Problems loading dynamic python module using init.py

Question

I'm trying to do an __init__.py that load module and instantiate dinamically internal namesake class. With a file tree like

 importer/
     __init__.py         # The file i'm writing
     asd.py              # It contains class asd
     bsd.py              # It contains class bsd

And __init__.py with

importername=__name__

def load(modname,paramlist = []):

  if modname in globals():
    print 'Module %s already loaded.' % (modname)
  else:
    imported_mod = __import__(importername + '.' + modname, fromlist = ["*"])
    try:
        globals()[modname]=getattr(imported_mod,modname)(*paramlist)
    except Exception, e:
        print 'Module %s not loaded: %s.' % (modname, e)

If I run

import importer
importer.load('asd')
print importer.asd.name
'ASD!'

It works like a charm, but if I run

from importer import *
load('asd')
print asd.name
Traceback (most recent call last):
  File "<string>", line 1, in <module>
NameError: name 'asd' is not defined

Can I fix it in some way? Thank you.

Dynamically updating global variables is considered bad practice in Python. What do you actually want this for? There's probably a better way. — Thomas K
– Thomas K, Commented May 10, 2011 at 21:37

Ned Batchelder · Accepted Answer · 2011-05-10 21:58:54Z

2

There is no such thing as truly global in Python. When you use globals() in importer/__init__.py, you are accessing the module's namespace. Your second non-working sample is adding 'asd' to importer, not to your test module.

The best you could is modify importer so you could do:

from importer import *
asd = load('asd')
print asd.name

But as Thomas K suggests: this is kind of odd, perhaps there's a simpler way to solve your problem?

answered May 10, 2011 at 21:58

Ned Batchelder

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7 Comments

Emilio Over a year ago

I need a sort of set of singleton classes, dynamically loadable, and reachable in every part of the program, like as import statement offer.

Thomas K Over a year ago

@Emilio: Um, that doesn't make it clearer, at least to me. What problem are you actually trying to solve?

Emilio Over a year ago

I need a sort of plugin factory, loaded a single time, that have to load and instantiate dinamically plugins in a directory (with .load() and .unload()). The 'import' way is ok because i need to reach this factory from everywhere in the code.

Thomas K Over a year ago

@Emilio: Can you, for example, store the loaded plugins in a list or a dict? You could even subclass dict, so that when you do asd = plugins['asd'], it loads the asd plugin if it isn't already loaded. Loading plugins is the easy bit: working out the best way to reference them in your code is what's interesting.

Emilio Over a year ago

If i initialize this plugin{} dict in a module moddict.py, i can import it from everywhere calling import moddict and using as moddict.plugin['pluginname'].method(). And it's ok, but is it cleaner than my solution?

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