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Can't I define a decorator in a separate file and import it?

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  Can't I define a decorator in a separate file and import it? Saqib Ali <saqib.ali.75@gmail.com> - 2011-12-22 12:53 -0800
    Re: Can't I define a decorator in a separate file and import it? Saqib Ali <saqib.ali.75@gmail.com> - 2011-12-22 13:09 -0800
      Re: Can't I define a decorator in a separate file and import it? Ben Finney <ben+python@benfinney.id.au> - 2011-12-23 09:33 +1100
    Re: Can't I define a decorator in a separate file and import it? Andrew Berg <bahamutzero8825@gmail.com> - 2011-12-22 15:20 -0600
    Re: Can't I define a decorator in a separate file and import it? Ethan Furman <ethan@stoneleaf.us> - 2011-12-22 13:31 -0800
    Re: Can't I define a decorator in a separate file and import it? Ethan Furman <ethan@stoneleaf.us> - 2011-12-22 13:07 -0800

#17749 — Can't I define a decorator in a separate file and import it?

I'm using this decorator to implement singleton class in python:

http://stackoverflow.com/posts/7346105/revisions

The strategy described above works if and only if the Singleton is
declared and defined in the same file. If it is defined in a different
file and I import that file, it doesn't work.

Why can't I import this Singleton decorator from a different file?
What's the best work around?

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#17750

BTW Here is the traceback:

>>> import myClass
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "myClass.py", line 6, in <module>
    @Singleton
TypeError: 'module' object is not callable



Here is Singleton.py:



class Singleton:

    def __init__(self, decorated):
        self._decorated = decorated

    def Instance(self):
        try:
            return self._instance
        except AttributeError:
            self._instance = self._decorated()
            return self._instance

    def __call__(self):
        raise TypeError(
            'Singletons must be accessed through the `Instance`
method.')



Here is myClass.py:

#!/usr/bin/env python
import os, sys, string, time, re, subprocess
import Singleton


@Singleton
class myClass:

    def __init__(self):
        print 'Constructing myClass'

    def __del__(self):
        print 'Destructing myClass'

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#17756

Saqib Ali <saqib.ali.75@gmail.com> writes:

> BTW Here is the traceback:
>
> >>> import myClass
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "myClass.py", line 6, in <module>
>     @Singleton
> TypeError: 'module' object is not callable

Yes. When you ‘import foo’, you have a module bound to the name ‘foo’. A
module is not callable.

> Here is Singleton.py:

You should name the file ‘singleton.py’ instead; modules should be named
in all lower-case, as PEP 8 recommends.

> Here is myClass.py:

Which should be named ‘my_class.py’, instead, by the same rationale.

Neither of those is necessary, but it will help dispel the confusion
you're experiencing, and help your code be more easily comprehensible to
other Python programmers.

> #!/usr/bin/env python
> import os, sys, string, time, re, subprocess
> import Singleton

So, you should (after the renames suggested) do either::

    import singleton

    @singleton.Singleton
    class my_class:

or::

    from singleton import Singleton

    @Singleton
    class my_class:

Notice how naming the module in lower-case makes it more easily
distinguishable from the class name.

More generally, remember that Python is not Java
<URL:http://dirtsimple.org/2004/12/python-is-not-java.html>, and you
should be grouping your classes into logically coherent files, not
one-file-per-class.

-- 
 \     “Reality must take precedence over public relations, for nature |
  `\       cannot be fooled.” —Richard P. Feynman, _Rogers' Commission |
_o__)                       Report into the Challenger Crash_, 1986-06 |
Ben Finney

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#17751

You have the same code for both files...

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#17753

Saqib Ali wrote:
> MYCLASS.PY:
> 
> #!/usr/bin/env python
> import os, sys, string, time, re, subprocess
> import Singleton

This should be 'from Singleton import Singleton'


> @Singleton
> class myClass:
>     
>     def __init__(self):
>         print 'Constructing myClass'

At this point, the *instance* of myClass has already been constructed 
and it is now being initialized.  The __new__ method is where the 
instance is actually created.

> 
>     def __del__(self):
>         print 'Destructing myClass'
> 
> 
 > class Singleton:
 >
 >     def __init__(self, decorated):
 >         self._decorated = decorated
 >
 >     def Instance(self):
 >         try:
 >             return self._instance
 >         except AttributeError:
 >             self._instance = self._decorated()
 >             return self._instance
 >
 >     def __call__(self):
 >         raise TypeError(
 >             'Singletons must be accessed through the `Instance`
 > method.')


~Ethan~

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#17754

Saqib Ali wrote:
> 
> I'm using this decorator to implement singleton class in python:
> 
> http://stackoverflow.com/posts/7346105/revisions
> 
> The strategy described above works if and only if the Singleton is
> declared and defined in the same file. If it is defined in a different
> file and I import that file, it doesn't work.
> 
> Why can't I import this Singleton decorator from a different file?
> What's the best work around?

Post the code, and the traceback.

~Ethan~

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