Python dictionary get keys with certain value

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Python dictionary get keys with certain value

In python, we can get the values present in a dictionary using the keys by simply using the syntax dict_name[key_name]. But there isn't any method to extract a key associated with the value when we have values. In this article, we will see the ways with help of which we can get the key from a given value in a dictionary. Get key from a value by searching the items in a dictionary This is the simplest way to get a key for a value. In this method we will check each key-value pair to find the key associated with the present value.For this task, we will use the items() method for a python dictionary. The items() method returns a list of tuples containing key value pairs. We will search each tuple to find the key associated with our value as follows. myDict={"name":"PythonForBeginners","acronym":"PFB"} print("Dictionary is:") print(myDict) dict_items=myDict.items() print("Given value is:") myValue="PFB" print(myValue) print("Associated Key is:") for key,value in dict_items: if value==myValue: print(key) Output: Dictionary is: {'name': 'PythonForBeginners', 'acronym': 'PFB'} Given value is: PFB Associated Key is: acronym In the above program, we have created a list of items in myDict using myDict.items() and then we check for each item in the list to find the key for our value. Get key from a value by using python lists We can create a separate list of keys and values and then we can retrieve the keys from the given value using index() method. For this task, we will first create a list of keys present in the dictionary using keys() method and then we will create the list of values present in the dictionary using values() method. Now we will get the index of the given value from the list of values using index() method. As we know that the list of keys has the same order of keys as values in the list of values, the index of the values in the list of values will be the same as the index of the associated key in the list of keys. So, after finding the index of the value in the list of values, we can find the key in the list of keys at the same index. This can be done as follows. myDict={"name":"PythonForBeginners","acronym":"PFB"} print("Dictionary is:") print(myDict) dict_keys=list(myDict.keys()) dict_values=list(myDict.values()) print("Given value is:") myValue="PFB" print(myValue) val_index=dict_values.index(myValue) print("Associated key is:") myKey=dict_keys[val_index] print(myKey) Output: Dictionary is: {'name': 'PythonForBeginners', 'acronym': 'PFB'} Given value is: PFB Associated key is: acronym Get key from a value by using list comprehension Instead of using index() method, we can use list comprehension to get the keys associated with the given value. To find the key, we will create a list of keys which have associated values equal to the given value using list comprehension as follows. myDict={"name":"PythonForBeginners","acronym":"PFB"} print("Dictionary is:") print(myDict) dict_items=myDict.items() print("Given value is:") myValue="PFB" print(myValue) print("Associated key is:") myKey=[key for key,value in dict_items if value==myValue] print(myKey) Output: Dictionary is: {'name': 'PythonForBeginners', 'acronym': 'PFB'} Given value is: PFB Associated key is: ['acronym'] Conclusion In this article, we have seen three ways to obtain a key from a python dictionary using list comprehension, items() method and list index() method. Stay tuned for more informative articles. For Python training, our top recommendation is DataCamp. In this article will be focusing on the 4 ways to Check if Key Exists in a Python Dictionary. A Python Dictionary is basically a data structure wherein the data items are stored in a key-value pair.Technique 1: `in' operator to Check if Key Exists in a Python DictionaryPython in operator along with if statement can be used to check whether a particular key exists in the input Python dictionary.Python in operator basically checks if a particular element or value is contained in a particular sequence such as list, tuple, dictionary, etc.Syntax: for/if value in iterable: Example: inp_dict = {'Python': "A", 'Java':"B", 'Ruby':"C", 'Kotlin':"D"} search_key = 'Ruby' if search_key in inp_dict: print("The key is present.") else: print("The key does not exist in the dictionary.") In the above example, we have used an if statement along with Python in operator to check whether the key `Ruby' is present in the dict or not.Output:Technique 2: Python keys() methodPython in-built keys() method can be used to check for the presence of a key in the existing dictionary.Syntax:The keys() method takes no arguments and returns an object that represents a list of all the keys present in a particular input dictionary.So, in order to check whether a particular key is present in the dict, we use Python if statement along with the keys() method to compare the search_key with the list of keys returned from the keys() method. If the key is present, it will follow the statement in the if portion, otherwise it will jump the statement in the else portion.Example: inp_dict = {'Python': "A", 'Java':"B", 'Ruby':"C", 