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116 lines
3.3 KiB
Markdown
116 lines
3.3 KiB
Markdown
+++
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title = "What's new in Python 3.9"
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description = "A list of major new features in python 3.9"
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date = 2020-11-10
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## Python 3.9
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Python 3.9 has been released on October 5, 2020.
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## Add Union Operators To dict (PEP 584)
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This allows the union operation to be performed on dicts:
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```python
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>>> a = {'x': 1, 'y': 2, 'z': 3}
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>>> e = {'w': 'hello world'}
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>>> a | e
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{'x': 1, 'y': 2, 'z': 3, 'w': 'hello world'}
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```
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And also:
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```python
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>>> x = a
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>>> x
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{'x': 1, 'y': 2, 'z': 3}
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>>> x |= e
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>>> x
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{'x': 1, 'y': 2, 'z': 3, 'w': 'hello world'}
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```
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## Type Hinting Generics In Standard Collections (PEP 585)
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This feature enables type hinting using the standard collections without having to rely on the `typings` module.
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Previously to type hint a list you would do:
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```python
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from typings import List
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def somefunc(a: List[int]):
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pass
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```
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Now you can use the standard type:
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```python
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def somefunc(a: list[int]):
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pass
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```
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From this version, importing **collections** from `typings` is deprecated, and they will be removed in 5 years.
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## Flexible function and variable annotations (PEP 593)
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This feature adds a new type `Annotated` which allows us to extend type annotations with metadata.
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This allows a type `T` to be annotated with metadata `x` like so:
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```python
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T1 = Annotated[T, x]
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# E.g
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UnsignedShort = Annotated[int, struct2.ctype('H')]
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SignedChar = Annotated[int, struct2.ctype('b')]
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# Multiple type annotations are supported
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T2 = Annotated[int, ValueRange(3, 10), ctype("char")]
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```
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The metadata can then be used for static or runtime analysis with tools such as [mypy](http://www.mypy-lang.org/)
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This feature allows authors to introduce new data types with graceful degradation,
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for example if mypy doesn't know how to parse X Annotation it should just ignore its metadata and use the annotated type.
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## Relaxing Grammar Restrictions On Decorators (PEP 614)
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Python currently requires that all decorators consist of a dotted name, optionally followed by a single call. This PEP proposes removing these limitations and allowing decorators to be any valid expression.
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An expression here means "anything that's valid as a test in if, elif, and while blocks".
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Basically this:
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```python
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button_0 = buttons[0]
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@button_0.clicked.connect
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def spam():
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pass
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```
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Can now be:
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```python
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@buttons[0].clicked.connect
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def spam():
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pass
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```
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## Support for the IANA Time Zone Database in the Standard Library
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This feature adds a new module `zoneinfo` that provides a concrte time zone implementation supporting the IANA time zone database.
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You can find more about this module here: [zoneinfo](https://docs.python.org/3/library/zoneinfo.html)
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Example:
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```python
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>>> from zoneinfo import ZoneInfo
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>>> from datetime import datetime, timedelta
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>>> dt = datetime(2020, 10, 31, 12, tzinfo=ZoneInfo("America/Los_Angeles"))
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>>> print(dt)
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2020-10-31 12:00:00-07:00
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>>> dt.tzname()
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'PDT'
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```
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## String methods to remove prefixes and suffixes
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Adds two new methods, [removeprefix()](https://docs.python.org/3/library/stdtypes.html?highlight=removeprefix#str.removeprefix) and [removesuffix()](https://docs.python.org/3/library/stdtypes.html?highlight=removeprefix#str.removesuffix), to the APIs of Python's various string objects. |