Assertions are a nice way to make your code ensure your assumptions are verified. If they are, all proceeds regularly but, if they are not, you get a nice exception that you can work with. Sometimes, instead of inspecting, it's quicker to drop a couple of assertions in the code just to exclude possibilities. Let's see an example:

# assertions.py
mylist = [1, 2, 3]  # this ideally comes from some place
assert 4 == len(mylist)  # this will break
for position in range(4):
    print(mylist[position])

This code simulates a situation in which mylist isn't defined by us like that, of course, but we're assuming it has four elements. So we put an assertion there, and the result is this:

$ python assertions.py
Traceback (most recent call last):
  File "assertions.py", line 3, in <module>
    assert 4 == len(mylist)  # this will break
AssertionError

This tells us exactly where the problem is.