concatenating list with tuple

[Skaperen]

i can do mylist+=mytuple but i cannot do mylist=mylist+mytuple. why is that?

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]
>>> b=(4,5,6)
>>> a+=b
>>> a=a+b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "tuple") to list
>>>

lt1/forums /home/forums 1> py2
Python 2.7.12 (default, Dec  4 2017, 14:50:18) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a=[1,2,3]
>>> b=(4,5,6)
>>> a+=b
>>> a=a+b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "tuple") to list
>>>

and mylist.extend(mytuple) also works.