Type Hints

Intermediate Object-Oriented Programming in Python

Jake Roach

Data Engineer

Code without type hints

class Student:
    def __init__(self, name, student_id, tuition_balance):
        self.name = name
        self.student_id = student_id
        self.tuition_balance = tuition_balance

# Trying to create a class can be tricky
walker = Student("Sarah Walker", 319921, 15000)

Is student_id supposed to be an integer?
Should tuition_balance be a float or an integer? Maybe a string?
After walker has been created, how can we remember what type it is later in code?

Type hints

Optional tool that allows for information about an object to be added to code

Easier to read, troubleshoot
One of the best ways to demonstrate enterprise-grade Python skills
Not enforced by the interpreter
Built-in type keywords, typing library, custom classes

# Add type hinting when creating a 
# variable
gpa: float = 3.92

# Add type hinting to a function/method 
# definition
def check_grades(year: str) -> List[int]:
    ...

# Can even use custom classes
students: Dict[str, Student] = {...}

Type hinting with built-in type keywords

# Type hinting for declarative logic
name: str = "Frost"  # Before: name = "Frost"
student_id: int = 91031367
tuition_balance: float = 17452.78

# Type hinting for function/method definitions
def get_schedule(semester: str) -> dict:
    ...

Use the syntax variable: type
Declarative, signature of function/method
def .... () -> type: to specify return type

typing Library

typing is a library used to provide more tools to type hint

List, Dict, Tuple
Hint top-level objects, and elements of objects

from typing import List, Dict

student_names: List[str] = ["Morgan", "Chuck", "Anna"]
student_gpas: Dict[str, float] = {
    "Casey": 3.71,
    "Sarah": 4.0
}

Any, Set, Iterator, Callable

Type hinting with custom classes

class Student:
    def __init__(self, name: str, student_id: int, tuition_balance: float) -> None:
        self.name: str = name
        self.student_id: int = student_id
        self.tuition_balance: float = tuition_balance

    def get_course(self, course_id: str) -> Course:
        ...
        return course

# Use Student and Course to type hint
walker: Student = Student("Sarah Walker", 319921, 15000)
data_science: Course = walker.get_course("TDM-20100")

Checking object types

# walker: Student = Student("Sarah Walker", 319921, 15000)
print(type(walker))

<class '__main__.Student'>

# data_science: Course = walker.get_course("TDM-20100")
print(type(data_science))

<class '__main__.Course'>

Let's practice!

Intermediate Object-Oriented Programming in Python