How to check submodules in Python with hasattr

Question

At runtime, the Python code gets the name of a submodule to load, which I don't know before. Now, I want to check, if this submodule exists inside an existing module. Consider this structure, where foo and bar can be specified:

master/
|
|- __init__.py
|
|- foo/
|  |
|  |- __init__.py
|
|- bar/
   |
   |- __init__.py

Now, usually I do this, which works for defs and variables:

import master

unknown_submodule = "foo"
if hasattr(master, unknown_submodule):
    pass # all's well

or I'm catching the AttributeError, which works equally.

However, with the above file structure, I'm not able to bring this approach up and working. hasattr() returns always False (that is, there is always an AttributeError thrown).

If I look at dir(master), I see this output:

['__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__']

and even explicitly specifying __all__ in master/__init__.py doesn't help, but changes the dir() to

['__all__', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__path__']

Any idea, what I'm doing wrong, or if there is a way to achieve these kinds of tests? (Btw: Python 2.6 on Win/Cygwin, if that's of any interest)

Answer 1

Submodules are not attributes of their parent modules unless stated explicitly. Simply try to import the module and catch ImportError:

try:
    __import__("os.peth", fromlist=[os])
except ImportError:
    pass

Answer 2

you can do

try:
 import module.submodule

except ImportError:
  print 'failed or whatever'

Answer 3

I recently had to check submodule existence and since I'm using python 3.4.1, I'm giving a new answer to the question:

In python 3.4.1 you can use importlib.util.find_spec(name, package=None) (https://docs.python.org/3.4/library/importlib.html#importlib.util.find_spec)

import importlib
module_exists = importlib.util.find_spec('path.to.my.module')

That easy =)