The document discusses the new features of Python 3.8, which was recently released. Some key updates include positional-only arguments, the walrus operator for variable assignment, improved static typing support, and performance enhancements. The document also notes additional developer changes and provides resources for obtaining Python 3.8.

1. Python 3.8
Henry Schreiner
October 18, 2019

2. New in Python
• Python 3.6
▶ Big update
▶ First version fast as 2.7
▶ New f-strings very popular
• Python 3.7
▶ First version faster than 2.7
▶ OpenSSL update has affected adoption
• Python 3.8
▶ Released just a few days ago!
▶ What’s new?
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3. Themes
• Performance
• ABI / Internals
• Static typing
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4. Python 3.8: Positional-only arguments
d = dict(one=1, two=2)
d = dict({'one':1, 'two':2}
Challenge: write dict yourself
3/13Henry Schreiner Python 3.8 October 18, 2019

5. Python 3.8: Positional-only arguments
d = dict(one=1, two=2)
d = dict({'one':1, 'two':2}
Challenge: write dict yourself
def dict(arg=None, **kargs):
...
dict(arg=3) # OOPS!
You have to use *args, and limit to 1 arg yourself. Or…
3/13Henry Schreiner Python 3.8 October 18, 2019

6. Python 3.8: Positional-only arguments (2)
If you look at the signatures of dict, pow, or some other builtins, you will see something kind of
like this:
def dict(arg=None, /, **kargs):
...
That is now valid Python in 3.8!
Why is it useful?
• Allow kwargs and positional arguments without overlap
• Force positional arguments without names
• You can change the internal name - not part of your API
def f(pos, /, pos_or_kw, *, kw_required, kw_optional=None):
...
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7. Python 3.8: Walrus operator
Assignment in Python is a statement (limited):
item = ... # simple
item[...] = ... # item
item.attr = ... # attr
a, b, = ... # tuple
a = b = ... # chained (special case)
But what about assigning in other places, like in the C languages?
if x = True: # Will never be allowed, too easy to make mistake
f(x=True) # Keyword argument
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8. Python 3.8: Walrus operator (2)
Solution: A new operator!
• Spelling: := (looks like a sideways walrus)
• Works almost anywhere normal = doesn’t (one way to do things)
• Often requires parenthesis for clarity
if res := check():
print(res)
a = [None, 0, 1, 2]
while a := b.pop():
print(a)
• Use carefully: could make code harder to read
• C++17/C++20 adding variable defines in loops for similar reasons
• Scope leaks, (un?)fortunately
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9. Python 3.8: f-string debugging
In Python 3.6, f-strings make string interpolation easy:
>>> x = 3
>>> print(f"x = {x}")
x = 3
Debugging is now DRY with the = specifier:
>>> print(f"{x = }")
x = 3
• Spaces respected
• Mix with complex expressions or formatting
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10. Python 3.8: Static typing
Static type hints are a big feature of Python 3, and now they are much more powerful:
Literals type (AKA make-shift enums)
def f(val : Literal['yes', 'no', 'auto']): ...
Final (AKA const)
x : Final[bool] = True
x = False # Invalid in type checker like mypy
Protocols (ACA C++20 Concepts / Java Interface)
class HasName(Protocol):
name: str
Now you can use HasName, will require a name attribute.
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11. TypedASDT
• TypedAST merged into Python!
• feature_version selector (3.4+)
>>> import ast
>>> s = ast.parse('x = 2 # type: Int', type_comments=True)
>>> ast.dump(s)
"Module(body=[Assign(
targets=[Name(id='x', ctx=Store())],
value=Constant(value=2, kind=None),
type_comment='Int')],
type_ignores=[])"
• ast.get_source_segment gets the source for a bit of ast, if location information is
present.
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12. Other features
• TypedDict gives types to dict parts
• importlib.metadata gives you info from installed packages (like importlib.resources)
• math and statistics have new functions
• namedtuple, pickle, and more are faster
• SyntaxError messages are more detailed in some common cases
• multiprocessing.shared_memory – can avoid pickle transfer of objects
• reversed works on dicts
• Unpacking in return/yield
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13. Other developer changes
• --libs no longer include libpython
• Single ABI for debug/release
• Runtime audit hooks
• New C API for initialization
• Provisional vectorcall protocol – fast calling of C functions
• Pickle support out-of-band data (multiple streams) (Protocol 5)
• __code__ now has .replace, like __signature__
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14. Final Words
Released
• Python.org released
• Docker: docker run --rm -it python:3.8
• Scikit-HEP GCC 9.2 ManyLinux1 containers
Not yet released
• Conda: Coming soon
• Numpy wheels
• Homebrew: In progress
• Still missing in Azure, etc.
• Scikit-HEP build tools (Windows, ManyLinux1 + GCC 9.2)
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15. Sources
• Official docs
• RealPython article
• DeepSource
This presentation available at /henryiii/python38.
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