Welcome to MadPy!

Organizers

Ed Rogers	David Hoese	Josh Karpel

Code of Conduct

MadPy is a community group and open to all experience levels. We are committed to a safe, professional environment

Our Commitment to You

We are enthusiastically focused on improving our event and making it a place that is welcoming to all. All reports will be taken seriously, handled respectfully, and dealt with in a timely manner.

Learn more about the MadPy Code of Conduct:

https://github.com/madison-python/code-of-conduct

Python Warm-Up

A get function with a default:

madpy_menu = {
    "pizza": "Ian's",
    "beverage": "Sprite",
}

def get_menu_item(key, default=None):
    value = madpy_menu.get(key)
    if value is None:
        return default
    return value

get_menu_item("pizza")

"Ian's"

get_menu_item("salad", default="Sorry, not tonight!")

'Sorry, not tonight!'

Trouble with the get function:

madpy_menu = {
    "pizza": "Ian's",
    "beverage": "Sprite",
    "dessert": None,
}

def get_menu_item(key, default=None):
    value = madpy_menu.get(key)
    if value is None:
        return default
    return value

get_menu_item("salad", default="Sorry, not tonight!")

'Sorry, not tonight!'

get_menu_item("dessert", default="Sorry, not tonight!")

'Sorry, not tonight!'

Can't distinguish between not being on the menu, and being on the menu and None-valued!

Using a "sentinel" value

madpy_menu = {
    "pizza": "Ian's",
    "beverage": "Sprite",
    "dessert": None,
}

MISSING = object()

def get_menu_item(key, default=None):
    value = madpy_menu.get(key, MISSING)
    if value is MISSING:
        return default
    return value

get_menu_item("salad", default="Sorry, not tonight!")

'Sorry, not tonight!'

get_menu_item("dessert", default="Sorry, not tonight!") is None

True

print(MISSING)

<object object at 0x7bf5c437d710>

Can we make it prettier?

madpy_menu = {
    "pizza": "Ian's",
    "beverage": "Sprite",
    "dessert": None,
}

from enum import StrEnum

class Missing(StrEnum):
    MISSING = "MISSING"

def get_menu_item(key, default=None):
    value = madpy_menu.get(key, Missing.MISSING)
    if value is Missing.MISSING:
        return default
    return value

get_menu_item("salad", default="Sorry, not tonight!")

'Sorry, not tonight!'

get_menu_item("dessert", default="Sorry, not tonight!") is None

True

print(Missing.MISSING)

MISSING

New in Python 3.15 (to be released in October 2026)

from sys import version

print(version)

3.15.0b2 (main, Jun  2 2026, 22:26:04) [Clang 22.1.3 ]

from builtins import sentinel

MISSING = sentinel("MISSING")

madpy_menu = {
    "pizza": "Ian's",
    "beverage": "Sprite",
    "dessert": None,
}

MISSING = sentinel("MISSING")

def get_menu_item(key, default=None):
    value = madpy_menu.get(key, MISSING)
    if value is MISSING:
        return default
    return value

get_menu_item("salad", default="Sorry, not tonight!")

'Sorry, not tonight!'

get_menu_item("dessert", default="Sorry, not tonight!") is None

True

print(MISSING)

MISSING

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