list(chain)返回的浅表副本chain，它等效于chain[:]。
如果您想要列表的浅表副本，请使用list()，它有时还用于从迭代器获取所有值。
y = list(x)和之间的区别y = x：
>>> x = [1,2,3]
>>> y = x #this simply creates a new referece to the same list object
>>> y is x
True
>>> y.append(4) # appending to y, will affect x as well
>>> x,y
([1, 2, 3, 4], [1, 2, 3, 4]) #both are changed
#shallow copy
>>> x = [1,2,3]
>>> y = list(x) #y is a shallow copy of x
>>> x is y
False
>>> y.append(4) #appending to y won't affect x and vice-versa
>>> x,y
([1, 2, 3], [1, 2, 3, 4]) #x is still same
请注意，如果x包含可变对象，则仅list()或[:]不足：
>>> x = [[1,2],[3,4]]
>>> y = list(x) #outer list is different
>>> x is y
False
但是内部对象仍然是x中对象的引用：
>>> x[0] is y[0], x[1] is y[1]
(True, True)
>>> y[0].append('foo') #modify an inner list
>>> x,y #changes can be seen in both lists
([[1, 2, 'foo'], [3, 4]], [[1, 2, 'foo'], [3, 4]])
由于外部列表不同，因此修改x不会影响y，反之亦然
>>> x.append('bar')
>>> x,y
([[1, 2, 'foo'], [3, 4], 'bar'], [[1, 2, 'foo'], [3, 4]])
为了处理这个使用copy.deepcopy。
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