TypeError: can only concatenate list (not "str") to list

[Skaperen]

Output:
lt1/forums /home/forums 1> py3
Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a=[1,2,3]+'foobar'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "str") to list
>>> a=[1,2,3]
>>> a+='foobar'
>>>

so, do you know what a now references? did you need to cheat?