Difference between 'append()' and list element without square brackets

Question

Instruction: The get_name_list() function returns a list of all the names in the parameter: name_list, with a given letter provided in parameter: to_look_for.

Question: If I remove the square brackets of 'name' in 'a_list += [name]' expression, it will show the following wrong output. However, if the square brackets or 'append()' method is used, the right output will be produced (2nd correct output below). I wonder why without the square brackets in [name], the right output is not produced?

Wrong output:

names with d ['J', 'a', 'd', 'e']
names with k ['M', 'i', 'k', 'e', 'y']

Wrong code:

def main():
    names = ["Jasper", "Jade", "Mikey", "Giani"]
    names_d = get_name_list(names, "d")
    names_k = get_name_list(names, "k")
    print("names with d", names_d)
    print("names with k", names_k)

    def get_name_list(name_list, to_look_for):

        a_list = [] 
        for name in name_list:
            #print(name)
            if to_look_for in name:
                print(name)
                a_list += name
                #a_list.append(name)

        return a_list

    main()

Correct output:

names with d ['Jade']
names with k ['Mikey']

Correct code:

def main():
    names = ["Jasper", "Jade", "Mikey", "Giani"]
    names_d = get_name_list(names, "d")
    names_k = get_name_list(names, "k")
    print("names with d", names_d)
    print("names with k", names_k)    

def get_name_list(name_list, to_look_for):

    a_list = [] 
    for name in name_list:
        #print(name)
        if to_look_for in name:
            print(name)
            #a_list += name
            a_list.append(name)

    return a_list

main()

Stephen Lin · Accepted Answer · 2014-09-22 03:19:30Z

3

'+=' operation is equal to extend() of list built-in method. It will change value right to equal-sign to a list first and then do extending.

In your case:

a_list += name  # which is the same as a_list.extend(name), although name is a string, it will be converted to a list first

When a string is converted to a list, it will act like changing 'abc' to ['a', 'b', 'c']. That's where your ['J', 'a', 'd', 'e'] comes from

edited Sep 22, 2014 at 3:19

answered Sep 22, 2014 at 3:13

Stephen Lin

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2 Comments

jmk Over a year ago

Hi, thank you for your answer. I kind of got your idea: When a string is converted to a list, it will act like changing 'abc' to ['a', 'b', 'c']. One question: The 'name' is an element of the list: 'name_list', therefore, 'name' should be a list not a string. I wonder why 'name' is a string?

Stephen Lin Over a year ago

@jmk 'for name in name_list' means you assign every element in name_list to variable name. And each element in name_list is a string. You can check with type(name_list[0]). If you want to get first element as a list, for instance, you should use name_list[0:1].

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