Background
In this post, we will see how we can use dictionaries and specifically how we can enable a custom class in Python to be used as a dict key. I am originally from a Java background and have written posts before on how HashMap/HashTable works in Java (See Related links section towards the end of this post).
If someone is new
- Dictionaries are data structures that store data of format key: value pairs
- Data stored in dict is unique (does not allow duplicate keys), mutable (can edit), and ordered
NOTE: As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
Dictionaries in Python
You can initialize a dictionary in Python using {} or dict keyword
- my_dict = {}
- my_dict_1 = {"Name": "Aniket", "Country": "India"}
- my_dict_2 = dict(name="Aniket")
You can print these and see the dictionary. It will print
{}
{'Name': 'Aniket', 'Country': 'India'}
{'name': 'Aniket'}
A normal dict like above is unordered which means the order in which you insert data in dict is not maintained, when you iterate over items in dict it may give you a different order. This is the same as HashMap in Java. However, similar to LinkedHashMap we have OrderedDict in Python (Please note that as of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered).
You can also merge one map to another using the update method. Remember key's of a dict are unique and similar to HashMap if you try to insert a key that is already present in the dict then it is going to override the key data with the new value (See below code).
my_dict = {}
my_dict["name"] = "Aniket"
my_dict["Country"] = "India"
for key in my_dict:
print(f"{key} : {my_dict[key]}")
print("--------------")
others_dict = {}
others_dict["name"] = "John"
others_dict["State"] = "NYC"
for key in others_dict:
print(f"{key} : {others_dict[key]}")
print("--------------")
my_dict.update(others_dict)
for key in my_dict:
print(f"{key} : {my_dict[key]}")
Output is:
name : Aniket
Country : India
--------------
name : John
State : NYC
--------------
name : John
Country : India
State : NYC
See how name key value was overridden by new dict key, Country key was not overridden so it stated the same and finally a new key called State was added.
Using a custom class as a key in dict
In Java for a new class, you would typically override equals and hashcode method to make it work, in Python we have to do something similar as well.
Let's create an Employee class with just name and age, try to create objects out of it, push to dict and print it.
class Employee:
def __init__(self, name, age):
self.name = name
self.age = age
def __str__(self):
return f"Employee with name: {self.name} & age: {self.age}"
my_dict = {}
e1 = Employee("Aniket", 30)
e2 = Employee("Aniket", 30)
my_dict[e1] = 1
my_dict[e2] = 2
for key, value in my_dict.items():
print(f"{key} : {value}")
Output is:
Employee with name: Aniket & age: 30 : 1
Employee with name: Aniket & age: 30 : 2
As you see from output it took the same key as different keys in dict and added separate entries. Since the employee (uniquely defined by name and age) is same we want just one entry in dict corresponding to a unique name and age. In this case it should have overridden value from 1 to 2 and just printed single entry with value 2. Let's see how we do that.
For the above to work similarly to Java (where we override equals and hashcode), in Python we override the two methods below
- __eq__()
- __hasg__()
So your code will now look like
class Employee:
def __init__(self, name, age):
self.name = name
self.age = age
def __str__(self):
return f"Employee with name: {self.name} & age: {self.age}"
def __eq__(self, other):
return (self.name, self.age) == (other.name, other.age)
def __hash__(self):
return hash((self.name, self.age))
my_dict = {}
e1 = Employee("Aniket", 30)
e2 = Employee("Aniket", 30)
my_dict[e1] = 1
my_dict[e2] = 2
for key, value in my_dict.items():
print(f"{key} : {value}")
This now prints:
Employee with name: Aniket & age: 30 : 2
which is in line with our expectations.
See the below diagram for all methods supported by Python dict
