Mok-Kong Shen wrote:
>
> In an earlier question about lists, I was told about the issue of
> creation of local names in a function. However, I still can't
> understand why the program below outputs:
>
> [999] sss
> [999]
>
> and not two identical lines of output. For both operators "+=" should
> anyway work in similar manner in the function xx in my view.
>
> Thanks for your help in advance.
>
> M. K. Shen
>
> ----------------------------------------------------------
>
> def xx(list,str):
> list+=[999]
> str+="sss"
a += b
is internally translated into to
a = a.__iadd__(b)
If the 'list' class were implemented in Python it would look like
class list:
def __iadd__(self, other):
for item in other:
self.append(item)
return self
i. e. the original list is modified when you perform += and you'll see the
modification when you look at any name bound to that original list:
b = a = [1, 2]
a += [3, 4]
print a # [1, 2, 3, 4]
print b # [1, 2, 3, 4]
Strings on the other hand are "immutable" -- they cannot be altered after
the initial creation. The hypothetical __iadd__() implementation is
class str:
def __iadd__(self, other):
return self + other
So += rebinds a name to a new string:
b = a = "first"
a += "second"
print b # first
print a # firstsecond
Armed with this knowledge
> lista=[]
> stra=""
> lista+=[999]
[999] is appended to lista and lista is rebound to itself.
> stra+="sss"
A new string "" + "sss" is created and stra is bound to that new string.
> print(lista,stra)
> listb=[]
> strb=""
> xx(listb,strb)
Inside xx()
(1) 999 is appended to listb and the local variable list is rebound.
(2) A new string "" + "sss" is created and bound to the local variable str.
> print(listb,strb)
If you have understood the above here's a little brain teaser:
>>> a = ([1,2,3],)
>>> a[0] += [4, 5]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment
>>> a[0]
What are the contents of a[0] now?
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