marcot/ posts/ Intuitive python 
>>> x = ['a', 'b']
>>> x
['a', 'b']
>>> x += 'c'
>>> x
['a', 'b', 'c']
>>> x += ''
>>> x
['a', 'b', 'c']
>>> x += ['']
>>> x
['a', 'b', 'c', '']
for better understanding

you might want to add:

>>> x += 'foo'
>>> x
['a', 'b', 'c', '', 'f', 'o', 'o']

and maybe

>>> x.append('foo')
>>> x
['a', 'b', 'c', '', 'f', 'o', 'o', 'foo']
Comment by Anonymous — Qui 02 Fev 2012 13:50:04 UTC
for better understanding

you might want to add:

>>> x += 'foo'
>>> x
['a', 'b', 'c', '', 'f', 'o', 'o']

and maybe

>>> x.append('foo')
>>> x
['a', 'b', 'c', '', 'f', 'o', 'o', 'foo']
Comment by Anonymous — Qui 02 Fev 2012 13:50:20 UTC
for better understanding

you might want to add:

>>> x += 'foo'
>>> x
['a', 'b', 'c', '', 'f', 'o', 'o']

and maybe

>>> x.append('foo')
>>> x
['a', 'b', 'c', '', 'f', 'o', 'o', 'foo']
Comment by Anonymous — Qui 02 Fev 2012 13:52:18 UTC
From chrysn

you might want to add:

x += 'foo' x ['a', 'b', 'c', '', 'f', 'o', 'o']

and maybe

x.append('foo') x ['a', 'b', 'c', '', 'f', 'o', 'o', 'foo']

Comment by Marco Túlio — Qui 02 Fev 2012 14:08:13 UTC
From chrysn

you might want to add:

x += 'foo' x ['a', 'b', 'c', '', 'f', 'o', 'o']

and maybe

x.append('foo') x ['a', 'b', 'c', '', 'f', 'o', 'o', 'foo']

Comment by Marco Túlio — Qui 02 Fev 2012 14:08:21 UTC
From chrysn

you might want to add:

x += 'foo' x ['a', 'b', 'c', '', 'f', 'o', 'o']

and maybe

x.append('foo') x ['a', 'b', 'c', '', 'f', 'o', 'o', 'foo']

Comment by Marco Túlio — Qui 02 Fev 2012 14:09:15 UTC
To a list you can += (__iadd__) any iterable, and a string is a iterable of chars.

It seems that for a list x, the statement: 

x += y
is internally translated into: 
for elem in y:
   x.append(elem)

Whereas for a list x, the statement 

x + y
requires y to be a list.

Python can iterate over a string, it's individual characters. 

x = ["a", "b"]
x += "cd"
   => x becomes ["a", "b", "c", "d"]

x = ["a", "b"]  
x = x + "cd"  
   => ERROR: can only concatenate list (not "str") to list

The same difference we see with sets: 

x = ["a", "b"]  
x += set(["c", "d"])  
   => x becomes ["a", "b", "c", "d"]  

x = ["a", "b"]  
x = x + set(["c", "d"])  
   => ERROR: can only concatenate list (not "str") to list

Also, since a string is not iterable, this does not work: 

x = ["a", "b"]  
x += 1  
   => ERROR: 'int' object is not iterable
Comment by Sander — Qui 02 Fev 2012 16:57:19 UTC
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