5.2. Logic None

  • Empty (null) or unknown (unset) value

  • It is not False value

  • First letter capitalized, other are lower cased

Example:

>>> name = None
>>> age = None
>>> result = None
>>> friends = None
>>> groups = None

5.2.1. Syntax

  • First letter capitalized, other are lower cased

  • none, ``NONE, null, Null, NULL, nil, NIL - does not exist in Python

>>> data = None

None, Null, Nil:

>>> data = none
Traceback (most recent call last):
NameError: name 'none' is not defined
>>>
>>> data = NONE
Traceback (most recent call last):
NameError: name 'NONE' is not defined

>>> data = null
Traceback (most recent call last):
NameError: name 'null' is not defined
>>>
>>> data = Null
Traceback (most recent call last):
NameError: name 'Null' is not defined
>>>
>>> data = NULL
Traceback (most recent call last):
NameError: name 'NULL' is not defined

>>> data = nil
Traceback (most recent call last):
NameError: name 'NIL' is not defined
>>>
>>> data = NIL
Traceback (most recent call last):
NameError: name 'NIL' is not defined

5.2.2. NoneType

  • NoneType

>>> type(None)
<class 'NoneType'>

5.2.3. Check If None

  • x is None - x is the same object as y

  • x is not None - x is not the same object as y

>>> data = None
>>>
>>>
>>> data is None
True
>>>
>>> data is not None
False

5.2.4. Is vs Eq

  • Do not use == or != to check None values

  • It works, but it is a subject to change

>>> data = None
>>>
>>>
>>> data == None
True
>>>
>>> data != None
False

5.2.5. Recap

  • Empty (null) or unknown (unset) value

  • It is not False value

  • First letter capitalized, other are lower cased

  • none, ``NONE, null, Null, NULL, nil, NIL - does not exist in Python

  • x is None - x is the same object as y

  • x is not None - x is not the same object as y

  • Do not use == or != to check None values

Definition:

>>> data = None

Check If None:

>>> data = None
>>>
>>>
>>> data is None
True
>>>
>>> data is not None
False

5.2.6. Use Case - 1

>>> adult = False  # Person is not adult
>>> adult = None   # We don't know is person is adult

5.2.7. Use Case - 2

>>> age = False  # False is invalid in this context
>>> age = None   # Age is unknown

5.2.8. Use Case - 3

>>> firstname = 'Alice'
>>> lastname = 'Apricot'
>>> age = None
>>>
>>> age is None
True

5.2.9. Assignments

# %% About
# - Name: Type None
# - Difficulty: easy
# - Lines: 5
# - Minutes: 3

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% English
# 1. Define `result1: bool` with result of `result1 is None == True`
# 2. Define `result2: bool` with result of `result2 is not None == False`
# 3. Define `result3: bool` with result of `bool(result3) is not bool(result3) == False`
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zdefiniuj `result1: bool` z wynikiem `result1 is None == True`
# 2. Zdefiniuj `result2: bool` z wynikiem `result2 is not None == False`
# 3. Zdefiniuj `result3: bool` z wynikiem `bool(result3) is not bool(result3) == False`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Expected
# >>> type(result1)
# <class 'NoneType'>
#
# >>> type(result2)
# <class 'NoneType'>
#
# >>> type(result3)
# <class 'NoneType'>

# %% Doctests
"""
>>> import sys; sys.tracebacklimit = 0

>>> assert sys.version_info >= (3, 9), \
'Python has an is invalid version; expected: `3.9` or newer.'

>>> assert 'result1' in globals(), \
'Variable `result1` is not defined; assign result of your program to it.'

>>> assert result1 is not Ellipsis, \
'Variable `result1` has an invalid value; assign result of your program to it.'

>>> assert 'result2' in globals(), \
'Variable `result2` is not defined; assign result of your program to it.'

>>> assert result2 is not Ellipsis, \
'Variable `result2` has an invalid value; assign result of your program to it.'

>>> assert 'result3' in globals(), \
'Variable `result3` is not defined; assign result of your program to it.'

>>> assert result3 is not Ellipsis, \
'Variable `result3` has an invalid value; assign result of your program to it.'

>>> assert type(result1) is type(None), \
'Variable `result1` has an invalid type; expected: `None`.'

>>> assert type(result2) is type(None), \
'Variable `result2` has an invalid type; expected: `None`.'

>>> assert type(result3) is type(None), \
'Variable `result3` has an invalid type; expected: `None`.'

>>> type(result1)
<class 'NoneType'>

>>> type(result2)
<class 'NoneType'>

>>> type(result3)
<class 'NoneType'>

>>> result1 is None
True

>>> result2 is not None
False

>>> bool(result3) is not bool(result3) == False
False
"""

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -f -v myfile.py`

# %% Imports

# %% Types
result1: None
result2: None
result3: None

# %% Data

# %% Result
result1 = ...
result2 = ...
result3 = ...