How to import a class from a module from a package dynamically?

Question

This is probably really simple, but I don't understand it. The following works:

foo.py

class Foo:
    pass

bar.py

module = __import__('foo')
foo = module.__dict__['Foo']

Afterwards, foo is the class foo.Foo as expected.

Yet, if I put a.py into a package, it stops working:

qux/init.py

(empty file)

qux/foo.py

class Foo:
    pass

bar.py

module = __import__('qux.foo')
foo = module.__dict__['Foo']

Running python bar.py gives me KeyError: 'Foo', but the module import is still successful.

What's going on, and how do I get it to work?

Walter Mundt · Accepted Answer · 2010-08-26 19:08:53Z

3

If you want to import a submodule, you can do something like this:

package = __import__('qux', fromlist=['foo'])
module = getattr(package, 'foo')

Note that with a fromlist the __import__ returns the most nested name in the first parameter, so you could substitute baz.bar.qux in the __import__ call to access the baz.bar.qux.foo module.

answered Aug 26, 2010 at 19:08

Walter Mundt

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1 Comment

Joel Mellon Over a year ago

I'm sure this is very Pythonic and makes perfect sense to many people, but man is this convoluted to me. Thanks a million.

Alexander Gessler · Accepted Answer · 2010-08-26 19:01:47Z

3

__import__ applied to nested module names returns the toplevel module/package - which is qux here. All modules are, however, inserted into sys.modules, so you can simply lookup qux.foo in there.

See the PyDoc on __import__() -it's all described there.

answered Aug 26, 2010 at 19:01

Alexander Gessler

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2 Comments

Walter Mundt Over a year ago

Note that 'import qux.foo' works analogously -- qux ends up in the local namespace, but foo is only accessible as an attribute of qux.

Thomas Over a year ago

Indeed, it suddenly and mysteriously appeared in the docs now that I look again: "When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned, not the module named by name. However, when a non-empty fromlist argument is given, the module named by name is returned."

jldupont · Accepted Answer · 2012-01-19 18:40:57Z

2

You can directly access a module, nested or not, using importlib:

import importlib
module_name="qux.foo"
mod=importlib.import_module(module_name)

See http://docs.python.org/dev/library/importlib.html for more details.

answered Jan 19, 2012 at 18:40

jldupont

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Comments

Bob · Accepted Answer · 2010-08-26 19:08:54Z

1

You need to use the fromlist parameter to reference a submodule:

temp = __import__('qux.foo', globals(), locals(), ['Foo'], -1)
foo = temp.Foo

edited Aug 26, 2010 at 19:08

answered Aug 26, 2010 at 19:03

Bob

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