The document outlines new features introduced in Python 3.9, including the dict union operator, enhanced type hinting with generics, and string methods like removeprefix() and removesuffix(). It also highlights the new parser based on PEG grammar, the addition of the graphlib module, and other updates such as the least common multiple function, new HTTP status codes, and the zoneinfo module for timezone handling. Overall, these enhancements aim to improve usability, readability, and functionality in Python programming.

1. What is new in Python 3.9
Haim Michael
March 8th
, 2021
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2. © 2008 Haim Michael 20150805
Python 3.9

3. © 2008 Haim Michael 20150805
The dict Union | Operator
 The dict union operator | return a new dict consisting of the
left operand merged with the right operand, each of which
must be a dict (or an instance of a dict subclass).
 When the key appears in both operands, the last-seen value
(i.e. that from the right-hand operand) wins.
https://www.python.org/dev/peps/pep-0584/

4. © 2008 Haim Michael 20150805
The dict Union | Operator
dict1 = {1:'one', 2:'two', 3:'three'}
dict2 = {3:'three',4:'four',5:'five',6:'six'}
dict3 = dict1 | dict2
print(dict3)

5. © 2008 Haim Michael 20150805
Type Hinting Generics in Collections
 We can now use the generics syntax when using the standard
collections in Python. We are no longer limited to using
generics with the collections the typing module includes.
 As of Python 3.9 we no longer need the standard collections
the typing module includes.
https://www.python.org/dev/peps/pep-0585/

6. © 2008 Haim Michael 20150805
Type Hinting Generics in Collections
numbers: list[int] = [23, 3, 6]
student: tuple[str, str, float, int] = ("mosh", "solomon", 98.2, 2343243)
nicknames: dict[str, str] = {"mosh": "moshe", "tal": "talia", "avi": "avraham"}
winners: set[str] = {"dave", "mark", "tal", "doron", "jane"}
print(numbers)
print(student)
print(nicknames)
print(winners)

7. © 2008 Haim Michael 20150805
The Prefix and Suffix Methods
 The methods, removeprefix() and removesuffix(), are
now available when using Python's various string objects.
 These methods would remove a prefix or suffix (respectively)
from a string, if present.
 These two methods will be available when using objects of the
types str, bytes, bytearray and collections.UserString.
https://www.python.org/dev/peps/pep-0616/

8. © 2008 Haim Michael 20150805
The Prefix and Suffix Methods
filenames:list[str] = [
"king.txt.txt",
"temp.txt",
"pep8song.txt",
"data.txt"]
for filename in filenames:
if filename.endswith(".txt.txt"):
print(filename.removesuffix(".txt"))
else:
print(filename)

9. © 2008 Haim Michael 20150805
The Prefix and Suffix Methods
names: list[str] = ["Dr.Mosh Levin", "Dr.Ronan Cohen", "Ran Roze"]
for name in names:
if name.startswith("Dr."):
print(name.removeprefix("Dr."))
else:
print(name)

10. © 2008 Haim Michael 20150805
Annotated Type Hints
 The possibility to annotate a function allows us to add
arbitrary metadata to functions we define.
def calc(distance: "meter", time: "second") -> "km per hour":
return 3600 * (distance / 1000) / time
 As of Python 3.9 we can use the typing.Annotated
from typing import Annotated
def calc(
distance: Annotated[int, "meter"],
time: Annotated[float, "second"]
) -> Annotated[float, "km per hour"]:
return 3600 * (distance / 1000) / time

11. © 2008 Haim Michael 20150805
Annotated Type Hints
 We can (as usual) access the annotations through
the .__annotations__ attribute.
from typing import Annotated
def calc(
distance: Annotated[int, "meter"],
time: Annotated[float, "second"]
) -> Annotated[float, "km per hour"]:
return 3600 * (distance / 1000) / time
print(calc.__annotations__)

12. © 2008 Haim Michael 20150805
Annotated Type Hints
 We can keep our code more readable by using type
aliases.
from typing import Annotated
Meter = Annotated[int, "meter"]
Second = Annotated[int, "second"]
KmPerHour = Annotated[float, "km per hour"]
def calc(distance: Meter, time: Second) -> KmPerHour:
return 3600 * (distance / 1000) / time
print(calc.__annotations__)

13. © 2008 Haim Michael 20150805
Powerful Parser
 One of the main components of the Python interpreter
is the parser.
 As of Python 3.9, we have a new parser. The new
parser was developed based on Guido's research of
PEG (Parsing Expression Grammar) parsers.

14. © 2008 Haim Michael 20150805
Powerful Parser
 The interpreter (in Python 3.9) includes both the new
PEG parser and the old LL(1) parser. The PEG parser
is the default one. It is still possible to use the LL(1) old
one by using the -X oldparser command-line flag:
$ python -X oldparser program.py

15. © 2008 Haim Michael 20150805
Powerful Parser
 The old parser will be removed in Python 3.10. This will
allow the implementation of new features without the
limitations of an LL(1) grammar (e.g. implementing
structural pattern matching).
 The performance of both parsers, the PEG parser and
the LL(1) parser, is similar.

16. © 2008 Haim Michael 20150805
The graphlib Module
 As of Python 3.9, the standard library includes the
graphlib new module.
from graphlib import TopologicalSorter
dependencies = {
"Java EE": {"Java", "Angular"},
"Angular": {"JavaScript"},
"Java": {"C++", "SQL"},
"C++": {"C"}
}
sorter = TopologicalSorter(dependencies)
print(list(sorter.static_order()))

17. © 2008 Haim Michael 20150805
The Least Common Multiple(LCM)
 As of Python 3.9, we can easily find the least common
multiple (LCM) by calling the math.lcm function.
import math
num1 = 5
num2 = 7
num3 = 3
result = math.lcm(num1,num2,num3)
print(result)

18. © 2008 Haim Michael 20150805
HTTP Status Codes
 As of Python 3.9, the http module includes two more
status codes: 103 (Early Hints) and 425 (Too Early).
from http import HTTPStatus
print(HTTPStatus.OK)
print(HTTPStatus.OK.description)
print(HTTPStatus.OK.value)
print(HTTPStatus.OK.phrase)
print(HTTPStatus.NOT_FOUND)
print(HTTPStatus.NOT_FOUND.description)
print(HTTPStatus.NOT_FOUND.value)
print(HTTPStatus.NOT_FOUND.phrase)
print(HTTPStatus.EARLY_HINTS)
print(HTTPStatus.TOO_EARLY)

19. © 2008 Haim Michael 20150805
The zoneinfo Module
 The zoneinfo was added in Python 3.9. It
complements the datetime module by allowing us
getting information about the time zone.
from datetime import datetime
from zoneinfo import ZoneInfo
vancouver = ZoneInfo("America/Vancouver")
release = datetime(2020, 11, 25, 2, 12, tzinfo=vancouver)
print(release)

20. © 2008 Haim Michael 20150805
The zoneinfo Module
 The following code sample prints out the available time
zones.
import zoneinfo
zones = zoneinfo.available_timezones()
print(len(zones))
print(zones)

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Questions & Answers
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