You're confusing loading a module into the current interpreter process and calling a Python script externally.
The former can be done by import
ing the file you're interested in. execfile is similar to importing but it simply evaluates the file rather than creates a module out of it. Similar to "sourcing" in a shell script.
The latter can be done using the subprocess module. You spawn off another instance of the interpreter and pass whatever parameters you want to that. This is similar to shelling out in a shell script using backticks.