Errata

If you override comparison functions, do not specify `order=True`, as that will raise a `ValueError`.

should be

If you override comparison functions, do not specify `order=True`, as that will raise a `TypeError`.

Note from the Author or Editor:
Good catch on this! The exception type should be changed to `TypeError`, jsut as Yuri suggested.

Text is: "If you want to be able to test equality ... between two data classes". I believe this should instead be "If you want to test equality ... between two _instances_ of a data class".

Note from the Author or Editor:
Thanks for the catch! Prod Team - please make the changes that Vadim suggested. "between two data classes" should be changed to "between two instances of a data class"

The code "def create_author_count_mapping(cookbooks: List[Cookbook]):"
should be


"def create_author_count_mapping(cookbooks: list[Cookbook]):"

Note from the Author or Editor:
Thank you for pointing this out.

There are two code examples on page 15 that need changing - the one under the defaultdict section and the one under Counter. In both cases, the List[Cookbook]` should be changed to list[Cookbook].

The lowercase "list" is consistent with the rest of the book.

“A datetime supports addition and subtraction, but not of other datetimes.”

This is statement is true with regard to the addition of datetimes, but false with regard to the subtraction of datetimes.

While datetime.datetime.__add__ supports addition only of a datetime.timedelta value, datetime.datetime.__sub__ supports subtraction of either a datetime.timedelta value or a datetime.datetime value. Subtracting a timedelta value from a datetime produces a datetime result; subtracting a datetime value from another datetime value produces a timedelta value:

>>> import datetime
>>> d1 = datetime.datetime(2022,1,31)
>>> d2 = datetime.datetime(2022,1,1)
>>> d1-d2
datetime.timedelta(days=30)
>>>

Note from the Author or Editor:
Thank you for pointing out this very incorrect generalization. The first two sentences of this paragraph should be changed from :

"A datetime supports addition and subtraction, but not of other datetimes. We only add time deltas (such as adding a day or subtracting a year)."

to

"A datetime supports addition, but not of other datetimes. We only add time deltas (such as adding a day or adding a week)."




Additionally, the bullet list right above this paragraph on page 26 has the second bullet as "Mathematical operations such as addition and subtraction of time deltas."

It should now read instead:

"Mathematical operations such as addition of time deltas and subtraction of both time deltas and other datetime objects."

The words "of time deltas" and "of both time deltas and other datetime objects" should be italicized, and datetime should be in the code font.

You import Union then use optional. should be import optional then use optional.

Note from the Author or Editor:
This is a technical mistake using the wrong import. The first line of the code block at the top of page 53 should change from "from typing import Union" to "from typing import Optional".

Thank you for pointing this out.

The numbers used in the end of this paragraph are wrong, which makes the point that is being made incorrect. Furthermore, some readers have highlighted this as a bit confusing, so I've also added a clarification. (Note I'm using tick marks (`) to indicate code font, they are not meant to be literal in the text.

What needs to be changed:
you cannot use the values 1 through 4 (inclusive) for your `Enum` because 4 will "bitwise and" for the values 1,2, and 4 which is probably not what you want.

Should be changed to:
you cannot use the values 1 through 3 (inclusive) for your `Flag`, because if you set the `Flag` to be value 3, performing a "bitwise and" (&) of 1,2, or 3 will all evaluate to `True` when converted to a boolean value, which makes it difficult to use the `Flag` effectively.

Definitions of SEAFOOD and ALL_NUTS are indented and are defined inside Allergen EnumType. This code will not work, raising a KeyError and a TypeError.

Note from the Author or Editor:
This is correct, the SEAFOOD and ALL_NUTS lines need to be dedented (the rest of the lines above it should stay indented)

....
....
DAIRY=auto()
SEAFOOD = Allergen.FISH | Allergen.SHELLFISH
ALL_NUTS = Allergen.TREE_NUTS | Allergen.PEANUTS

I am using a technically incorrect term throughout the book by using the term "constructor" when talking about an __init__ method. Special thanks to Henrik Strøm for pointing this out (twitter.com/henrikstroem/status/1528345763230654465)

Note, all of this text uses backticks (`abcdef`) to indicate code font.
There are many fixes that need to be put in to correct this usage of incorrect term.

On page 86, the callout numbered #2 should be changed from "A description of the constructor" to "A description of the `__init___` method."

On page 136, in the paragraph that starts "That's really frustrating", the sentence that goes "You need to explicitly define how a class gets constructed, which is done through a special method called a constructor." needs to be changed to "You need to explicitly define how a class gets initialized, which is done through a special method called `__init__`. Please note that "constructors" in the index corresponds to this page; that should be changed to "__init__ method" in the index.

On page 136, the heading "Constructors" should be up dated to "Class initialization". The first paragraph of this section should be changed from "A constructor describes how to initialize your class. You define a constructor with an `__init_`_ method" to "When you construct a new instance of a class, you control how that class is initialized with an `__init__` method. Please note that "constructors" in the index corresponds to this page; that should be changed to "__init__ method" in the index.

