Python Dictionaries

Dictionary

Data values are stored in key : value pairs using dictionaries.

A dictionary is an ordered collection that can be changed and it will not support duplicate key-values.

Dictionaries are ordered with Python version 3.7. Dictionaries are not ordered in Python 3.6 and earlier.

Curly brackets {} are used to create dictionaries.

Example :- A simple dictionary

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
print(mydict)

Output :-

{'id': '101', 'name': 'Suresh', 'salary': 2500}

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Dictionary Items

Dictionary items are ordered, modifiable and don't allow duplicates.

In key : value pairs the dictionary items will be presented and can be referred to by the key name.

Example :- Print the dictionary's 'salary' value :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
print(mydict["salary"])

Output :-

2500

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Ordered or Unordered?

Dictionaries are ordered as of Python version 3.7. Dictionaries are unordered in Python 3.6 and earlier.

If we say the ordering of dictionaries, it means that there is a definition of the commands, and that order won't change.

Unordered means that you cannot refer to an item by using an index without a defined order.

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Changable

The dictionaries can be modified so that after the dictionary is created we may change, add or remove items.

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Duplicates Not Allowed

Dictionaries cannot have the same key for two items. If same key found, existing values overwrite the duplicate values

Example :- A simple dictionary with duplicate key "salary" :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500,
  "salary": 3500
}
print(mydict)

Output :-

{'id': '101', 'name': 'Suresh', 'salary': 3500}

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Dictionary Length

Use the len() function to find out the number of items in a dictionary.

Example :- Print the dictionary's number of items :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
print(len(mydict))

Output :-

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Check if Key Exists

For example, you may use the in keyword to find out whether a given key is in a dictionary :

Example :- Look in the dictionary to see if "id" is present :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
if "id" in mydict:
  print("Yes, 'id' is one of the keys in the mydict dictionary")

Output :-

Yes, 'id' is one of the keys in the mydict dictionary

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Dictionary Items - Data Types

Dictionary items can have values of all types of data :

Example :- List of dictionaries with strings, int, booleans, and lists datatypes :

mydict = {
  "id": 101,
  "name": "xy",
  "salary": 2500.56,
  "married": True,
  "dept": ["sales", "service", "accounts"]
}
print(mydict)

Output :-

{'id': 101, 'name': 'xy', 'salary': 2500.56, 'married': True, 'dept': ['sales', 'service', 'accounts']}

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type()

Dictionaries are defined from a Python perspective like objects with the 'dict' data type:

Example :- Print the data type of a dictionary :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
print(type(mydict))

Output :-

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Python - Access Dictionary Items

There are two to access a dictionary items in python, which is

[] :- Get a dictionary content by refferring to the key name in the square brackets.
get() :- This method is used to get content of a dictionary by providing key name in the method argument.

Accessing Items

Using square brackets, you can retrieve a dictionary's contents by referring to the key name within the brackets and also you can use a method named get(), which returns the same result.

Example 1 :- Retrieve the value of the "id" key :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
x = mydict["id"]
y = mydict.get("id")
print(x)
print(y)

Output :-

101
101

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Get Keys and Values

The keys() method is used to return all key names from a dictionary.

The values() method is used to return all key values from a dictionary.

Example :- Get all keys and values from a dictionary:

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
x = mydict.keys()
y = mydict.values();
print(x)
print(y)

Output :-

dict_keys(['id', 'name', 'salary'])
dict_values(['101', 'Suresh', 2500])

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Get Items

The items() method returns each item in a dictionary as a list of tuples.

Example 1 :- Get a list of the key:value pairs :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
x = mydict.items()
print(x)

Output :-

dict_items([('id', '101'), ('name', 'Suresh'), ('salary', 2500)])

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Python - Change Dictionary Items

Python provide facilities to change a item value in a dictionary.

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Change Values

A specific item's value can be changed by referring to its key name :

Example :- Change the "name" to "Babu" :

mydict =	{
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
mydict["name"] = "Babu"
print(mydict)

Output :-

{'id': '101', 'name': 'Babu', 'salary': 2500}

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Update Dictionary

The dictionary will be updated with the elements from the supplied input using the update() method.

A dictionary or an iterable object containing key:value pairs must be used as an argument to the function.

