixlocal.UnixLocal'}}})
  • You may use devices or create commands manually

  • You can take connection or build it yourself:

    from moler.threaded_moler_connection import ThreadedMolerConnection
    from import ThreadedTerminal
    terminal_connection = ThreadedTerminal(moler_connection=ThreadedMolerConnection())
  • You can even install your own implementation in place of default implementation per connection type

API design reasoning

The main goal of command is its usage simplicity: just run it and give me back its result.

Command hides from its caller:

  • a way how it realizes "runs"
  • how it gets data of output to be parsed
  • how it parses that data

Command shows to its caller:

  • API to start/stop it or await for its completion
  • API to query for its result or result readiness

Command works as Futures and promises

After starting, we await for its result which is parsed out command output provided usually as dict. Running that command and parsing its output may take some time, so till that point result computation is yet incomplete.

Command as future

  • it starts some command on device/shell over connection (as future-function starts it's execution)
  • it parses data incoming over such connection (as future-function does it's processing)
  • it stores result of that parsing (as future-function concludes in calculation result)
  • it provides means to return that result (as future-function does via 'return' or 'yield' statement)
  • it's result is not ready "just-after" calling command (as it is with future in contrast to function)

So command should have future API.

Quote from "Professional Python" by Luke Sneeringer:

The Future is a standalone object. It is independent of the actual function that is running. It does nothing but store the state and result information.

Command differs in that it is both:

  • function-like object performing computation
  • future-like object storing result of that computation.

Command vs. Connection-observer

Command is just "active version" of connection observer.

Connection observer is passive since it just observes connection for some data; data that may just asynchronously appear (alarms, reboots or anything you want). Intention here is split of responsibility: one observer is looking for alarms, another one for reboots.

Command is active since it actively triggers some output on connection by sending command-string over that connection. So, it activates some action on device-behind-connection. That action is "command" in device terminology. Like ping on bash console/device. And it produces that "command" output. That output is what Moler's Command as connection-observer is looking for.

Most well known Python's futures

API concurrent.futures.Future asyncio.Future
storing result :white_check_mark: set_result() :white_check_mark: set_result()
result retrieval :white_check_mark: result() :white_check_mark: result()
storing failure cause :white_check_mark: set_exception() :white_check_mark: set_exception()
failure cause retrieval :white_check_mark: exception() :white_check_mark: exception()
stopping :white_check_mark: cancel() :white_check_mark: cancel()
check if stopped :white_check_mark: cancelled() :white_check_mark: cancelled()
check if running :white_check_mark: running() :no_entry_sign: (but AbstractEventLoop.running())
check if completed :white_check_mark: done() :white_check_mark: done()
subscribe completion :white_check_mark: add_done_callback() :white_check_mark: add_done_callback()
unsubscribe completion :no_entry_sign: :white_check_mark: remove_done_callback()

Starting callable to be run "as future" is done by entities external to future-object

API concurrent.futures
start via Executor objects (thread/proc)
start via module-lvl functions or ev-loop
start callable submit(fn, *args, **kwargs)
Schedules callable to be executed as
fn(*args **kwargs) -> Future
ensure_future(coro_or_future) -> Task
future = run_coroutine_threadsafe(coro, loop)
start same callable
on data iterator
map(fn, *iterables, timeout) -> iterator join_future = asyncio.gather(*map(f, iterable))

Awaiting completion of future is done by entities external to future-object

API concurrent.futures
awaiting by module level functions
awaiting by module-lvl functions or ev-loop
await completion done, not_done = wait(futures, timeout) -> futures done, not_done = await wait(futures)
results = await gather(futures)
result = await future
result = yield from future
result = await coroutine
result = yield from coroutine
result = yield from wait_for(future, timeout)
loop.run_until_complete(future) -> blocking run
process as they
for done in as_completed(futures, timeout) -> futures for done in as_completed(futures, timeout) -> futures

Fundamental difference of command

Contrary to concurrent.futures and asyncio we don't want command to be run by some external entity. We want it to be self-executable for usage simplicity. We want to take command and just say to it:

  • "run" or "run in background"
  • and not "Hi, external runner, would you run/run-background that command for me"

Designed API

  1. create command object
command = Command()
  1. run it synchronously/blocking and get result in one shot behaves like function call since Command is callable.

Run-as-callable gives big advantage since it fits well in python ecosystem.

result = command()

function example:

map(ping_cmd, all_machines_to_validate_reachability)
  1. run it asynchronously/nonblocking
command_as_future = command.start()
  1. shift from background to foreground

asyncio: variant looks like:

result = await future
done_futures, pending = yield from asyncio.wait(futures)
result = yield from asyncio.wait_for(future, 60.0)

and concurrent.futures variant looks like:

done_futures, pending = wait(futures)

Moler's API maps to above well-known API

result = command.await_done(timeout)
  • it is "internal" to command "Hi command, that is what I want from you" (above APIs say "Hi you there, that is what I want you to do with command")
  • it directly (Zen of Python) shows what we are awaiting for
  • timeout is required parameter (not as in concurrent.futures) since we don't expect endless execution of command (user must know what is worst case timeout to await command completion)

Video introduction

You can watch videos how to use Moler on YouTube.

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