On page 137, the paragraph that starts with "Notice that I tweaked" has two uses of the word constructor: "...variables in a constructor." and "The constructor is a special method..." These should be changed to "...variables in `__init__` and "This special method...", respectively.

On page 138, there is a sentence at the top of the page: "Let's start with the constructor" should be changed to "Let's start with the `__init__` method."

On page 138, the first bullet that starts with "`dough_radius_in_inches` is an integer has the phrase: "into the constructor". This should be "into the `__init__` method" . The next bullet has a phrase "preventing construction of the class" to "preventing initialization of the class"

On page 143, The first paragraph under "Consuming Your Class" has two mentions to constructors: "... in the constructor is a great way..." and "A constructor is typically...". These should be changed to "...in the `__init__` method is a great way..." and "The `__init__` method is typically...", respectively. Please note that "constructors" in the index corresponds to this page; that should be changed to "__init__ method" in the index.

On page 147, The paragraph that starts with "So far, I've covered two parts of the API, needs two changes: Remove the parenthetical "(constructor)" and change "I don't have much more to say about the constructor;" to "I don't have much more to say about the `__init__` method;"

On page 166, the bullet point "In Chapter 10" should" say "to initialize a class" instead of "to construct a class". The term "constructed" should be changed "instantiated"

On page 174, the chapter that starts with "Suppose I want my food truck" calls out" `FoodTruck`'s constructor", which should be changed to `FoodTruck`'s `__init__` method

On page 175, in the paragraph starting with "I am using a special function", has a phrase "`Restaurant`'s constructor" which should be changed to "`Restaurants`'s `__init__` method.

On page 183, bottom paragraph, "Each of the member fields set in the constructor" should be changed to "Each of the member fields set in `__init__`"

On page 196, the note mentions "concerning setting variables in constructors" which should be set to "concerning setting variables in `__init__` methods".

On page 203, the bottom paragraph has a phrase " could you imagine opening up a constructor" to "could you imagine opening up an `__init__` method"

On page 210, the sentence "However, pydantic protects the invariant, even when you call a constructor or set a field" should be changed to "However, pydantic protects the invariant, even when you initialize a type or set a field"

On page 290, the last paragraph starts with "In this case, I've defined a constructor". This should be changed to "In this case, I've defined an `__init__` method."

On page 334, the last paragraph has a phrase "In the constructor, I set up `self.recommender`" which should be changed to "In the `__init__` method, I set up `self.recommender`"

def order_items(store: Store, item: items: Iterable[Item]) -> bool:

should be

def order_items(store: Store, items: Iterable[Item]) -> bool:

Note from the Author or Editor:
Thank you for catching this. The word "item:" should be removed from this line, just as you suggested.

def split_dish(order: Splittable) -> tuple[Splittable, Splittable]:

should be

def split_dish(dish: Splittable) -> tuple[Splittable, Splittable]:

Note from the Author or Editor:
Thanks for the catch. This should be changed from "order" to "dish" as Yuri suggested

def split_in_half(self) -> ('BLTSandwich', 'BLTSandwich'):

is wrong and invalid Python syntax and should be

def split_in_half(self) -> tuple['BLTSandwich', 'BLTSandwich']:

Note from the Author or Editor:
Thanks for the catch. This should be changed as described

def split_in_half(self) -> ('BLTSandwich', 'BLTSandwich'):

should be changed to

def split_in_half(self) -> tuple['BLTSandwich', 'BLTSandwich']:

In method "send_text" the last "raise NotImplementedError(...)" should probably be put unter an "else:" statement: Otherwise each of the "pass" blocks require a "return" statement to exit early from the method to NOT execute the "raise" statement at the end of the method.

Note from the Author or Editor:
Thank you for pointing that out.

The line in question should be changed from

raise NotImplementedError("Unsupported Notification Method")


to


else:
raise NotImplementedError("Unsupported Notification Method")


(the (raise NotImplementedError...) is indented underneath the else block

def repeat(func: Callable, times: int = 1) -> Callable:
''' this is a function that calls the wrapped function
a specified number of times
'''
def _wrapper(*args, **kwargs):
for _ in range(times):
func(*args, **kwargs)
return _wrapper

should be

def repeat(times: int = 1) -> Callable:
"""this is a function that calls the wrapped function a specified number of times"""
def _repeat(func: Callable):
@functools.wraps(func)
def _wrapper(*args, **kwargs):
for _ in range(times):
func(*args, **kwargs)
return _wrapper
return _repeat

Note from the Author or Editor:
Thank you for the catch of the missing wrapper function. The change that you have suggested is accurate and is what should show up in the book.

Production team, if the whitespace was not preserved in Yuri's message, please reach out to me and I can make the change, as the levels of indentation are important for this code.

The 2nd s et of parentheses should be removed from
test_database.cleanup()()

Note from the Author or Editor:
Thank you for another catch. Prod team, please apply the suggested change by removing the second set of parentheses

Line 2-4 should be indented right to make only the 1st line stay left:
Scenario: ...
Given ...
When ...
Then ...

Note from the Author or Editor:
Thank you for the catch.

Prod team: The lines starting with "Given", "When", and "Then" (3 lines) should all be indented, similar to the code example preceding it on the page.