Example :- The update() method can be used to update the "salary" of the employee :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
mydict.update({"salary": 5000})
print(mydict)

Output :-

{'id': '101', 'name': 'Suresh', 'salary': 5000}

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Python - Add Dictionary Items

Adding Items

Each entry in the dictionary is added using an index key that has a value assigned to the new item.

Example :- Adding a new key "dept" :

mydict =	{
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
mydict["dept"] = "sales"
print(mydict)

Output :-

{'id': '101', 'name': 'Suresh', 'salary': 2500, 'dept': 'sales'}

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Update Dictionary

The update() method updates the dictionary with items from a provided input. If the item does not already exist, it will be added to the system.

A dictionary or an iterable object with key:value pairs must be used as the argument to the function

Example :- The update() method can be used to add a dept item to the dictionary :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
mydict.update({"dept": "sales"})
print(mydict)

Output :-

{'id': '101', 'name': 'Suresh', 'salary': 2500, 'dept': 'sales'}

Python - Remove Dictionary Items

Remove things from a dictionary can be done in a variety of ways :

The pop() method and del keyword are used to delete an item from a dictionary.

Example 1 :- Removes an item using pop() method and del keyword :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
mydict.pop("name")
print(mydict)
del mydict["salary"]
print(mydict)

Output :-

{'id': '101', 'salary': 2500}
{'id': '101'}

The clear() method is used to delete all items from a dictionary and make it empty dictionary.

Example 2 :- Using the clear() method, the dictionary is emptied of all items :

mydict =	{
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
mydict.clear()
print(mydict)

Output :-

{}

The del keyword is used to delete all items in a dictionary and also delete the dictionary itself.

Example 3 :- The del keyword can also be used to remove the entire dictionary :

mydict =	{
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
del mydict
print(mydict) #this will cause an error because "mydict" no longer exists.

Output :-

Traceback (most recent call last):
File "demo_dictionary_del3.py", line 7, in < module>
print(mydict) #this will cause an error because "mydict" no longer exists.
NameError: name 'mydict' is not defined

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Python - Loop Dictionaries

Using a for loop, you may loop through a dictionary.

In a dictionary looping through, the return value is the dictionary keys, but there are several methods to return the dictionary's actual values.

Example 1 :- Print out the dictionary's key names one by one :

mydict =	{
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
for x in mydict:
  print(x)

Output :-

id
name
salary

Example 2 :- Print all dictionary values one by one :

mydict =	{
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
for x in mydict:
  print(mydict[x])

Output :-

101
Suresh
2500

Example 3 :- Use the items() method to loop through both keys and values :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
for x, y in mydict.items():
  print(x, " = ", y)

Output :-

id = 101
name = Suresh
salary = 2500

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Python - Copy Dictionaries

By entering dict2 = dict1, you will just be copying the dictionary's contents, and any modifications made to dict1 will immediately be made to dict2.

Making a copy can be done in several ways, including by using the built-in dictionary method copy() and dict().

Example :- make a copy of a dictionary's contents using copy() and dict() methods :

mydict = {
  "id": "101",
  "name": "Suresh",
  "salary": 2500
}
mydict2 = mydict.copy()
mydict3 = dict(mydict)
print(mydict2)
print(mydict3)

Output :-

{'id': '101', 'name': 'Suresh', 'salary': 2500}
{'id': '101', 'name': 'Suresh', 'salary': 2500}

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Nested Dictionaries

When a dictionary contains another dictionary, this is called nesting dictionary.

Exmaple 1 :- A dictionary that contains three dictionaries should be constructed :

employees = {
  "emp1" : {
    "name" : "aa",
    "year" : 2004
  },
  "emp2" : {
    "name" : "bb",
    "year" : 2007
  },
  "emp3" : {
    "name" : "cc",
    "year" : 2011
  }
}
print(employees)

Output :-

{'emp1': {'name': 'aa', 'year': 2004}, 'emp2': {'name': 'bb', 'year': 2007}, 'emp3': {'name': 'cc', 'year': 2011}}

If you wish to combine three dictionaries into one, For example.

Example 2 :- Construct two dictionaries, then create a dictionary that contains the other two dictionaries :

emp1 = {
  "name" : "aa",
  "year" : 2004
}
emp2 = {
  "name" : "bb",
  "year" : 2007
}
employees = {
  "emp1" : emp1,
  "emp2" : emp2
}
print(employees)

Output :-

{'emp1': {'name': 'aa', 'year': 2004}, 'emp2': {'name': 'bb', 'year': 2007}}

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