Python Implementation: Summer of 69
Your code looks good. It's always nice to see people following PEP 8. There is a small issue though with how you check if a list is empty or not. Currently you write 
elif not len(exclude_stack): or elif num == 9 and len(exclude_stack): but PEP 8 actually has the following recommendation for this kind of tests:


  For sequences, (strings, lists, tuples), use the fact that empty
  sequences are false.

Yes: if not seq:  
     if seq:

No:  if len(seq):  
     if not len(seq):



So, in your case, it should be elif not exclude_stack: and elif num == 9 and exclude_stack:.

Additionally, the details on implementation in the docstring look redundant, I would remove them and leave only the task description.

The main question I have regarding your implementation is: Do you really need to have the stack to keep track if you are inside the 6-9 section? If I print exclude_stack for this input summer_sum([6] * 10 + [9] * 10), it will look like this:

[0]
[0, 0]
[0, 0, 0]
[0, 0, 0, 0]
[0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0]
[0, 0, 0, 0]
[0, 0, 0]
[0, 0]
[0]
[]


This doesn't really look like the most optimal solution memory-wise. How about keeping a track of the 6-9 sections by simply incrementing/decrementing some integer variable instead? Something like this:

def summer_sum(a_list):
    """Solution for Summer 69 challenge"""
    section_depth = 0
    total = 0
    for num in a_list:
        if num == 6:
            section_depth += 1
        elif section_depth == 0:
            total += num
        elif num == 9 and section_depth &gt; 0:
            section_depth -= 1
    return total


I assume this will be also a bit faster as you won't have to pop or append to a list.

My hands are also itching to remove the total variable and make a generator function, but this may be a bit overboard:

def summer_sum(a_list):
    """Solution for Summer 69 challenge"""

    def free_numbers(numbers):
        section_depth = 0
        for num in numbers:
            if num == 6:
                section_depth += 1
            elif section_depth == 0:
                yield num
            elif num == 9 and section_depth &gt; 0:
                section_depth -= 1

    return sum(free_numbers(a_list))