Python追加循环工作之谜

Question

I found some "mystery" problem when I'm trying filling list in "for in" loop.

Here is example:

>>> params = [1,2,3,4,5]
>>> el = {'param':0}
>>> el
{'param': 0}
>>> list = []
>>> for param in params:
...     el['param'] = param
...     list.append(el)
... 
>>> print(list)
[{'param': 5}, {'param': 5}, {'param': 5}, {'param': 5}, {'param': 5}]
>>> 

But I think result should be

>>> print(list)
[{'param': 1}, {'param': 2}, {'param': 3}, {'param': 4}, {'param': 5}]
>>> 

How to resolve this problem? And why real result isn't similar with result in my mind?

Answer 1

Every element in your list contains a reference to the same dict. That is why you are seeing the same number in every element. Perhaps you want to do:

params = [1,2,3,4,5]

a = []
for param in params:
    el = {'param': param}
    a.append(el)

Also, be careful of shadowing builtins such as list with your variable names.

Answer 2

el refers to the same dictionary throughout, and list.append(el) adds a reference to that dictionary, not a copy of its current value, to the list. You don't have a list of 5 different dictionaries, but a list of 5 references to one dictionary.

To see what is happening in closer detail, print list and el each time through the loop. You'll see a growing list that always refers to the current value of el .

Answer 3

The problem is that you are using the same dictionary for each entry in your list. To fix this problem, add el = {} above el['param'] = param in your for loop.

Answer 4

You modify the same dict in every iteration and add it to the list . You'll get the other result when adding copies of this dict to the list .

Just call list.append(dict(el)) instead.

Answer 5

You have one dict object that you are modifying. You add a reference to that one dict to your list multiple times. So your list contains 5 references to the same dict object.