“Pythonic”替换列表中的一个或多个项目的方法

Question

I want to replace one or more items in a list, in place. My gut reaction would be something like this:

mylist = [...]
other_object = dict(spam='eggs', a_list=mylist)

def replacer(item):
   ... contents not shown, returns a replacement ...

mylist[:] = [replacer(item) for item in mylist]

It needs to be in-place, so that if I have other objects (eg other_object in the above example) with a reference to mylist, then those other objects will see the updated value. (Clarification: I do not want to require any changes to other_object because it is managed elsewhere from the code I maintain.)

Is this the most idiomatic way to do it in Python? If not, is there a better way?

Answer 1

well, your own solution sounds pretty fine to me:

mylist[:] = [replacer(item) for item in mylist]

or you can do it in a more explicit but verbose way:

for idx, val in enumerate(mylist):
    mylist[idx] = replacer(val)

I can imagine many other ways, but I would be burnt and banned to hell if I suggest them 🔥🔥🔥

---

Demonstrating that other_object sees the change:

>>> def replacer(x):
...     return x+2
... 
>>> mylist = [1,2,3]
>>> other_object = dict(spam='eggs', a_list=mylist)
>>> mylist[:] = [replacer(item) for item in mylist]
>>> mylist, other_object['a_list']
([3, 4, 5], [3, 4, 5])

you're welcome ♥

Answer 2

You can do it directly:

>>> my_list=[1,2,3]
>>> other_obj={'eggs':my_list}
>>> my_other_list=[[1,2,3],my_list]
>>> other_obj['eggs'][:]=[e+2 for e in other_obj['eggs']]
>>> other_obj
{'eggs': [3, 4, 5]}
>>> my_other_list
[[1, 2, 3], [3, 4, 5]]
>>> my_list
[3, 4, 5]

Notice that anywhere the original my_list is used, it is changed by using a slice assignment to any one of those names:

>>> my_other_list[1][:]=[20,30,40]
>>> my_list
[20, 30, 40]
>>> other_obj
{'eggs': [20, 30, 40]}

More at Ned Batchelder: Facts and myths about Python names and values

---

Based on your edit: Just change mylist inplace and it will change in the other objects where mylist is used (but not a copy of mylist ). You can use any method that does in place modification.

Example:

>>> my_list[:]=['new','inserted','items']

The other two objects change too:

>>> other_obj
{'eggs': ['new', 'inserted', 'items']}
>>> my_other_list
[[1, 2, 3], ['new', 'inserted', 'items']]

Just beware if these are 'shared' objects since other programs / processes can change the name reference without your knowledge. Potentially sneaky bugs.