Alternate Event Loops¶
Throughout this documentation we have focused on the 90% use case, that is
the use case we believe around 90+% of our user base is looking for. This
focuses on ease of use and the best out-of-the-box experience where developers
get the most functionality for the least amount of effort. We are talking
about running cmd2
applications with the cmdloop()
method:
from cmd2 import Cmd
class App(Cmd):
# customized attributes and methods here
app = App()
app.cmdloop()
However, there are some limitations to this way of using cmd2
, mainly that
cmd2
owns the inner loop of a program. This can be unnecessarily
restrictive and can prevent using libraries which depend on controlling their
own event loop.
Many Python concurrency libraries involve or require an event loop which they are in control of such as asyncio, gevent, Twisted, etc.
cmd2
applications can be executed in a fashion where cmd2
doesn’t own
the main loop for the program by using code like the following:
import cmd2
class Cmd2EventBased(cmd2.Cmd):
def __init__(self):
cmd2.Cmd.__init__(self)
# ... your class code here ...
if __name__ == '__main__':
app = Cmd2EventBased()
app.preloop()
# Do this within whatever event loop mechanism you wish to run a single command
cmd_line_text = "help history"
app.runcmds_plus_hooks([cmd_line_text])
app.postloop()
The runcmds_plus_hooks()
method runs multiple commands via
onecmd_plus_hooks()
.
The onecmd_plus_hooks()
method will do the following to
execute a single command in a normal fashion:
- Parse user input into a
Statement
object - Call methods registered with
register_postparsing_hook()
- Redirect output, if user asked for it and it’s allowed
- Start timer
- Call methods registered with
register_precmd_hook()
- Call
precmd()
- for backwards compatibility withcmd.Cmd
- Add statement to History
- Call do_command method
- Call methods registered with
register_postcmd_hook()
- Call
postcmd()
- for backwards compatibility withcmd.Cmd
- Stop timer and display the elapsed time
- Stop redirecting output if it was redirected
- Call methods registered with
register_cmdfinalization_hook()
Running in this fashion enables the ability to integrate with an external event loop. However, how to integrate with any specific event loop is beyond the scope of this documentation. Please note that running in this fashion comes with several disadvantages, including:
- Requires the developer to write more code
- Does not support transcript testing
- Does not allow commands at invocation via command-line arguments