Fixed.

Okay to be honest, I didn't fully understand the google explanation of mutation. Thanks for the clarification. I solved it without now.

correct! ;)

I based my assumption in the expected results message ;)

Mutating the input means...

...or using pop, append, extend (the last two are not probable usages here, but...) on one of the lists (which should actually be tuples... EDIT: that's the perfect indication: if one single thing you do with the input couldn't be done with tuples, you're mutating the inputs)

If you mutate the input those cases change from those values to these (an additional of 150 points added to whoever catched the snitch):

Testing for [50, 'yes'] and [300, 'no'] -> after the end of the game it's 350 vs 300 # Gryffindor wins
Testing for [200, 'no'] and [50, 'yes'] -> here it's 200 vs 350 # Slytherin wins
Testing for [300, 'no'] and [50, 'yes'] -> and here 300 vs 350 # Slytherin wins

Mutating the input (in this case) means you're doing gryffindor[0] = ... and slytherin[0] = ... in your code (or +=).
Because the control function uses the mutated values as input. I've solved it without mutating the input and the tests passed. Could you try doing that? Or just wait for the issue to be resolved.

I take the list-elements as they are, no casting or anything.

but still, as I add the numbers as a person would do (not a computer) the answer is still wrong (Gryffindor wouldn't win when it's 100 points behind)

Translation to fix, please (see issue above)

2 posts below, someone reported an issue about input mutation in Python, are you doing that?

I believe in python that your random tests don't always work. I copied some of my output:
Testing for [50, 'yes'] and [300, 'no'] -> after the end of the game it's 200 vs 300
It should work for random inputs too: 'Slytherin wins!' should equal 'Gryffindor wins!'
Testing for [200, 'no'] and [50, 'yes'] -> here it's 200 vs 200
It should work for random inputs too: "It's a draw!" should equal 'Slytherin wins!'
Testing for [300, 'no'] and [50, 'yes'] -> and here 300 vs 200
It should work for random inputs too: 'Gryffindor wins!' should equal 'Slytherin wins!'