Flash cards
Review the key moves
What is the main idea behind Python Dictionaries?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ = {Put the learning moves in the order that makes the concept easiest to apply.
Dictionary
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.
As of Python version 3.7, dictionaries are ordered . In Python 3.6 and earlier, dictionaries are unordered .
Dictionaries are written with curly brackets, and have keys and values:
Example
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
print(thisdict)Dictionary Items
Dictionary items are ordered, changeable, and do not allow duplicates.
Dictionary items are presented in key:value pairs, and can be referred to by using the key name.
Example
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 do 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:
Example
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:
Example
print(len(thisdict))Dictionary Items - Data Types
The values in dictionary items can be of any data type:
Example
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':
Example
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
print(type(thisdict))The dict() Constructor
It is also possible to use the dict() constructor to make a dictionary.
Example
thisdict =
dict(name = "John", age = 36, country = "Norway")
print(thisdict)Python Collections (Arrays)
There are four collection data types in the Python programming language:
- List is a collection which is ordered and changeable. Allows duplicate members.
- Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
- Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
- Dictionary is a collection which is ordered** and changeable. No duplicate members.
*Set items are unchangeable, but you can remove and/or add items whenever you like.
**As of Python version 3.7, dictionaries are ordered . In Python 3.6 and earlier, dictionaries are unordered .
When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.