get Key by value, dict, python

问题

How can I get a key from a value?

my dict:

countries = {
        "Normal UK project" : "1",
        "UK Omnibus project" : "1-Omni",
        "Nordic project" : ["11","12","13","14"],
        "German project" : "21",
        "French project" : "31"
        }

my semi functioning code:

for k, v in countries.items():
    if "1" in v:
        print k

expected output:

Normal UK project

actual output:

French project
UK Omnibus project
German project
Normal UK project

How can I fix my code?

回答1:

The problem is that the types of the values in the dictionary are not the same, making it much more difficult to use the dictionary, not only in this scenario. While Python allows this, you really should consider unifying the types in the dictionary, e.g. make them all lists. You can do so in just one line of code:

countries = {key: val if isinstance(val, list) else [val] 
                        for key, val in countries.items()}

Now, each single string is wrapped into a list and your existing code will work correctly.

Alternatively, if you have to use the dictionary in it's current form, you can adapt your lookup function:

for k, v in countries.items():
    if "1" == v or isinstance(v, list) and "1" in v:
        print k

回答2:

c={} - any dict
a(value) - need know this key

key=list(c.keys())[list(c.values()).index(a)]]

回答3:

def keys_of_value(dct, value):
    for k in dct:
        if isinstance(dct[k], list):
            if value in dct[k]:
                return k
        else:
            if value == dct[k]:
                return k

assert keys_of_value(countries, "12") == "Nordic project"
assert keys_of_value(countries, "1")  == "Normal UK project"

If you want me to shorten it a little bit, I might do

from operator import eq, contains

def keys_of_value(dct, value, ops = (eq, contains)):
    for k in dct:
        if ops[isinstance(dct[k], list)](dct[k], value):
            return k

assert keys_of_value(countries, "12") == "Nordic project"
assert keys_of_value(countries, "1") == "Normal UK project"

回答4:

The following code provide another and short version of getting dictionary keys by some value, by using list comprehensions and dic.items():

keys_have_value = [k for k,v in dic.items() if v=="1"]

回答5:

Your semi-functioning code is returning the other values because with entries like:

    "Normal UK project" : "1",

..then "1" in v checks if the string contains the "1" character, whereas with entries like:

    "Nordic project" : ["11","12","13","14"],

..then it will check if the list contains an element "1".

The in operator works on both strings and lists, but in a different manner:

>>> "1" in "123"
True
>>> "1" in ["123", "blah"]
False
>>> "1" in ["1", "blah"]
True

Ideally your data would be more consistent - all lists, or all strings:

countries = {
        "Normal UK project" : ["1"],
        "UK Omnibus project" : ["1-Omni"],
        "Nordic project" : ["11","12","13","14"],
        "German project" : ["21"],
        "French project" : ["31"]
        }

for k, v in countries.items():
    if "1" in v:
        print k

回答6:

I personally think using in and the function values is easier.

print 1 in {1:"123", 2:"blah"}
print "blah" in {1:"123", 2:"blah"}.values()

Output is:

True
True

回答7:

def get_Value(dic,value):
    for name in dic:
        if dic[name] == value:
            return name

来源：https://stackoverflow.com/questions/23295315/get-key-by-value-dict-python

标签

python

dictionary

key-value