'Kotlin':"D"} search_key = 'Ruby' if search_key in inp_dict.keys(): print("The key is present.") else: print("The key does not exist in the dictionary.") Output:Example 2: inp_dict = {'Python': "A", 'Java':"B", 'Ruby':"C", 'Kotlin':"D"} search_key = 'Cpp' if search_key in inp_dict.keys(): print("The key is present.") else: print("The key does not exist in the dictionary.") Output: The key does not exist in the dictionary. Technique 3: get() method to Check if Key Exists in a Python Dictionary Python get() method can be used to check whether a particular key is present in the key-value pairs of the dictionary.The get() method actually returns the value associated with the key if the key happens to be present in the dictionary, else it returns `None`.Syntax: dict.get(key, default=None) We pass the key to be searched as an argument to the get() method, and if the get() function does not return None i.e. if the key is present in the dict, we print it.Example 1: inp_dict = {'Python': "A", 'Java':"B", 'Ruby':"C", 'Kotlin':"D"} if inp_dict.get('Python')!=None: print("The key is present.") else: print("The key does not exist in the dictionary.") Output:Technique 4: Python has_key() methodNote: The has_keys() method has been omitted from Python version 3 and above.Python has_key() method checks whether a particular key is available in the dict and returns True, else it returns false.Syntax:Example: inp_dict = {'Python': "A", 'Java':"B", 'Ruby':"C", 'Kotlin':"D"} search_key = 'Kotlin' if inp_dict.has_key(search_key): print("The key is present.") else: print("The key does not exist in the dictionary.") ConclusionThus, in this article, we have unveiled and understood the various techniques to check if key exists in a Python dictionary.I recommend all the readers to go through the below post to know more about Python Dictionary in a detailed manner.ReferencesPython DictionaryPython if statement James Gallagher is a self-taught programmer and the technical content manager at Career Karma. He has experience in range of programming languages and extensive expertise in Python, HTML, CSS, and JavaScript. James has written hundreds of programming tutorials, and he frequently contributes to publications like Codecademy, Treehouse, Repl.it, Afrotech, and others. He also serves as a researcher at Career Karma, publishing comprehensive reports on the bootcamp market and income share agreements. Read more Comments (0) Something went wrong. Wait a moment and try again. The dict.get() method returns the value of the specified key. Syntax: dict.get(key, value) Parameters: key: (Required) The key to be searched in the dictionary. value: (Optional) The value that should be returned, in case the key is not found. By default, it is None. Return Value: Returns the value of the specified key if a key exists in the dictionary. If a key is not found, then it returns None. If a key is not found, and the value parameter is specified, it will return the specified value. The following example demonstrates the dict.get() method. romanNums = {'I':1, 'II':2, 'III':3, 'IV':4, 'V':5 } value = romanNums.get('I') print("I = ",value) If the key is not found in the dictionary, and the value parameter is not specified, it will return None. romanNums = {'I':1, 'II':2, 'III':3, 'IV':4, 'V':5 } value = romanNums.get('VI') print("VI = ", value) If the key is not found in the dictionary and the value parameter is specified, it will return the specified value. romanNums = {'I':1, 'II':2, 'III':3, 'IV':4, 'V':5 } value = romanNums.get('VI','Not in the dictionary.') print("VI = ", value) A dictionary is a set of unordered key, value pairs. In a dictionary, the keys must be unique and they are stored in an unordered manner. In this tutorial you will learn the basics of how to use the Python dictionary. By the end of the tutorial you will be able to - Create Dictionaries - Get values in a Dictionary - Add and delete elements in a Dictionary - To and For Loops in a Dictionary Creating a Dictionary Let's try to build a profile of three people using dictionaries. To do that you separate the key-value pairs by a colon(":"). The keys would need to be of an immutable type, i.e., data-types for which the keys cannot be changed at runtime such as int, string, tuple, etc. The values can be of any type. Individual pairs will be separated by a comma(",") and the whole thing will be enclosed in curly braces({...}). For example, you can have the fields "city", "name," and "food" for keys in a dictionary and assign the key,value pairs to the dictionary variable person1_information. >>> person_information = {'city': 'San Francisco', 'name': 'Sam', "food": "shrimps"} >>> type(person1_information) >>> print(person1_information) {'city': 'San Francisco', 'name': 'Sam', 'food': 'shrimps'} Get the values in a Dictionary To get the values of a dictionary from the keys, you can directly reference the keys. To do this, you enclose the key in brackets [...] after writing the variable name of the dictionary. So, in the following example, a dictionary is initialized with keys "city", "name," and "food" and you can retrieve the value corresponding to the key "city." >>> create a dictionary person1_information >>> person1_information = {'city': 