I'm doing some exercises in Python and I came across a doubt. I have to set a list containing the first three elements of list, with the .append method. The thing is, I get an assertion error, lists don't match. If I print list_first_3 I get "[['cat', 3.14, 'dog']]", so the double square brackets are the problem. But how can I define the list so the output matches?
list = ["cat", 3.14, "dog", 81, 6, 41] list_first_3 = [] list_first_3.append(list[:3]) assert list_first_3 == ["cat", 3.14, "dog"] append can only add a single value. I think what you may be thinking of is the extend method (or the += operator)
list1 = ["cat", 3.14, "dog", 81, 6, 41] list_first_3 = [] list_first_3.extend(list1[:3]) assert list_first_3 == ["cat", 3.14, "dog"] or
list1 = ["cat", 3.14, "dog", 81, 6, 41] list_first_3 = [] list_first_3 += list1[:3] assert list_first_3 == ["cat", 3.14, "dog"] otherwise you'll need a loop:
list1 = ["cat", 3.14, "dog", 81, 6, 41] list_first_3 = [] for value in list1[:3]: list_first_3.append(value) assert list_first_3 == ["cat", 3.14, "dog"] with append but without a loop would be possible using a little map() trickery:
list1 = ["cat", 3.14, "dog", 81, 6, 41] list_first_3 = [] any(map(list_first_3.append,list1[:3])) assert list_first_3 == ["cat", 3.14, "dog"] When appending a list to a list, the list becomes a new item of the original list:
list_first_3 == [["cat", 3.14, "dog"]] You are looking for:
list_first_3 += list[:3] # ["cat", 3.14, "dog"] This adds every item from list to list_first_3.
Also you shouldn't name your variables like inbuilt types like list.
If you NEED to append, you could use a for-loop:
list_first_three = [] for item in list[:3]: list_first_three.append(item) Your problem is that you try to append a list to another list.
list[:3] will return you the result ["cat", 3.14, "dog"] Then, you take that result as a whole and put it as an item in list_first_3. If you want to fix that, you can do:
list = ["cat", 3.14, "dog", 81, 6, 41] list_first_3 = [] list_first_3 += list[:3] assert list_first_3 == ["cat", 3.14, "dog"] # Return True And if you insist using append method:
list = ["cat", 3.14, "dog", 81, 6, 41] list_first_3 = [] for item in list[:3] list_first_3.append(item) assert list_first_3 == ["cat", 3.14, "dog"] # Return True 
list_first_3 = list[:3]. This will set the value oflist_first_3to what you want it to be.