In Geomatica there is no concept of a workspace, like in ArcPy. However, there are several ways of doing something similar in python:
1) You can change the current working directory using os.chdir() - https://docs.python.org/2/library/os.html#os-file-dir
2) You can set a variable that represents the working directory, and then use this to create other variables that represent files using os.path.join - https://docs.python.org/2/library/os.path.html#os.path.join
root_dir = 'path/to/root'
my_file = os.path.join(root_dir, 'mytestfile.pix')
You may wish to take a look at the online python documentation (https://www.python.org/doc/) to learn about the os module, and some of the other python standard library modules, as these are very useful. One of the best features of python is its community. There are many great resources available on the web. For example, here is a good quick tutorial on the python standard library- https://docs.python.org/2/tutorial/stdlib.html.
Hope this helps,