如何遍历字典并在Python中返回键？

Question

I have a dictionary in Python with people's last names as the key and each key has multiple values linked to it. Is there a way to iterate over the dictionary using a for loop to search for a specific value and then return the key that the value is linked to?

for i in people:
      if people[i] == criteria: #people is the dictionary and criteria is just a string
            print dictKey #dictKey is just whatever the key is that the criteria matched element is linked to

There maybe multiple matches as well so I need to people to output multiple keys.

Answer 1

You can use list comprehension

print [key
          for people in peoples
          for key, value in people.items()
          if value == criteria]

This will print out all the keys for which the value matches the criteria. If people is the dictionary,

print [key
          for key, value in people.items()
          if value == criteria]

Answer 2

for i in people:
  if people[i] == criteria:
        print i

i is your key. That's how iterating over dictionary works. Remember, though, that if you want to print keys in any specific order - you need to keep results in a list and sort it before printing. Dictionaries don't keep their entries in any guaranteed order.

Answer 3

Use this:

for key, val in people.items():
    if val == criteria:
        print key

Answer 4

Given a dictionary of lastnames and traits:

>>> people = {
    'jones': ['fast', 'smart'],
    'smith': ['slow', 'dumb'],
    'davis': ['slow', 'smart'],
}

A list comprehension nicely finds all lastnames matching some criteria:

>>> criteria = 'slow'
>>> [lastname for (lastname, traits) in people.items() if criteria in traits]
['davis', 'smith']

However, if you're going to do many such lookups, it would be faster to build a reverse dictionary that maps traits to a list of matching last names:

>>> traits = {}
>>> for lastname, traitlist in people.items():
        for trait in traitlist:
            traits.setdefault(trait, []).append(lastname)

Now, the criteria searches can be done quickly and elegantly:

>>> traits['slow']
['davis', 'smith']
>>> traits['fast']
['jones']
>>> traits['smart']
['jones', 'davis']
>>> traits['dumb']
['smith']

Answer 5

Try this one,

for key,value in people.items():
        if value == 'criteria':
                print key