为什么字典不可以调用？

Question

It's a usual programming problem to have a list of objects that you want to map into another. This is usually done with a dictionary:

l1=[1,2,3]
d={1:'_ONE', 2:'_TWO', 3:'_THREE'}
l2= [d[i] for i in l1]

In many cases, it's natural to see a dictionary as a function that maps keys to values. Then, you'd simply l2= map(d,l1) instead of using the list comprehension (or the ugly map(d.__getitem__,l1) .

Of course, since python is so flexible, you can get this behaviour easily:

class CallableDict(dict):
    def __call__(self, k):
        return self[k]

but this obviously doesn't work when accessing dicts you don't create yourself.

Are there good reasons why the base python dictionaries are not callable by default?

Disclaimer: The motivation for this question is mostly ruby, where dictionaries work this way

Answer 1

There should be one-- and preferably only one --obvious way to do it. http://legacy.python.org/dev/peps/pep-0020/

If you make dict callable to subscript a dictionary, now you have two ways of doing the same thing:

d[k]  # Existing
d(k)  # Proposed

Even the case that you bring up, having a callable to pass to map , already has one obvious solution:

map(d.get, it)  # Existing
map(d, it)      # Proposed

There doesn't seem to be any particular advantage to your proposal, and it duplicates what are already obvious ways.