Use an actions queue instead of a string

[0xjac]

Use a Queue rather than a string to handle the actions may be more adapted (L118)

It's clearer and you can get rid of ACTION_SIZE and i logic.

Here is an example on how you would handle it:

from queue import Queue


def __init__(self, *args, **kwargs):
    self.actions = Queue()

def do_actions(self):
    while not self.actions.empty():
        action = self.actions.get()
        # do something with action
        
def add_action(action):
    self.actions.put(action)  # instead of self.actions += action

This is also much easier to make thread-proof in the future, when you have ATMs with octo-cores... 😁

[tshabs]

Note on using Queue, it might be better to use six.moves Queue (for py2 & py3 compatibility).

eg:
rtu-dataframe/escposprinter@4e13ee2

[0xjac]

I wasn't aware you needed compatibility with Python 2. In that case, yes use six.moves.

Using Enum in #4 may also be problematic. I would write a simple Enum class for Python 2 in this case.