Get list of all combinations from a set
Style

I suggest you check PEP0008 https://www.python.org/dev/peps/pep-0008/ the official Python style guide while writing your code and the following goes accordingly:


Docstrings: Python Docstring is the documentation string which is string literal, and it occurs in the class, module, function or method definition, and it is written as a first statement. Docstrings are accessible from the doc attribute for any of the Python object and also with the built-in help() function can come in handy. I suggest you include docstrings to your functions indicating what they do and what they return instead of writing a comment above each function.
Python naming conventions: 

def getCombinations(elementTupleParam, rParam):

def partialCombinations(prevTuple, prevVisitedIndex):

Function names should be lowercase, with words separated
by underscores as necessary to improve readability same goes for
parameter names.
Descriptive variable names: thisIndex thisTuple prevTuple are examples of bad non-descriptive names. Names should reflect the significance of the variable/function/parameter they represent and the less ambiguous they are, the more readable is your code.


Code


Global variables: are bad in Python and programming languages in general and are advised against. I suggest enclosing variables inside functions. The reason they are bad that they may allow functions to have hidden/hard to detect side effects leading to an increased complexity potentially leading to Spaghetti code. Examples of good use of global variables include: algorithm optimization - caching and memoization. 
There is a built-in Python library itertools that does the same functionality using itertools.combinations() which saves you the effort of implementing the thing yourself.

Code can be shortened into the following:
And according to GZO's comment, there is no need for the warpper function get_combinations(), use combinations() directly. I included it just for the sake of the example.

from itertools import combinations


def get_combinations(n: list, r: int):
    """Return combinations iterator."""
    return combinations(n, r)


if __name__ == '__main__':
    print(list(get_combinations([2, 3, 4], 2)))



Check these references(regarding global variables):


https://softwareengineering.stackexchange.com/questions/148108/why-is-global-state-so-evil
https://stackoverflow.com/questions/19158339/why-are-global-variables-evil