Add vs in-place add of str to list

[rs387]

Hi

I'm trying to understand why it is that I can do

>>a = []
a += 'stuff'
a

['s', 't', 'u', 'f', 'f']

but not

>>a = []
a = a + 'stuff'

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

Can someone explain the logic? Why is the in-place add not a type
error but the usual add is? (This applies to both Python 2.6rc1 and
3.0b2)

Thanks
Roman

[Erik Max Francis]

rs387 wrote:

I'm trying to understand why it is that I can do

>>>a = []
a += 'stuff'
a

['s', 't', 'u', 'f', 'f']

but not

>>>a = []
a = a + 'stuff'

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

Can someone explain the logic? Why is the in-place add not a type
error but the usual add is? (This applies to both Python 2.6rc1 and
3.0b2)

It's because the `+=` operator is doing the equivalent of calling the
`extend` method, which treats its argument as a generic sequence, and
doesn't enforce type. The same thing happens with any other sequence
type as the right-hand operand; for instance, tuples:

>>a = []
a += (1, 2, 3)
a

[1, 2, 3]

>>a = []
a = a + (1, 2, 3)

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

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[rs387]

On Oct 2, 8:11*am, Erik Max Francis <[e-mail]:

It's because the `+=` operator is doing the equivalent of calling the
`extend` method, which treats its argument as a generic sequence, and
doesn't enforce type.

I see. Do you know whether this is seen as a problem with the language
design?

If so, this could be fixed by changing list.__add__ to call .extend on
the left argument if it's a list... Or, by changing __iadd__ to throw
a TypeError, whichever way around is seen as more correct/desirable.

[Mel]

rs387 wrote:

On Oct 2, 8:11Â*am, Erik Max Francis <[e-mail]:

>It's because the `+=` operator is doing the equivalent of calling the
`extend` method, which treats its argument as a generic sequence, and
doesn't enforce type.

I see. Do you know whether this is seen as a problem with the language
design?

No.

[1,2,3] + [4,5,6]

produces

[1,2,3,4,5,6]

not

[1,2,3,[4,5,6]]

Mel.

[rs387]

On Oct 2, 3:50*pm, Mel <[e-mail]:

rs387 wrote:

I see. Do you know whether this is seen as a problem with the language
design?

No.

OK, I get it now. I was assuming that the "+" could be implemented in
terms of "+=" as follows:

def add(x,y):
temp = list(x)
temp += y
return temp

But it turns out that this is not what "+" means at all. A bit of a
gotcha...

Roman