Get key by value in dictionary

Question

I made a function which will look up ages in a Dictionary and show the matching name:

dictionary = {\'george\' : 16, \'amber\' : 19}
search_age          


        

Answer 1

You need to use a dictionary and reverse of that dictionary. It means you need another data structure. If you are in python 3, use enum module but if you are using python 2.7 use enum34 which is back ported for python 2.

Example:

from enum import Enum

class Color(Enum): 
    red = 1 
    green = 2 
    blue = 3

>>> print(Color.red) 
Color.red

>>> print(repr(Color.red)) 
<color.red: 1=""> 

>>> type(Color.red) 
<enum 'color'=""> 
>>> isinstance(Color.green, Color) 
True 

>>> member = Color.red 
>>> member.name 
'red' 
>>> member.value 
1 

Answer 2

my_dict = {'A': 19, 'B': 28, 'carson': 28}
search_age = 28

take only one

name = next((name for name, age in my_dict.items() if age == search_age), None)
print(name)  # 'B'

get multiple data

name_list = [name for name, age in filter(lambda item: item[1] == search_age, my_dict.items())]
print(name_list)  # ['B', 'carson']

Answer 3

we can get the Key of dict by :

def getKey(dct,value):
     return [key for key in dct if (dct[key] == value)]

Answer 4

I thought it would be interesting to point out which methods are the quickest, and in what scenario:

Here's some tests I ran (on a 2012 MacBook Pro)

>>> def method1(list,search_age):
...     for name,age in list.iteritems():
...             if age == search_age:
...                     return name
... 
>>> def method2(list,search_age):
...     return [name for name,age in list.iteritems() if age == search_age]
... 
>>> def method3(list,search_age):
...     return list.keys()[list.values().index(search_age)]

Results from profile.run() on each method 100000 times:

Method 1:

>>> profile.run("for i in range(0,100000): method1(list,16)")
     200004 function calls in 1.173 seconds

Method 2:

>>> profile.run("for i in range(0,100000): method2(list,16)")
     200004 function calls in 1.222 seconds

Method 3:

>>> profile.run("for i in range(0,100000): method3(list,16)")
     400004 function calls in 2.125 seconds

So this shows that for a small dict, method 1 is the quickest. This is most likely because it returns the first match, as opposed to all of the matches like method 2 (see note below).

---

Interestingly, performing the same tests on a dict I have with 2700 entries, I get quite different results (this time run 10000 times):

Method 1:

>>> profile.run("for i in range(0,10000): method1(UIC_CRS,'7088380')")
     20004 function calls in 2.928 seconds

Method 2:

>>> profile.run("for i in range(0,10000): method2(UIC_CRS,'7088380')")
     20004 function calls in 3.872 seconds

Method 3:

>>> profile.run("for i in range(0,10000): method3(UIC_CRS,'7088380')")
     40004 function calls in 1.176 seconds

So here, method 3 is much faster. Just goes to show the size of your dict will affect which method you choose.

Notes: Method 2 returns a list of all names, whereas methods 1 and 3 return only the first match. I have not considered memory usage. I'm not sure if method 3 creates 2 extra lists (keys() and values()) and stores them in memory.

Answer 5

it's answered, but it could be done with a fancy 'map/reduce' use, e.g.:

def find_key(value, dictionary):
    return reduce(lambda x, y: x if x is not None else y,
                  map(lambda x: x[0] if x[1] == value else None, 
                      dictionary.iteritems()))

Answer 6

Sometimes int() may be needed:

titleDic = {'Фильмы':1, 'Музыка':2}

def categoryTitleForNumber(self, num):
    search_title = ''
    for title, titleNum in self.titleDic.items():
        if int(titleNum) == int(num):
            search_title = title
    return search_title