I cannot find the error in my code


Traceback (most recent call last):
File "python", line 12, in
File "python", line 7, in flatten
AttributeError: 'str' object has no attribute 'append'

Replace this line with your code. 
n = [[1, 2, 3], [4, 5, 6, 7, 8, 9]]
# Add your function here
def flatten(lists) :
    results =''
    for number in lists :
        for i in range(len(number)) :
    return results

print flatten(n)


append() is for lists. You created results as an empty string so append() won't work for it.
Do you mean to create it as an empty list?


results = '' should be results = [].

For every sublist in lists and for every number in sublist, append the number to results, like so:

for sublist in lists:
    for num in sublist:


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