I installed pytest into a virtual environment (using virtualenv) and am running it from that virtual environment, but it is not using the packages that I installed in that virtual environment. Instead, it is using the main system packages. (Using python -m unittest discover, I can actually run my tests with the right python and packages, but I want to use the py.test framework.)
Is it possible that py.test is actually not running the pytest inside the virtual environment and I have to specify which pytest to run?
How to I get py.test to use only the python and packages that are in my virtualenv?
Also, since I have several version of Python on my system, how do I tell which Python that Pytest is using? Will it automatically use the Python within my virtual environment, or do I have to specify somehow?
There is a bit of a dance to get this to work:

activate your venv : source venv/bin/activate
install pytest : pip install pytest
re-activate your venv: deactivate && source venv/bin/activate

The reason is that the path to pytest is set by the sourceing the activate file only after pytest is actually installed in the venv. You can't set the path to something before it is installed.
Re-activateing is required for any console entry points installed within your virtual environment.
How do I use pytest with virtualenv?