How can python call a class that is never defined in the code?

Question

I don't know if it is feasable to paste all of the code here but I am looking at the code in this git repo.

If you look at the example they do:

ec2 = EC2('access key id', 'secret key')

...but there is no EC2 class. However, it looks like in libcloud\providers.py there is a dict that maps the EC2 to the EC2NodeDriver found in libcloud\drivers\ec2.py. The correct mapping is calculated by get_driver(provider), but that method doesn't appear to be called anywhere.

I am new to python, obviously, but not to programming. I'm not even sure what I should be looking up in the docs to figure this out.

Answer 1

example.py includes an import statement that reads:

from libcloud.drivers import EC2, Slicehost, Rackspace

This means that the EC2 class is imported from the libcloud.drivers module. However, in this case, libcloud.drivers is actually a package (a Python package contains modules), which means that EC2 should be defined in a file __init__.py in libcloud/drivers/, but it's not. Which means that in this specific case, their example code is actually wrong. (I downloaded the code and got an import error when running example.py, and as you can see, the file libcloud/drivers/__init__.py does not contain any definitions at all, least of all an EC2 definition.)

Answer 2

Checking out the libcloud\examples.py might be helpful. I saw this:

from libcloud.drivers import EC2, Slicehost, Rackspace

The python 'import' statement brings in the class from other python module, in this case from the libcloud.drivers module.