How to set a class attribute from an instance method in python?

Question

For previous versions of the question see the revision history - managed to come up with a minimal executable example that reproduces the problem

# module test.py
import shlex
from test2 import Parser

class Test(object):
    sync_client = None

    def __init__(self):
        self.__class__.sync_client = 1
        parser = Parser(description='desc') # was before the assignment - egal
        while True:
            cmd = shlex.split(raw_input('> ').strip())
            parser.parse_args(cmd)

    @classmethod
    def newRequestClient(cls):
        print cls.sync_client # None

if __name__ == "__main__":
    Test()

# module test2.py
import argparse

class Share(object):
    class _ShareAction(argparse.Action):
        def __call__(self, parser, namespace, values, option_string=None):
            from test import Test
            print Test.sync_client # None
            Test.newRequestClient()

    def __call__(self, subparsers):
        parser_a = subparsers.add_parser('name')
        parser_a.add_argument(dest='dest', help='help2',
                              action=Share._ShareAction)

class Parser(argparse.ArgumentParser):
    def __init__(self, description, add_h=True):
        super(Parser, self).__init__(description=description, add_help=add_h)
        subparsers = self.add_subparsers(title='Commands')
        subparsers._parser_class = argparse.ArgumentParser
        Share()(subparsers)

Run test.py and type name 123 in the prompt to see the Nones printed. Well maybe it is something obvious - it's been hours :)

EDIT: For the reproducer posted here this:

if __name__ == "__main__":
    from test import Test
    Test()

works. Adding from sync import Sync (and variations) in my main did not help however. I finally "solved" it by:

class Share(Command):
    class _ShareAction(argparse.Action):
        def __call__(self, parser, namespace, values, option_string=None):
            sync = sys.modules['__main__']
            print sync.Sync.sync_client # <SyncClient(SyncClient, started 6084)>
            sync.Sync.newRequestClient(host=namespace.host, repo=values)

But I do not yet fully understand why from sync import Sync did not work.

FINALLY: thanks to a comment by @MartijnPieters:

# module watcher.sync

if __name__ == "__main__":
    from watcher.sync import Sync
    Sync().main()

This looks ugly though so feel free to comment (or even add an answer) on how I could avoid it.

Your new example involves a lot of other complications. How are you using those classes? — BrenBarn
– BrenBarn, Commented Sep 21, 2014 at 21:09
I don't see an instance of Sync ever being created in your new code. — Lukas Graf
– Lukas Graf, Commented Sep 21, 2014 at 21:14
@BrenBarn: will edit with more details but what especially you want to know ? — Mr_and_Mrs_D
– Mr_and_Mrs_D, Commented Sep 21, 2014 at 21:19
the minimal executable example that reproduces your problem — joaquin
– joaquin, Commented Sep 21, 2014 at 21:21

Community · Accepted Answer · 2017-05-23 12:18:31Z

3

You misspelled __init__:

def __int__(self):

You are missing an i and the method is simply not called. Correct the error and your test works.

Demo:

>>> class Test(object):
...     class_attr = None
...     def __init__(self):  # note the spelling
...         self.__class__.class_attr = 1
...     @staticmethod
...     def static_meth():
...         print Test.class_attr
...     @classmethod
...     def class_meth(cls):
...         print cls.class_attr
... 
>>> t = Test()
>>> Test.class_attr
1
>>> Test.static_meth()
1
>>> Test.class_meth()
1
>>> t.class_meth()
1
>>> t.static_meth()
1

In you updated code you have two issues:

First you create an instance of Parser before the class attribute has been set. The self.__class__.sync_client = 1 line simply hasn't executed yet when Parser.__init__ is being called.

You then confuse the main script and the test module. Python imports the main script as __main__, not test. If you move the Test class out to a separate module or use from __main__ import Test your code will work.

See Importing modules: __main__ vs import as module

edited May 23, 2017 at 12:18

CommunityBot

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answered Sep 21, 2014 at 20:36

Martijn Pieters

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16 Comments

Mr_and_Mrs_D Over a year ago

Well this is true - in my real class hadn't mispelled __init__ though - let me see a minute

Martijn Pieters Over a year ago

@Mr_and_Mrs_D: you misspelled something; see my demo (from a 2.7.8 session).

Mr_and_Mrs_D Over a year ago

I edited the question with my original code - will keep looking but I hope you see something I don't :)

Mr_and_Mrs_D Over a year ago

Well - may be an import issue ? I do from watcher.sync import Sync and this is run before Sync.__init__() runs - at that time the debugger shows me a Sync class that has sync_client == None - could it be this ?

Martijn Pieters Over a year ago

@Mr_and_Mrs_D: no, that is not frowned upon. What won't work is putting the script file inside a package, and then expect to be part of the package namespace.

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