Assign Constructor argument with static method pointer in python?

Question

Getting undefined error with the below program

class SignalHandler(object):  # pylint:  disable=too-few-public-methods
"""
Handles various user generated signals
"""

    def __init__(self,
             sigint_signalhandler=SignalHandler.signal_handler,
             sigquit_signalhandler=SignalHandler.signal_handler,
             sighup_signalhandler=SignalHandler.signal_handler):

        signal.signal(signal.SIGINT, sigint_signalhandler)
        signal.signal(signal.SIGQUIT, sigquit_signalhandler)
        signal.signal(signal.SIGHUP, sighup_signalhandler)

    @staticmethod
    def signal_handler(signalnum):
        print ('Ignoring signal : ', signalnum)

This is what the error looks like

import signalhandler
File "/usr/local/sandvine/scripts/upgrade-assistance/signalhandler.py", line 10, in <module>
class SignalHandler(object):  # pylint:  disable=too-few-public-methods
File "/usr/local/sandvine/scripts/upgrade-assistance/signalhandler.py", line 22, in SignalHandler
sigint_signalhandler=SignalHandler.signal_handler,
NameError: name 'SignalHandler' is not defined

So eventually i want to pass some custom methods, if not i will use signal_handler method provided by SignalHandler class.

Possible duplicate of assigning class variable as default value to class method argument — Taku
– Taku, Commented Apr 23, 2018 at 9:59

Tom Dalton · Accepted Answer · 2018-04-23 10:03:13Z

1

Define signal_handler above the class as a plain method, not inside it as a static method. You can't refer to a class before it's been defined, which is what your __init__ is trying to do.

Alternatively, use self.signal_handler in the init body instead of SignalHandler.signal_handler:

class Foo(object):
    def __init__(self, bar_printer=None):
        self.bar_printer = bar_printer if bar_printer else self._default_bar_printer

    @staticmethod
    def _default_bar_printer():
        print("bar")


f = Foo()
f.bar_printer()  # Prints "bar"

def better_bar_printer():
    print("The best bar!")

f2 = Foo(better_bar_printer)
f2.bar_printer()  # Prints "The best bar!"

edited Apr 23, 2018 at 10:03

answered Apr 23, 2018 at 9:56

Tom Dalton

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4 Comments

a_guest Over a year ago

And after class creation the function can be added to the class using SignalHandler.signal_handler = staticmethod(signal_handler).

a_guest Over a year ago

Ok now it's clear. By the way you could use simplified version self.bar_printer = bar_printer or self._default_bar_printer in __init__.

Tom Dalton Over a year ago

I have a personal (probably unfair/irrational) dislike of using or for that purpose. Possibly due to childhood exposure to C constructs that use or alongside functions with side-effects :-)

a_guest Over a year ago

Okay, but actually or in Python does always the same thing, no matter what the context is (i.e. it is the same when you use it for that purpose and when you use it within if clauses for example): or yields the left-hand side if it evaluates to True and the right-hand side otherwise. So it is exactly the same thing as you coded it explicitly.

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