Python: Data Collections - Dictionary


In a previous article, we looked at the features of the list data collection in Python. In this article we look at the dictionary collection type in Python. As in other languages, the dictionary is a list of key-value pairs and just like other variables in Python we do not explicitly need to define the data type of key and value. It is easy to do bulk updates in Python dictionaries using other dictionary objects, but unlike a list the slice(:) operator is not supported. Hence some bulk updates have to be done using a loop.

The following code snippets demonstrate how dictionaries are created, modified and deleted. Inline comments (starting with #) clarify the purpose of specific lines of code.

# Code for demonstration of Python Dictionary
class DemoDictClass:
    # List Initialization
    def Initialize(self):
        # Dictionaries are defined between curly brackets '{}'
        self._dictStr={"One" : 1, "Two":2, "Three":3, "Four":4}
        self._dictNumber =  {1:"One", 2:"Two", 3:"Three", 4:"Four"}

    # List Print -  values at specific indexes
    def PrintInit(self):
        print("------ PrintInit");
        #Loop to print all elements
        print("------ All key,value pairs")
        for key, value in self._dictStr.items():
            print(key + "," + str(value))
        #Single command to print all Keys
        print("------ All keys")
        print(self._dictStr.keys())
        # Single command to print all values
        print("------ All values")
        print(self._dictStr.values())
        print("String dictionary element with key 'One' :" + str(self._dictStr["One"]))
        print("String dictionary element with key 'Three' :" + str(self._dictStr["Three"]))
        print("Number dictionary element with key 2 :" + self._dictNumber[2])
        print("Number dictionary element with key 4 :" + self._dictNumber[4])

    # List Modify - Add
    def ModifyElements_Add(self):
        print("------ ModifyElements_Add");
        # Add 4 elements directly
        self._dictNumber.update({5:"Five", 6:"Six", 7:"Seven", 8:"Eight"})
        print("------ All values of dicNumber:",self._dictNumber.values())
        # Add 4 elements through another dictionary
        localDict1 = {9:"Nine", 10:"Ten", 11:"Eleven", 12:"Twelve"}
        self._dictNumber.update(localDict1)
        print("------ All values of dicNumber:", self._dictNumber.values())
        #Adding existing element has no impact
        self._dictNumber.update({9:"Nine"})
        print("------ All values of dicNumber:", self._dictNumber.values())
        # Add 8 elements by adding 2 dictionaries
        localDict2 = {13:"Thirteen", 14: "Fourteen", 15:"Fifteen", 16:"Sixteen"}
        localDict3 = {17: "Seventeen", 18: "Eighteen", 19:"Nineteen", 20:"Twenty"}
        #{**localDict2, **localDict3} will merge dictionaries
        self._dictNumber.update({**localDict2, **localDict3});
        print("------ All values of dicNumber:", self._dictNumber.values())

    # List Modify - Delete
    def ModifyElements_Delete(self):
        print("------ ModifyElements_Delete");
        print("------ All values of dicNumber:", self._dictNumber.values())
        # Delete 1 element with specified key
        self._dictNumber.pop(2)
        print("------ All values of dicNumber:", self._dictNumber.values())
        # Delete 1 element with specified key - another way
        del self._dictNumber[5]
        print("------ All values of dicNumber:", self._dictNumber.values())
        # Remove all elements of a dictionary
        self._newDictStr = self._dictStr.copy() # create a new memory copy of dictionary
        self._dictStr.clear() #Delete two more elements (in slices)
        print("------ All values of dicStr:", self._dictStr.values())
        print("------ All values of newDicStr:", self._newDictStr.values())

    # List Modify - Update
    def ModifyElements_Update(self):
        print("------ ModifyElements_Update");
        print("------ All values of newDicStr:", self._newDictStr.values())
        # Update element with key "One"
        self._newDictStr["One"]=10
        print("------ All values of newDicStr:", self._newDictStr.values())
        # Update using another dictionary
        dictTemp = {"Two": 20, "Three": 30, "Four": 40}
        self._newDictStr.update(dictTemp)
        print("------ All values of newDicStr:", self._newDictStr.values())
        # Update dictionary to restore original values. Values with same key will be overwritten
        dictTemp = {"One":1, "Two": 2, "Three": 3, "Four": 4}
        self._newDictStr.update(dictTemp)
        print("------ All values of newDicStr:", self._newDictStr.values())
        # Update values using a loop
        for k in self._newDictStr: #iterate through every key in k
            self._newDictStr[k] = self._newDictStr[k] * 10
        print("------ All values of newDicStr:", self._newDictStr.values())

# Instantiate the class and call member functions
ddc=DemoDictClass()
ddc.Initialize()
ddc.PrintInit()
ddc.ModifyElements_Add()
ddc.ModifyElements_Delete()
ddc.ModifyElements_Update()

The output for the above code is:
------ PrintInit
------ All key,value pairs
One,1
Two,2
Three,3
Four,4
------ All keys
dict_keys(['One', 'Two', 'Three', 'Four'])
------ All values
dict_values([1, 2, 3, 4])
String dictionary element with key 'One' :1
String dictionary element with key 'Three' :3
Number dictionary element with key 2 :Two
Number dictionary element with key 4 :Four
------ ModifyElements_Add
------ All values of dicNumber: dict_values(['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight'])
------ All values of dicNumber: dict_values(['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve'])
------ All values of dicNumber: dict_values(['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve'])
------ All values of dicNumber: dict_values(['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen', 'Twenty'])
------ ModifyElements_Delete
------ All values of dicNumber: dict_values(['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen', 'Twenty'])
------ All values of dicNumber: dict_values(['One', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen', 'Twenty'])
------ All values of dicNumber: dict_values(['One', 'Three', 'Four', 'Six', 'Seven', 'Eight', 'Nine', 'Ten', 'Eleven', 'Twelve', 'Thirteen', 'Fourteen', 'Fifteen', 'Sixteen', 'Seventeen', 'Eighteen', 'Nineteen', 'Twenty'])
------ All values of dicStr: dict_values([])
------ All values of newDicStr: dict_values([1, 2, 3, 4])
------ ModifyElements_Update
------ All values of newDicStr: dict_values([1, 2, 3, 4])
------ All values of newDicStr: dict_values([10, 2, 3, 4])
------ All values of newDicStr: dict_values([10, 20, 30, 40])
------ All values of newDicStr: dict_values([1, 2, 3, 4])
------ All values of newDicStr: dict_values([10, 20, 30, 40])

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