Why python shows error for a similar method but with different syntax where one of them works fine but another one give error as shown below

Question

letters=["A","B","C"..............,"X","Y"]
password_list=[]

for i in range(1 , 5):
         password_list += random.choice(letters) <--- No error

is working but,

password_list=[]

for i in range(1 , 5):
         password_list = password_list +  random.choice(letters) <--- error

giving error of cannot concatenate string, if both are same just syntax is different then why it is giving error?

can someone explain this briefly?

PS:- if required i will attach the Screenshot of problem.

Thank you

not a minimal reproducible example: NameError on nr_letters, letters + missing imports. Why iterate over for char in range(1,nr_letters+1) and never use char (which is a number, not a char) instead of iterating for _ in range(nr_letters): wich iterates as much. — Patrick Artner
– Patrick Artner, Commented Jun 23, 2021 at 7:37

rdas · Accepted Answer · 2021-06-23 07:40:08Z

3

They are not equivalent.

The += operator on a list calls the __iadd__ magic method, which does accept single valued arguments.

The + operator on a list calls the __add__ magic method, that just expects another iterable.

l = []

l.__iadd__('s') # no error
l.__add__('s') # Error

answered Jun 23, 2021 at 7:40

rdas

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Mustafa Aydın · Accepted Answer · 2021-06-23 07:52:08Z

In the first case __iadd__ i.e., in-place add method of the list password_list is called and it can work with any iterable e.g., a string in your case.

In the other one __add__ method of password_list is called and it can only work with lists, which random.choice(letters) isn't one.

Another difference is that first one doesn't make any new list; it changes the place where password_list looks at. Second one, however, generates a brand new list and then makes password_list look at there.

samples:

>>> a = [12, 51]
>>> a += range(2)    # *any* iterable
>>> a
[12, 51, 0, 1]

>>> a += ("this", "that")
>>> a
[12, 51, 0, 1, "this", "that"]

>>> a += "strings"
>>> a
[12, 51, 0, 1, "this", "that", "s", "t", "r", "i", "n", "g", "s"]

>>> a = a + (64, 23)
# TypeError... Even tuples are not acceptable; it must be a list

Patrick Artner · Accepted Answer · 2021-06-23 07:41:10Z

You are most probably trying to concattenate a string to a list - wich is an error!

import random       # interpolated - missing in your post

password_list = []  # this is a list
letters = "abcdefg" # interpolated - missing in your post
nr_letters = 25     # interpolated - missing in your post

for _ in range(nr_letters):
     # tries to concat a random string to your list
     password_list = password_list + random.choice(letters)

Fix:

 import random

 password_list = []  # this is a list
 letters = "abcdefg" # interpolated - missing in your post
 nr_letters = 25     # interpolated - missing in your post

 for _ in range(nr_letters):
      password_list = password_list + [random.choice(letters)] 
      # password_list.append(random.choice(letters))

 # or better:

 password_list = random.choices(letters, k=nr_letters)

That password_list += random.choice(letters) works is because it is similar to password_list.append(random.choice(letters))

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