Alan Gauld via Tutor wrote:
> On 29/06/17 03:14, shubham goyal wrote:
>
>> This Question is asked in some exam. i am not able to figure it out.
>>
>> a = [0, 1, 2, 3]
>> for a[-1] in a:
>> print(a[-1])
>>
>> its giving output 0 1 2 2
>>
>> it should be 3 3 3 3 as a[-1] belongs to 3.
>> can anyone help me figuring it out.
>
> This is quite subtle and it took me a few minutes to figure
> it out myself.
>
> It might be clearer if we print all of 'a' instead
> of a[-1]:
>
>>>> for a[-1] in a:
> ... print(a)
> ...
> [0, 1, 2, 0]
> [0, 1, 2, 1]
> [0, 1, 2, 2]
> [0, 1, 2, 2]
>
> What is happening is that a[-1] is being assigned the value
> of each item in a in turn. The final iteration assigns a[-1]
> to itself, thus we repeat the 2.
>
> Another way to see it is to convert the for loop to
> a while loop:
>
> for variable in collection:
> process(variable)
>
> becomes
>
> collection = [some sequence]
> index = 0
> while index < len(collection):
> variable = collection[index]
> process(variable)
> index += 1
>
> Now substitute your values:
>
> collection = [0,1,2,3]
> index = 0
> while index < len(collection):
> collection[-1] = collection[index]
> print(collection[-1])
> index += 1
>
> Does that help?
The simplest rewrite is probably
>>> a = [0, 1, 2, 3]
>>> for tmp in a:
... a[-1] = tmp
... print(tmp)
...
0
1
2
2
which should clarify how the list is modified while iterating over it.
The takeaway is that
for ... in [value]:
pass
is equivalent to
... = value
There are a few other implicit ways to assign a value, with varying
generality:
>>> with open("tmp.txt") as a[0]: pass
...
works, but
>>> import os as a[0]
File "<stdin>", line 1
import os as a[0]
^
SyntaxError: invalid syntax
and
>>> try: 1/0
... except Exception as a[0]: pass
File "<stdin>", line 2
except Exception as a[0]: pass
^
SyntaxError: invalid syntax
are forbidden by the language.
The only case -- other than standard name binding -- I find useful is
unpacking in a for loop:
>>> for hi, lo in ["Aa", "Zz"]:
... print(hi, "-", lo)
...
A - a
Z - z
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