Hacking on FlashFlow

Unit testing

Bare assert versus self.assert*

Use bare asserts in unit tests for things that you expect to be true but are not about the behavior being tested. Use self.assert* for things that are being tested.

For example, you have some test that verifies initial state of your state machine is START. This test creates a state machine and uses self.assertEqual(state_machine.state, START) to test this behavior.

In other unit tests where you want to make certain and/or document for the reader that the current state is START when you are adding a widget to the state machine’s list of widgets, use a bare assert.

class TestWidgetStateMachine(unittest.TestCase):
   def test_add_widget(self):
      # Make explicit we are in state START. Not behavior being tested.
      assert state_machine.state == START
      w = Widget()
      # Verify the widget was added. Behavior being tested.
      self.assertEqual(len(state_machine.widgets), 1)

Here is another example showing proper use of assert versus self.assert*. The test is not about correctly adding a widget, but removing one.

class TestWidgetStateMachine(unittest.TestCase):
   def test_remove_not_exist(self):
      # Create two widgets and add one to the list
      w_listed = Widget()
      w_unlisted = Widget()
      # Make explicit there is no widgets listed. Not being tested.
      assert not state_machine.widgets
      # Add one
      # Make explicit there is indeed a widget listed now. Not being tested.
      assert len(state_machine.widgets) == 1
      # Here is the core of the test: try removing a non-existant widget
      # Verify the listed widget still exists. Behavior being tested.
      self.assertEqual(len(state_machine.widgets), 1)

Integration Tests

Generating a new network

  1. Navigate to tests/integration/.

  2. Create a new directory for the network. E.g. mynet/.

  3. Copy the scripts from net-gen-scripts/ into the new directory. Navigate to the new directory.

  4. Modify ./01-gen-configs.sh as necessary and run it.

This generates the network, and it should bootstrap just fine when you start it assuming your modifications to ./01-gen-configs.sh don’t prevent that somehow. Continue followings these steps to test that it bootstraps.

  1. Start the network with ./02-start-network.sh. Quickly verify that:

    • All relay processes are running.

    • They are producing logs that indicate successful startup.

  2. Verify that all relays can boostrap with ./03-network-in-ready-state.py:

    ./03-network-in-ready-state.py -d auth* guard* middle* exit*
  3. Stop the network with ./04-stop-network.sh.

You now have a verified-working network. If you want to pack it into a tarball with the minimal files necessary to recreate it, run ./01-gen-configs.sh first. Note: this will change relays’ keys, fingerprints, etc.. The structure of the network will be the same, which is all that should matter.


Network generation scripts are adapted for pastly’s tor-network-on-localhost scripts most recently published here.

These scripts were originally released under the Unlicense, dedicating them to the public domain. For consistency with the rest of FlashFlow in this repository, the copy(s) here, with all modifications (if any), are released under the same license, which last time this paragraph was updated, is the CC0 dedicating them to the public domain.

Adding a new command

Like flashflow coord and flashflow measurer.

The working example here is a new command called jeff.

Create a new file in flashflow/cmd/


Add boilerplate to new file

You need to:

  • create a log object so you can log things

  • define a gen_parser(...) function to call to attach your command’s arguments to the global argument parser

  • define a main(...) function

Here’s a clean example that works at the time of writing. Look at the existing FlashFlow commands for examples that (1) definitely stay updated with how FlashFlow functions, and (2) may be more complex than this.

from argparse import ArgumentParser
import logging

log = logging.getLogger(__name__)

def gen_parser(sub) -> ArgumentParser:
    ''' Add the cmd line options for this FlashFlow command '''
    d = 'The example FlashFlow command known as jeff'
    p = sub.add_parser('jeff', description=d)
    return p

# This function needs **some sort** of type annotation so that mypy will check
# the things it does. Adding the return value (e.g. '-> None') is enough
def main(args, conf) -> None:
    log.error('Hi I'm jeff, and I am boilerplate. Make me do something useful')

Import new file in flashflow/flashflow.py

In the imports at the top of flashflow/flashflow.py find where other commands are being imported. Import yours too. For example:

# ... other imports of flashflow commands
import flashflow.cmd.jeff

Call your new gen_parser(...) in flashflow/flashflow.py

Find create_parser(...) in flashflow/flashflow.py and find where other commands are getting their subparsers added. Do the same.

# ... other lines calling commands' gen_parser() function
return p

Add a call to your new main(...) in flashflow/flashflow.py

Find call_real_main(...) in flashflow/flashflow.py. In it find the dictionary of all possible commands and the arguments to pass to them. Add your new command.

cmds = {
    # ... other commands
    'jeff': {
        'f': flashflow.cmd.jeff.main,
        'a': def_args, 'kw': def_kwargs,


That’s it. You should be able to do things such as the following and see jeff show up.

flashflow jeff
flashflow jeff -h
flashflow -h