It's a bit of a lie to call this “Week 4,” because we spent so much time on new things for web scraping in the fourth week, we didn’t have time to talk about new Python concepts.
The slides cover two chapters in Sweigart:
- Chapter 5: Dictionaries and Structuring Data
- Chapter 7: Pattern Matching with Regular Expressions
The crucial concept to understand is that a dictionary consists of key-value pairs.
{"Title":"Harry Potter and the Goblet of Fire","Author":"J. K. Rowling","Year":2001}The keys in that dictionary are "Title", "Author" and "Year".
The values in that dictionary are "Harry Potter and the Goblet of Fire", "J. K. Rowling" and 2001.
Different from a list, a dictionary does not have indexes. You can't count on the order of items in a dictionary to stay the same. You cannot insert a new item into a particular position in a dictionary because the items do not have positions, or places. Thus we refer to lists as ordered and to dictionaries as unordered.
Although Sweigart refers to the keys as indexes, you shouldn't think of them behaving exactly as indexes do in lists.
The following ways to loop through a dictionary are demonstrated in the file looping_keys_values.py in this folder:
for k in dict.keys():for v in dict.values():for k, v in dict.items():
Sweigart does an excellent job of explaining dictionaries in his chapter 5.
You can use Mr. Data Converter to do this.
Alternatively, you can use Python's csv library and the DictReader() method to create a dictionary of dictionaries or a list of dictionaries from a CSV file. See dictreader_example.py in this folder.
- Differences between lists and dictionaries
- Take care with use of quotes when both keys and values are strings
- How to loop through keys, values, or both at once
- Use
list( dict.keys() )to get a list of all keys in a dictionary - Check for presence of a key or a value with
inornot in - Provide a default value (here, 0) in case a key is not present:
dict.get(key, 0) - Use
dict.setdefault(key, value)to set a new key but prevent overwriting an existing value if the key is already present - Use
import pprintif you need to print out a large dictionary's contents in the Terminal
We stop on page 112, “Using Data Structures to Model Real-World Things.” On page 117, Sweigart starts to discuss “dictionaries and lists that contain other dictionaries and lists,” also known as nested dictionaries. It's a rather brief discussion and not tremendously helpful.
Let's look at a nested dictionary that contains four other dictionaries.
person =
{"gender":"female","name":{"title":"mrs","first":"jeanette","last":"thomas"},
"location":{"street":"7446 hickory creek dr","city":"caldwell",
"state":"wyoming","postcode":73617},
"dob":"1959-03-21 14:19:01",
"id":{"name":"SSN","value":"578-92-7338"},
"picture":{"large":"https://randomuser.me/api/portraits/women/56.jpg",
"medium":"https://randomuser.me/api/portraits/med/women/56.jpg",
"thumbnail":"https://randomuser.me/api/portraits/thumb/women/56.jpg"}}Just a note: We can have lists and/or tuples inside dictionaries as well. We can have multiples inside of multiples inside of multiples. In this case, though, we have only one added level of complexity.
Note too that the key cannot be a list or a dictionary. The key must be an immutable type, such as a string or a number. (Some tuples can be used as keys, but I don't know how that would work.) The value may be a list or a dictionary, or just a string or a number. We use the key to find the value, so it follows that each key must be unique within a dictionary.
In the example above, these are the (main or top-level) keys:
- gender
- name !
- location !
- dob
- id !
- picture !
The keys marked with ! above have values that are embedded dictionaries.
Keys in the inner dictionary (person["name"]) that is the value of "name":
- title
- first
- last
Keys in the inner dictionary (person["location"]) that is the value of "location":
- street
- city
- state
- postcode
Keys in the inner dictionary (person["id"]) that is the value of "id":
- name
- value
Keys in the inner dictionary (person["picture"]) that is the value of "picture":
- large
- medium
- thumbnail
The example file complex_dicts.py demonstrates how to access the values inside the embedded dictionaries.
There are several ways to use Python dictionaries to create a larger data store.
