On Sat, 15 Oct 2016 11:35 pm, Uday J wrote:
> Hi,
>
> Here is the code, which I would like to understand.
>
>>>> l=['a','b','c']
>>>> bm=dict.fromkeys(l,['-1','-1'])
fromkeys() doesn't make a copy of the list each time it is used. It uses the
exact same list each time. Watch:
py> L = []
py> d = dict.fromkeys((1, 2, 3), L)
py> print(id(d[1]), id(d[2]), id(d[3]), id(L))
3081897484 3081897484 3081897484 3081897484
You get the same ID number four times: there's only one dict here, in four
places: L, d[1], d[2] and d[3] all refer to the same dict. So what what
happens:
py> print(d)
{1: [], 2: [], 3: []}
py> L.append('abc')
py> print(d)
{1: ['abc'], 2: ['abc'], 3: ['abc']}
That's because this is NOT four copies of the same list, but just the same
list, in four places.
Here are two options:
# Python 3 only: use a dict comprehension
py> d = {x:[] for x in (1, 2, 3)}
py> d
{1: [], 2: [], 3: []}
py> d[1].append('abc')
py> d
{1: ['abc'], 2: [], 3: []}
# any version of Python
d = {}
for x in (1, 2, 3):
d[x] = []
--
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.
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https://mail.python.org/mailman/listinfo/python-list
