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Python Dictionary

Video link: https://youtu.be/_4wOvc-vt4k

In this video, we learned about Python dictionary that allows us to work with key/value pairs.

Programs in the Video

Create Dictionaries

A dictionary is a collection of key/value pairs. It is similar to associative arrays in other programming languages.

To create a dictionary, we put the key/value pairs separated by a colon : inside the curly braces {}.

person1 = {"name": "Linus", "age": 21}
print(person1)

Output

{"name": "Linus", "age": 21}
Key Value
"name" "Linus"
"age" 21

Notes:

  • Keys of a dictionary can be any immutable objects like numbers, strings and tuples. However, they cannot be objects that can be modified like lists.
  • Keys must be unique for identification.

Access Dictionary Elements

Dictionaries are optimized to get values when the key is known.

Similar to numbered indexes like 0, 1, 2 to get elements from sequences like lists and tuples, keys are used as indices for dictionaries.

person1 = {"name": "Linus", "age": 21}
print(person1["name"])
print(person1["age"])

Output

Linus
21

If we try to access a key that is not in the dictionary, we will get KeyError.

person1 = {"name": "Linus", "age": 21}
print(person1["hobbies"])

Output

Traceback (most recent call last):
  File "<string>", line 2, in <module>
KeyError: 'hobbies'

Sometimes instead of getting this error, we may just want to know if the key exists or not and decide what to do based on it

In that case we can use the dictionary's get() method:

person1 = {"name": "Linus", "age": 21}
print(person1.get("namr"))
print(person1.get("hobbies"))

Output

Linus
None

Instead of an error, we get None which denotes empty or no value. This value can be used with if statement to make different decision as per the need.

We can also pass a second default argument to the get() method that will be returned instead of None if the key is not found.

person1 = {"name": "Linus", "age": 21}
print(person1.get("hobbies", ["dancing", "fishing"]))

Output

["dancing", "fishing"]

Add and Change Dictionary Elements

person1 = {"name": "Linus", "age": 21}

# changing existing keys
person1["name"] = "Dennis"
print(person1)

# adding new keys
person1["hobbies"] = ["dancing", "fishing"]
print(person1)

Output

{'name': 'Dennis', 'age': 21}
{'name': 'Dennis', 'age': 21, 'hobbies': ['dancing', 'fishing']}

Remove Elements From a Dictionary

To remove an item from the dictionary, we can use the dictionary's pop() method. The pop() method also returns the value of the removed key.

For example,

person1 = {"name": "Linus", "age": 21}
print(person1.pop("name"))

print(person1)

Output

Linus
{"age": 21}

Iterating Through a Dictionary

Similar to sequences, we can easily iterate through items of a dictionary by using a for loop. We get one key in every iteration:

person1 = {"name": "Linus", "age": 21}

for key in person1:
   print(key)
   print(person1[key])

Output

name
Linus
age
21

Note: Starting from Python 3.7, the order of items in a dictionary is preserved. So when we iterate through a dictionary, we get the keys in the order in which they are inserted in the dictionary.

Programming Task

Can you guess the output of this program?

synonyms = {"mountain": "peak", "forest": "jungle"}
print("1.", synonyms["mountain"])

synonyms["terrain"] = "land"
print("2.", synonyms)

synonyms.pop("forest")
print("3.", synonyms)

Output

1. peak
2. {'mountain': 'peak', 'forest': 'jungle', 'terrain': 'land'}
3. {'mountain': 'peak', 'terrain': 'land'}