'San Francisco', 'name': 'Sam', "food": "shrimps"} >>> print the dictionary >>> print(person1_information["city"]) San Francisco You can also use the get method to retrieve the values in a dict. The only difference is that in the get method, you can set a default value. In direct referencing, if the key is not present, the interpreter throws KeyError. >>> # create a small dictionary >>> alphabets = {1: `a'} >>> # get the value with key 1 >>> print(alphabets.get(1)) 'a' >>> # get the value with key 2. Pass "default" as default. Since key 2 does not exist, you get "default" as the return value. >>> print(alphabets.get(2, "default")) 'default' >>> # get the value with key 2 through direct referencing >>> print(alphabets[2]) Traceback (most recent call last): File "", line 1, in KeyError: 2 Looping over dictionary Say, you got a dictionary, and you want to print the keys and values in it. Note that the key-words for and in are used which are used when you try to loop over something. To learn more about looping please look into tutorial on looping. >>> person1_information = {'city': 'San Francisco', 'name': 'Sam', "food": "shrimps"} >>> for k, v in person1_information.items(): ... print("key is: %s" % k) ... print("value is: %s" % v) ... print("###########################") ... key is: food value is: shrimps ########################### key is: city value is: San Francisco ########################### key is: name value is: Sam ########################### Add elements to a dictionary You can add elements by updating the dictionary with a new key and then assigning the value to a new key. >>> # initialize an empty dictionary >>> person1_information = {} >>> # add the key, value information with key "city" >>> person1_information["city"] = "San Francisco" >>> # print the present person1_information >>> print(person1_information) {'city': 'San Francisco'} >>> # add another key, value information with key "name" >>> person1_information["name"] = "Sam" >>> # print the present dictionary >>> print(person1_information) {'city': 'San Francisco', 'name': 'Sam'} >>> # add another key, value information with key "food" >>> person1_information["food"] = "shrimps" >>> # print the present dictionary >>> print(person1_information) {'city': 'San Francisco', 'name': 'Sam', 'food': 'shrimps'} Or you can combine two dictionaries to get a larger dictionary using the update method. >>> # create a small dictionary >>> person1_information = {'city': 'San Francisco'} >>> # print it and check the present elements in the dictionary >>> print(person1_information) {'city': 'San Francisco'} >>> # have a different dictionary >>> remaining_information = {'name': 'Sam', "food": "shrimps"} >>> # add the second dictionary remaining_information to personal1_information using the update method >>> person1_information.update(remaining_information) >>> # print the current dictionary >>> print(person1_information) {'city': 'San Francisco', 'name': 'Sam', 'food': 'shrimps'} Delete elements of a dictionary To delete a key, value pair in a dictionary, you can use the del method. >>> # initialise a dictionary with the keys "city", "name", "food" >>> person1_information = {'city': 'San Francisco', 'name': 'Sam', "food": "shrimps"} >>> # delete the key, value pair with the key "food" >>> del person1_information["food"] >>> # print the present personal1_information. Note that the key, value pair "food": "shrimps" is not there anymore. >>> print(person1_information) {'city': 'San Francisco', 'name': 'Sam'} A disadvantage is that it gives KeyError if you try to delete a nonexistent key. >>> # initialise a dictionary with the keys "city", "name", "food" >>> person1_information = {'city': 'San Francisco', 'name': 'Sam', "food": "shrimps"} >>> # deleting a non existent key gives key error. >>> del person1_information["non_existent_key"] Traceback (most recent call last): File "", line 1, in KeyError: 'non_existent_key' So, instead of the del statement you can use the pop method. This method takes in the key as the parameter. As a second argument, you can pass the default value if the key is not present. >>> # initialise a dictionary with key, value pairs >>> person1_information = {'city': 'San Francisco', 'name': 'Sam', "food": "shrimps"} >>> # remove a key, value pair with key "food" and default value None >>> print(person1_information.pop("food", None)) 'Shrimps' >>> # print the updated dictionary. Note that the key "food" is not present anymore >>> print(person1_information) {'city': 'San Francisco', 'name': 'Sam'} >>> # try to delete a nonexistent key. This will return None as None is given as the default value. >>> print(person1_information.pop("food", None)) None More facts about the Python dictionary You can test the presence of a key using the has_key method. >>> alphabets = {1: `a'} >>> alphabets.has_key(1) True >>> alphabets.has_key(2) False A dictionary in Python doesn't preserve the order. Hence, we get the following: >>> call = {'sachin': 4098, 'guido': 4139} >>> call["snape"] = 7663 >>> call {'snape': 7663, 'sachin': 4098, 'guido': 4139} You see that even though "snape" was given later, it appeared at the top. Contributed by: Joydeep Bhattacharjee

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