Processing Collections in Python Using Lambdas

To sort our list of dictionaries we're going to use the sorted function. Not all of the values for the name key in our dictionaries start with a capital letter, and because of that, we're going to want to convert all of the strings to lowercase before making the comparison. Thankfully, we can do this by passing a lambda for the key parameter. Here's how we can complete the first task:

collection_processing.py

# 1) Sort the `people` list of dictionaries alphabetically based on the
# 'name' key from each dictionary using the `sorted` function and store
# the new list as `sorted_by_name`

people = [
    {"name": "Kevin Bacon", "age": 61},
    {"name": "Fred Ward", "age": 77},
    {"name": "finn Carter", "age": 59},
    {"name": "Ariana Richards", "age": 40},
    {"name": "Victor Wong", "age": 74},
]

sorted_by_name = sorted(people, key=lambda d: d['name'].lower())

assert sorted_by_name == [
    {"name": "Ariana Richards", "age": 40},
    {"name": "finn Carter", "age": 59},
    {"name": "Fred Ward", "age": 77},
    {"name": "Kevin Bacon", "age": 61},
    {"name": "Victor Wong", "age": 74},
], f"Expected sorted_by_name to equal '{sorted_by_name}' to equal '{[{'name': 'Ariana Richards', 'age': 40}, {'name': 'finn Carter', 'age': 59}, {'name': 'Fred Ward', 'age': 77}, {'name': 'Kevin Bacon', 'age': 61}, {'name': 'Victor Wong', 'age': 74}]}''"

Running the script, we should see the following error. This indicates that we've successfully met the requirements for the first task.

$ python3.7 collection_processing.py
Traceback (most recent call last):
  File "collection_processing.py", line 36, in <module>
    ], f"Expected name_declarations to equal '{name_declarations}' to equal '{['Ariana Richards is 40 years old', 'finn Carter is 59 years old', 'Fred Ward is 77 y
ears old', 'Kevin Bacon is 61 years old', 'Victor Wong is 74 years old']}'"
AssertionError: Expected name_declarations to equal 'None' to equal '['Ariana Richards is 40 years old', 'finn Carter is 59 years old', 'Fred Ward is 77 years old', 'Kevin Bacon is 61 years old', 'Victor Wong is 74 years old']'

Our second task is to create a list where each string uses the name and age parameters to build strings of <NAME> is <AGE> years old. Let's create the name_declarations variable:

collection_processing.py

# 2) Use the `map` function to iterate over `sorted_by_name` to generate a
# new list called `name_declarations` where each value is a string with
# `<NAME> is <AGE> years old.` where the `<NAME>` and `<AGE>` values are from
# the dictionaries.

name_declarations = list(
    map(lambda d: f"{d['name']} is {d['age']} years old", sorted_by_name)
)

assert name_declarations == [
    "Ariana Richards is 40 years old",
    "finn Carter is 59 years old",
    "Fred Ward is 77 years old",
    "Kevin Bacon is 61 years old",
    "Victor Wong is 74 years old",
], f"Expected name_declarations to equal '{name_declarations}' to equal '{['Ariana Richards is 40 years old', 'finn Carter is 59 years old', 'Fred Ward is 77 years old', 'Kevin Bacon is 61 years old', 'Victor Wong is 74 years old']}'"

Note that we need to convert the result of map to be a list, otherwise our name_declarations variable will be a map object.

Running our script again, this is what we should see:

python3.7 collection_processing.py
Traceback (most recent call last):
  File "collection_processing.py", line 50, in <module>
    ], f"Expected under_seventy to equal '{under_seventy}' to equal '{[{'name': 'Ariana Richards', 'age': 40}, {'name': 'finn Carter', 'age': 59}, {'name': 'Kevin Bacon', 'age': 61}]}'"
AssertionError: Expected under_seventy to equal 'None' to equal '[{'name': 'Ariana Richards', 'age': 40}, {'name': 'finn Carter', 'age': 59}, {'name': 'Kevin Bacon', 'age': 61}]'

Our last task is to filter the values in sorted_by_name so our output list only includes dictionaries that have an age value less than 70. To achieve this, we're going to combine filter and sorted to create our final list.

collection_processing.py

# 3) Combine the `filter` and `sorted` functions to iterate over `sorted_by_name` to generate a
# new list called `under_seventy` that only contains the dictionaries where the
# 'age' key is less than 70, sorting the list by age.

under_seventy = sorted(
    filter(lambda d: d['age'] < 70, sorted_by_name), key=lambda d: d['age']
)

assert under_seventy == [
    {"name": "Ariana Richards", "age": 40},
    {"name": "finn Carter", "age": 59},
    {"name": "Kevin Bacon", "age": 61},
], f"Expected under_seventy to equal '{under_seventy}' to equal '{[{'name': 'Ariana Richards', 'age': 40}, {'name': 'finn Carter', 'age': 59}, {'name': 'Kevin Bacon', 'age': 61}]}'"

Running our script one last time we should see no output because all of the assertions were true.