I used list, def. why li += [100,200] , and li = li + [100,200] is different

[Bill]

Rob Gaddi wrote:
> On 10/23/2017 09:29 AM, 임현준 wrote:
>> I am a Korean student, and I am a beginner in English and Python.;(
>>
>> I can't understand about this def
>>
>> If I want to print
>>
>> [1,2,3,4,5]
>> [1,2,3,4,5,100,200]
>>
>> I will make code like this, and I can understand code.
>>
>>
>> def modify(li):
>>       li += [100,200]
>>
>> list = [1,2,3,4,5]
>> print(list)
>> modify(list)
>> print(list)
>>
>>
>> BUT, when I make code like this.
>>
>> I will make code like this, and I can understand code.
>>
>>
>> def modify(li):
>>       li = li + [100,200]
>>
>> list = [1,2,3,4,5]
>> print(list)
>> modify(list)
>> print(list)
>>
>>
>>
>> python print
>>
>> [1,2,3,4,5]
>> [1,2,3,4,5]
>>
>> why 'li+= [100,200]'and 'li = li + [100,200]' 's print is different
>> please help me
>>
>
> Lists are mutable, they can be changed.  Your call to += is equivalent 
> to a call to li.extend([100, 200]), which changes the single existing 
> object pointed to by the "li" reference.  The second time, however, 
> you take the existing value that "li" refers to [1,2,3,4,5], create a 
> new object that is ([1,2,3,4,5] + [100,200]), and reassign the local 
> reference "li" to point to that new object.  Then your function ends, 
> "li" goes out of scope, nothing points to that newly created object 
> and it gets lost.
>


The problem and both solutions are great!  Thanks for posting!

Bill