CODEWITHSUNDEEP
pythonpython-3.xlistappendelement
Jonathan Francis
Jonathan Francis

Reputation: 113

How come using += to add to a list in python without using square brackets adds one character at a time?

I'm trying to understand why, computationally, using += to add to a list, where square brackets have not been used to encapsulate the value, results in a single character at a time being added to the list as an element.

I hope that the question is clear; here is an example:

In:


def generate_sentences(subjects, predicates, objects):
    lst1 = []
    lst2 = []
    lst3 = []
    lst4 = []
    lst5 = []
    subjects = sorted(subjects)
    predicates = sorted(predicates)
    objects = sorted(objects)
    for i in subjects:
        for j in predicates:
            for k in objects:
                lst1 += i + " "
                lst2 += (i + " ")
                lst3 += [i + " "]
                lst4.append(i + " ")
                lst5.append([i + " "])


    print("+= no paren: ")
    print(lst1)
    print(" ")
    print("+= paren: ")
    print(lst2)
    print(" ")
    print("+= brackets: ")
    print(lst3)
    print(" ")
    print("append standard: ")
    print(lst4)
    print(" ")
    print("append with brackets: ")
    print(lst5)



generate_sentences(["John", "Mary"], ["hates", "loves"],\
    ["apples", "bananas"])

generate_sentences(["Vlad", "Hubie"], ["drives"],\
["car", "motorcycle", "bus"])

and

Out:


+= no paren: 
['H', 'u', 'b', 'i', 'e', ' ', 'H', 'u', 'b', 'i', 'e', ' ', 'H', 'u', 'b', 'i', 'e', ' ', 'V', 'l', 'a', 'd', ' ', 'V', 'l', 'a', 'd', ' ', 'V', 'l', 'a', 'd', ' ']

+= paren: 
['H', 'u', 'b', 'i', 'e', ' ', 'H', 'u', 'b', 'i', 'e', ' ', 'H', 'u', 'b', 'i', 'e', ' ', 'V', 'l', 'a', 'd', ' ', 'V', 'l', 'a', 'd', ' ', 'V', 'l', 'a', 'd', ' ']

+= brackets: 
['Hubie ', 'Hubie ', 'Hubie ', 'Vlad ', 'Vlad ', 'Vlad ']

append standard: 
['Hubie ', 'Hubie ', 'Hubie ', 'Vlad ', 'Vlad ', 'Vlad ']

append with brackets: 
[['Hubie '], ['Hubie '], ['Hubie '], ['Vlad '], ['Vlad '], ['Vlad ']]

Upvotes: 0

Views: 76

Answers (1)

Quentin EmacsFTW
Quentin EmacsFTW

Reputation: 126

I think I understand your issue, let me know if I got this correct or not :). So there are several things to this behavior, First, the difference between append and += is that :

+= merges 2 lists into one list.

append adds an element to a list.

So fundamentally they are not the same thing.

Second, a string is basically a list of characters, so when you are doing this :

mylist += "a word"

You are basically doing this :

mylist += ['a', ' ', 'w', 'o', 'r', 'd']

Try adding an int to your list with a +=, this will occur :

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

On the other end if you use append to add an integer, it will work because it is not adding a list but an item that can be any type.

I think I answered your question, let me know if you need clarification :)

Upvotes: 1

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