Type Hinting & Checking

Definitions

Some terms to help you navigate the post

Type hinting

Is the mechanism in python that allows you to know what type a variable is. Each created object in python has a type including the builtin types like dict and list. See this cheatsheet as reference for full details here.

Type Checking

Is a method of validating that a variable is of a certain expected type. There are multiple ways of doing this. For Example:

my_list = ["hello"] # Returns <class 'list'>
type(my_list) == list # Returns True

Some types require a more exotic form of type checking such as generators, to type check generators for example:

import types
def yield_numbers(amount = 10):
    for i in range(amount):
        yield i

isinstance(yield_numbers(), types.GeneratorType) # Returns True

Usage

In this repo you can see the demo code and actually run it by running python type_cheking.py or python3 type_cheking.py

Real World Applications

If you are writing API's that are meant to work with with various data types then type checking is a good way to validate input. KEEP IN MIND type checking is expensive in terms of computation so avoid using it as much as possible. A common pattern to avoid is checking types in a loop.

For example:

data = {"URL": ["google.ca", "youtube.com", "facebook.com"], 
    "Company": ["Google", "Google", "Facebook"]}

for counter in range(len(list(data.keys())[0])): # Iterates over whole list/generator
    for key in data:
        # Check if value of key is a list or generator
        if type(data[key]) is list:
            # Do stuff

        elif isinstance(data[key], types.GeneratorType):
            # Do stuff

Because python is checking on every iteration this loop would be VERY slow. A better option would be something like this:

data = {"URL": ["google.ca", "youtube.com", "facebook.com"], 
    "Company": ["Google", "Google", "Facebook"]}

# Set up boolean values to keep track of value types
is_list = False
is_generator = False


for key in data: # Do an initial pass to check types
    if type(data[key]) is list:
        is_list = True
    elif isinstance(data[key], types.GeneratorType):
        is_generator = True
    else:
        raise TypeError("An invalid type was provided")
for counter in range(len(list(data.keys())[0])): # Iterates over whole list/generator
for key in data:
        # Check if value of key is a list or generator
        if is_list:
            # Do stuff

        elif is_generator:
            # Do stuff

Files

type_checking_demo.py

"""
Description:
A demonstration of type checking
"""
__author__ = "Canadian Coding"
# Type Hinting/Checking on a list
my_list = ["hello"] # Returns <class 'list'>
type(my_list) == list # Returns True
# Type Checking Generators
import types # Need to explicitly import types to check generators
def yield_numbers(amount = 10):
    """A dummy function that yields a generator of ints as an example
    Yields:
    Incrementing integers up to amount provided
    """
    for i in range(amount):
        yield i