Adding a string to a list using augmented assignment

Question

>>> b = []
>>> c = '1234'
>>> b += c
>>> b
['1', '2', '3', '4']
>>> 

What is happening here? This should not work, right? Or am I missing something obvious?

>>> b = []
>>> c = '1234'
>>> b + c
Traceback (most recent call last):
  File "<pyshell#7>", line 1, in <module>
    b + c
TypeError: can only concatenate list (not "str") to list
>>> 

Then a += b is not always equivalent to a = a + b ?

Answer 1

Strings are iterable: the elements are the string's characters. When you add an iterable to a list, the iterable's elements get appended to the list.

Either of the following will do what you're expecting (i.e. append the string, not extend the list with the string's characters):

b += [c]

or

b.append(c)

Answer 2

The += operator extends a list instead of appending to it:

>>> b = []
>>> c = "1234"
>>> b.append(c)
>>> b
['1234']
>>> b.extend(c)
>>> b
['1234', '1', '2', '3', '4']
>>> b += c
>>> b
['1234', '1', '2', '3', '4', '1', '2', '3', '4']
>>> b += [c]
>>> b
['1234', '1', '2', '3', '4', '1', '2', '3', '4', '1234']