Python Dictionaries

Dictionaries

Dictionaries are used to store data values in key:value pairs.

A dictionary is a collection which is ordered*, changeable and do not allow duplicates.

Dictionaries are written with curly brackets, and have keys and values:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict)

Dictionary Items

Dictionary items are ordered, changeable, and does not allow duplicates.

Dictionary items are presented in key:value pairs, and can be referred to by using the key name.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict["brand"])

Ordered or Unordered?

As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

When we say that dictionaries are ordered, it means that the items have a defined order, and that order will not change.

Unordered means that the items does not have a defined order, you cannot refer to an item by using an index.

Changeable

Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.

Duplicates Not Allowed

Dictionaries cannot have two items with the same key:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964,
  "year": 2020
}
print(thisdict)

Dictionary Length

To determine how many items a dictionary has, use the len() function:

print(len(thisdict))

Dictionary Items - Data Types

The values in dictionary items can be of any data type:

thisdict = {
  "brand": "Ford",
  "electric": False,
  "year": 1964,
  "colors": ["red", "white", "blue"]
}

type()

From Python's perspective, dictionaries are defined as objects with the data type 'dict':

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(type(thisdict))

Accessing Items

You can access the items of a dictionary by referring to its key name, inside square brackets:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
x = thisdict["model"]

There is also a method called get() that will give you the same result:

x = thisdict.get("model")

Get Keys

The keys() method will return a list of all the keys in the dictionary.

x = thisdict.keys()

Get Values

The values() method will return a list of all the values in the dictionary.

x = thisdict.values()

Get Items

The items() method will return each item in a dictionary, as tuples in a list.

x = thisdict.items()

Check if Key Exists

To determine if a specified key is present in a dictionary use the in keyword:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
if "model" in thisdict:
  print("Yes, 'model' is one of the keys in the thisdict dictionary")

Change Values

You can change the value of a specific item by referring to its key name:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict["year"] = 2018

Update Dictionary

The update() method will update the dictionary with the items from the given argument.

The argument must be a dictionary, or an iterable object with key:value pairs.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.update({"year": 2020})

Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict["color"] = "red"
print(thisdict)

Update Dictionary

The update() method will update the dictionary with the items from a given argument. If the item does not exist, the item will be added.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.update({"color": "red"})

Removing Items

There are several methods to remove items from a dictionary:

The pop() method

The pop() method removes the item with the specified key name:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.pop("model")
print(thisdict)

The popitem() method

The popitem() method removes the last inserted item (in versions before 3.7, a random item is removed instead):

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.popitem()
print(thisdict)

The del keyword

The del keyword removes the item with the specified key name:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
del thisdict["model"]
print(thisdict)

The del keyword can also delete the dictionary completely:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
del thisdict
print(thisdict) #this will cause an error because "thisdict" no longer exists.

The clear() method

The clear() method empties the dictionary:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.clear()
print(thisdict)

Loop Through a Dictionary

You can loop through a dictionary by using a for loop.

When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well.

for x in thisdict:
  print(x)

for x in thisdict:
  print(thisdict[x])

for x in thisdict.values():
  print(x)

for x in thisdict.keys():
  print(x)

for x, y in thisdict.items():
  print(x, y)

Safe Access with get()

Indexing a missing key raises KeyError. get() returns a default instead.

person = {"name": "John", "age": 36}

# print(person["city"])       # KeyError!
print(person.get("city"))      # None
print(person.get("city", "Unknown"))   # Unknown
print("age" in person)         # True

Looping Over a Dictionary

person = {"name": "John", "age": 36}

for key in person:                 # keys
    print(key)

for value in person.values():      # values
    print(value)

for key, value in person.items():  # both
    print(f"{key} = {value}")

Dict Comprehensions and Counting

squares = {n: n ** 2 for n in range(5)}   # {0:0, 1:1, 2:4, 3:9, 4:16}

# count letters
from collections import Counter
print(Counter("banana"))   # Counter({'a': 3, 'n': 2, 'b': 1})

# merge two dicts (Python 3.9+)
a = {"x": 1}; b = {"y": 2}
print(a | b)               # {'x': 1, 'y': 2}

Key Takeaways

• Dicts map unique, hashable keys to values with O(1) lookup.
• Use get(key, default) to avoid KeyError.
• Iterate with .items() for key/value pairs.
• Dicts keep insertion order; Counter tallies effortlessly.