All dictionaries contain the same keys, and they are all in one list, which can be looped through:
presidents_list = [ {"Presidency":1,"President":"George Washington","Wikipedia_entry":"http://en.wikipedia.org/wiki/George_Washington","Took_office":"4/30/1789","Left_office":"3/4/1797","Party":"Independent ","Home_state":"Virginia","Occupation":"Planter","College":"None","Age_when_took_office":57,"Birth_date":"2/22/1732","Birthplace":"Westmoreland County, Virginia","Death_date":"12/14/1799","Location_death":"Mount Vernon, Virginia"},
{"Presidency":2,"President":"John Adams","Wikipedia_entry":"http://en.wikipedia.org/wiki/John_Adams","Took_office":"3/4/1797","Left_office":"3/4/1801","Party":"Federalist ","Home_state":"Massachusetts","Occupation":"Lawyer","College":"Harvard","Age_when_took_office":61,"Birth_date":"10/30/1735","Birthplace":"Quincy, Massachusetts","Death_date":"7/4/1826","Location_death":"Quincy, Massachusetts"},
{"Presidency":3,"President":"Thomas Jefferson","Wikipedia_entry":"http://en.wikipedia.org/wiki/Thomas_Jefferson","Took_office":"3/4/1801","Left_office":"3/4/1809","Party":"Democratic-Republican ","Home_state":"Virginia","Occupation":"Planter, Lawyer","College":"William and Mary","Age_when_took_office":57,"Birth_date":"4/13/1743","Birthplace":"Albemarle County, Virginia","Death_date":"7/4/1826","Location_death":"Albemarle County, Virginia"},
{"Presidency":4,"President":"James Madison","Wikipedia_entry":"http://en.wikipedia.org/wiki/James_Madison","Took_office":"3/4/1809","Left_office":"3/4/1817","Party":"Democratic-Republican ","Home_state":"Virginia","Occupation":"Lawyer","College":"Princeton","Age_when_took_office":57,"Birth_date":"3/16/1751","Birthplace":"Port Conway, Virginia","Death_date":"6/28/1836","Location_death":"Orange County, Virginia"},
{"Presidency":5,"President":"James Monroe","Wikipedia_entry":"http://en.wikipedia.org/wiki/James_Monroe","Took_office":"3/4/1817","Left_office":"3/4/1825","Party":"Democratic-Republican ","Home_state":"Virginia","Occupation":"Lawyer","College":"William and Mary","Age_when_took_office":58,"Birth_date":"4/28/1758","Birthplace":"Westmoreland County, Virginia","Death_date":"7/4/1831","Location_death":"New York, New York"},
... ]All dictionaries contain an identifying key (in this case, the name of a president) and a value that is itself a dictionary, containing the same keys as all the others. Note where the curly braces are used.
presidents_dict = { { "George Washington": {"Presidency":1,"Wikipedia_entry":"http://en.wikipedia.org/wiki/George_Washington","Took_office":"4/30/1789","Left_office":"3/4/1797","Party":"Independent ","Home_state":"Virginia","Occupation":"Planter","College":"None","Age_when_took_office":57,"Birth_date":"2/22/1732","Birthplace":"Westmoreland County, Virginia","Death_date":"12/14/1799","Location_death":"Mount Vernon, Virginia"} },
{ "John Adams": {"Presidency":2,"Wikipedia_entry":"http://en.wikipedia.org/wiki/John_Adams","Took_office":"3/4/1797","Left_office":"3/4/1801","Party":"Federalist ","Home_state":"Massachusetts","Occupation":"Lawyer","College":"Harvard","Age_when_took_office":61,"Birth_date":"10/30/1735","Birthplace":"Quincy, Massachusetts","Death_date":"7/4/1826","Location_death":"Quincy, Massachusetts"} },
{ "Thomas Jefferson": {"Presidency":3,"Wikipedia_entry":"http://en.wikipedia.org/wiki/Thomas_Jefferson","Took_office":"3/4/1801","Left_office":"3/4/1809","Party":"Democratic-Republican ","Home_state":"Virginia","Occupation":"Planter, Lawyer","College":"William and Mary","Age_when_took_office":57,"Birth_date":"4/13/1743","Birthplace":"Albemarle County, Virginia","Death_date":"7/4/1826","Location_death":"Albemarle County, Virginia"} },
{ "James Madison": {"Presidency":4,"Wikipedia_entry":"http://en.wikipedia.org/wiki/James_Madison","Took_office":"3/4/1809","Left_office":"3/4/1817","Party":"Democratic-Republican ","Home_state":"Virginia","Occupation":"Lawyer","College":"Princeton","Age_when_took_office":57,"Birth_date":"3/16/1751","Birthplace":"Port Conway, Virginia","Death_date":"6/28/1836","Location_death":"Orange County, Virginia"} },
{ "James Monroe": {"Presidency":5,"Wikipedia_entry":"http://en.wikipedia.org/wiki/James_Monroe","Took_office":"3/4/1817","Left_office":"3/4/1825","Party":"Democratic-Republican ","Home_state":"Virginia","Occupation":"Lawyer","College":"William and Mary","Age_when_took_office":58,"Birth_date":"4/28/1758","Birthplace":"Westmoreland County, Virginia","Death_date":"7/4/1831","Location_death":"New York, New York"} },
... }