Checking for a series of uniform non-zero digits def standvastig(reeks): 
    """
    Checks if the given input (list or number) is a series of one number (eg. 444444, 2222, etc.)
    Remark: raise an error if the input is a series of 0s or non-integers

    &gt;&gt;&gt; standvastig(44444444)
    True
    &gt;&gt;&gt; standvastig('44544444')
    False
    """
    if str(reeks).isdigit() == False or int(str(reeks)) == 0:
        raise AssertionError('ongeldig serienummer')# gives an error when the input is not a list of integers or 0s

    if any([str(reeks) == '1'*len(str(reeks)), str(reeks) == '2'*len(str(reeks)), str(reeks) == '3'*len(str(reeks)), str(reeks) == '4'*len(str(reeks)), str(reeks) == '5'*len(str(reeks)), str(reeks) == '6'*len(str(reeks)), str(reeks) == '7'*len(str(reeks)), str(reeks) == '8'*len(str(reeks)), str(reeks) == '9'*len(str(reeks))]):
        return(True)
    else: return(False)

if __name__ == "__main__":
    import doctest
    doctest.testmod()


Can this code be written more pythonic? Mainly the if condition.
Here are a couple of suggestions for you to implement:


You convert reeks into a string far more frequently than is necessary; just do it once, at the top of the function.
The correct exception to raise if a value is not acceptable is ValueError. 
Rather than thing == False, you can use the more readable not thing.
Your approach to the solution seems strange to me; rather than "are all of the digits one, or are all of the digits two, or are all of the digits...", why not "are all of the digits the same as the first digit"?
Python has a style guide, which discourages e.g. redundant parentheses and multi-statement lines:

else:
    return False