Python: get key of index in dictionary

Question

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Inverse dictionary lookup - Python
reverse mapping of dictionary with Python

How do i get key of index in dictionary?

For example like:

i = {'a': 0, 'b': 1, 'c': 2}

so if i want to get key of i[0], it will return 'a'

Answer 1

You could do something like this:

i={'foo':'bar', 'baz':'huh?'}
keys=i.keys()  #in python 3, you'll need `list(i.keys())`
values=i.values()
print keys[values.index("bar")]  #'foo'

However, any time you change your dictionary, you'll need to update your keys,values because dictionaries are not ordered in versions of Python prior to 3.7. In these versions, any time you insert a new key/value pair, the order you thought you had goes away and is replaced by a new (more or less random) order. Therefore, asking for the index in a dictionary doesn't make sense.

As of Python 3.6, for the CPython implementation of Python, dictionaries remember the order of items inserted. As of Python 3.7+ dictionaries are ordered by order of insertion.

Also note that what you're asking is probably not what you actually want. There is no guarantee that the inverse mapping in a dictionary is unique. In other words, you could have the following dictionary:

d={'i':1, 'j':1}

In that case, it is impossible to know whether you want i or j and in fact no answer here will be able to tell you which ('i' or 'j') will be picked (again, because dictionaries are unordered). What do you want to happen in that situation? You could get a list of acceptable keys ... but I'm guessing your fundamental understanding of dictionaries isn't quite right.

Answer 2

Python dictionaries have a key and a value, what you are asking for is what key(s) point to a given value.

You can only do this in a loop:

[k for (k, v) in i.items() if v == 0]

Note that there can be more than one key per value in a dict; {'a': 0, 'b': 0} is perfectly legal.

As of Python 3.6 the dictionary implementation retains the order you inserted keys (it became part of the language specification in 3.7). However, if you need more control over the order you could use a list:

items = ['a', 'b', 'c']
items.index('a') # gives 0
items[0]         # gives 'a'

Answer 3

By definition dictionaries are unordered, and therefore cannot be indexed. For that kind of functionality use an ordered dictionary. Python Ordered Dictionary