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

Steven D'Aprano steven at REMOVE.THIS.cybersource.com.au
Mon Jan 4 09:22:44 CET 2010


On Mon, 04 Jan 2010 04:59:02 -0300, Gabriel Genellina wrote:

> Is there any reason for this error? Apart from "nobody cared to write
> the code"

Yes, because such implicit conversions would be a bad idea.



> py> [1,2,3] + (4,5)


What result are you expecting? A list or a tuple?



> Traceback (most recent call last):
>      File "<stdin>", line 1, in <module>
> TypeError: can only concatenate list (not "tuple") to list


Apart from the different error message, this is essentially the same 
error as this:

>>> 2 + "2"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'



> In-place addition += does work:
> 
> py> a = [1,2,3]
> py> a += (4,5)
> py> a
> [1, 2, 3, 4, 5]


I call that an impressive gotcha. I believe that is because in-place 
addition of lists is implemented as functionally equivalent to the extend 
method:



>>> a += "abc"  # same as a.extend("abc")
>>> a
[1, 2, 3, 4, 5, 'a', 'b', 'c']
>>> a += {None: -1}
>>> a
[1, 2, 3, 4, 5, 'a', 'b', 'c', None]



-- 
Steven



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