"""

A `collections.defaultdict` is a subclass of the built-in dictionary (`dict`)

that automatically initializes missing keys with a default value.

It overrides the `__missing__` method and calls a user-specified factory

function (e.g., `list`, `int`, `set`) to provide a default value when a key

is accessed but not found. This eliminates the need for boilerplate checks

using `dict.setdefault()` or `if key not in dict` blocks, improving both

readability and efficiency.

"""

from collections import defaultdict

# Module-level constants

_GPA_MIN = 0.0

_GPA_MAX = 4.0

_EPS = 0.000001

def main() -> None:

# Let's create a defaultdict with student keys and GPA values. The first

# parameter is called default_factory, and it is the initialization value for

# first use of a key. It can be a common type or a function

student_gpa = defaultdict(float, [("john", 3.5), ("bob", 2.8), ("mary", 3.2)])

# There are three student records in this dictionary

assert len(student_gpa) == 3

# Each student has a name key and a GPA value

assert len(student_gpa.keys()) == len(student_gpa.values())

# We can get the names in isolation. Note that in Python 3.7 and

# above, dictionary entries are sorted in the order that they were

# defined or inserted

student_names = []

for student in student_gpa.keys():

student_names.append(student)

assert student_names == ["john", "bob", "mary"]

# We can get the GPA for a specific student

assert abs(student_gpa["john"] - 3.5) < _EPS

# And the defaultdict allow us to get the GPA of a student that is not in

# the data structure yet, returning a default value for float that is 0.0

assert student_gpa["jane"] == _GPA_MIN

# And now there are four student records in this dictionary

assert len(student_gpa) == 4

# You can set the default value in default_factory attribute

def set_default_to_gpa_max() -> float:

return _GPA_MAX

student_gpa.default_factory = set_default_to_gpa_max

assert student_gpa["rika"] == _GPA_MAX

# And now there are five student records in this dictionary

assert len(student_gpa) == 5

if __name__ == "__main__":

main()