Python reverse / reverse mapping (but with multiple values ​​for each key)

Question

This is really a variation of this question, but not a duplicate:

Python reverse / invert display

Given such a dictionary:

mydict= { 'a': ['b', 'c'], 'd': ['e', 'f'] }

How can you invert this dict to get:

inv_mydict = { 'b':'a', 'c':'a', 'e':'d', 'f':'d' }

Note that the values ​​differ for each key.

Note I previously had the syntax map = ... and dict = ... Reminder not to use map and dict as they are built-in functions, see excellent comments and answers below :)

Answer 1

TL DR

Use a dictionary understanding like

 >>> my_map = { 'a': ['b', 'c'], 'd': ['e', 'f'] } >>> {value: key for key in my_map for value in my_map[key]} {'c': 'a', 'f': 'd', 'b': 'a', 'e': 'd'} 

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The above dictionary understanding is functionally equivalent to the following loop structure, which fills an empty dictionary

 >>> inv_map = {} >>> for key in my_map: ... for value in my_map[key]: ... inv_map[value] = key ... >>> inv_map {'c': 'a', 'f': 'd', 'b': 'a', 'e': 'd'} 

Note. . Using map obscures the built-in map function. Therefore, do not use this as a variable name unless you know what you are doing.

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Other similar ways to do the same.

Python 3.x

You can use dict.items for example

 >>> {value: key for key, values in my_map.items() for value in values} {'c': 'a', 'f': 'd', 'b': 'a', 'e': 'd'} 

We use the items() method here, which would create a view object from a dictionary that would give pairs of key values ​​in an iteration. Therefore, we simply iterate over it and build a new dictionary with reverse mapping.

Python 2.x

You can use dict.iteritems like this

 >>> {value: key for key, values in my_map.iteritems() for value in values} {'c': 'a', 'b': 'a', 'e': 'd', 'f': 'd'} 

We do not prefer the items() method in 2.x because it will return a list of key-value pairs. We do not want to create a list only for iterating and building a new dictionary. Therefore, we prefer iteritems() , which returns an iterator object that gives a pair of key values ​​in an iteration.

Note: The actual equivalent of Python 3.x items will be Python 2.x viewitems , which returns a view object. Read more about the object of presentation in the official documentation, here .

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iter* vs view* in Python 2.x

The main difference between the iter* functions and the view* functions in Python 2.x is that view objects reflect the current state of the dictionary. For instance,

 >>> d = {1: 2} >>> iter_items = d.iteritems() >>> view_items = d.viewitems() 

now we add a new element to the dictionary

 >>> d[2] = 3 

If you try to check if (2, 3) (key-value pair) is in iter_items , this will cause an error

 >>> (2, 3) in iter_items Traceback (most recent call last): File "<input>", line 1, in <module> RuntimeError: dictionary changed size during iteration 

but the viewer will display the current state of the dictionary. This way it will work fine

 >>> (2, 3) in view_items True 

Answer 2

 mp = { 'a': ['b', 'c'], 'd': ['e', 'f'] } pm={} for x in mp.iterkeys(): for d in mp[x]: pm[d]=x print pm 

Output:

{'c': 'a', 'b': 'a', 'e': 'd', 'f': 'd'}

Note. A dictionary is a data structure that is not ordered, so the result cannot be ordered the way you want.