Daniel Urban <urban.dani...@gmail.com> added the comment:
The reason of this behaviour is that x += 1 basically is the same as x =
x.__iadd__(1). In the tuple case: t[1] = t[1].__iadd__([6]). The __iadd__
call mutates the list, then the tuple item assignment raises the TypeError.
See these examples:
>>> import dis
>>> dis.dis('x += 1')
1 0 LOAD_NAME 0 (x)
3 LOAD_CONST 0 (1)
6 INPLACE_ADD
7 STORE_NAME 0 (x)
10 LOAD_CONST 1 (None)
13 RETURN_VALUE
>>>
>>> dis.dis('t[1] += [6]')
1 0 LOAD_NAME 0 (t)
3 LOAD_CONST 0 (1)
6 DUP_TOP_TWO
7 BINARY_SUBSCR
8 LOAD_CONST 1 (6)
11 BUILD_LIST 1
14 INPLACE_ADD
15 ROT_THREE
16 STORE_SUBSCR
17 LOAD_CONST 2 (None)
20 RETURN_VALUE
----------
nosy: +durban
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue11562>
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