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Slice infinite generators with this Python 3.7 feature
Learn more about this and two other underutilized but still useful
Python features.
This is the eighth in a series of articles about features that first appeared in a version of Python 3.x. Python 3.7 was first released in 2018, and even though it has been out for a few years, many of the features it introduced are underused and pretty cool. Here are three of them.
Postponed evaluation of annotations
In Python 3.7, as long as the right __future__ flags are activated, annotations are not evaluated during runtime:
from __future__ import annotations
def another_brick(wall: List[Brick], brick: Brick) -> Education:
pass
`another_brick.__annotations__`
` {'wall': 'List[Brick]', 'brick': 'Brick', 'return': 'Education'}`
This allows recursive types (classes that refer to themselves) and other fun things. However, it means that if you want to do your own type analysis, you need to use ast explictly:
import ast
raw_type = another_brick.__annotations__['wall']
[parsed_type] = ast.parse(raw_type).body
subscript = parsed_type.value
f"{subscript.value.id}[{subscript.slice.id}]"
` 'List[Brick]'`
itertools.islice supports index
Sequence slices in Python have long accepted all kinds of int-like objects (objects that have __index__()) as valid slice parts. However, it wasn't until Python 3.7 that itertools.islice, the only way in core Python to slice infinite generators, gained this support.
For example, now it is possible to slice infinite generators by numpy.short-sized integers:
import numpy
short_1 = numpy.short(1)
short_3 = numpy.short(3)
short_1, type(short_1)
` (1, numpy.int16)`
import itertools
list(itertools.islice(itertools.count(), short_1, short_3))
` [1, 2]`
functools.singledispatch() annotation registration
If you thought singledispatch couldn't get any cooler, you were wrong. Now it is possible to register based on annotations:
import attr
import math
from functools import singledispatch
@attr.s(auto_attribs=True, frozen=True)
class Circle:
radius: float
@attr.s(auto_attribs=True, frozen=True)
class Square:
side: float
@singledispatch
def get_area(shape):
raise NotImplementedError("cannot calculate area for unknown shape",
shape)
@get_area.register
def _get_area_square(shape: Square):
return shape.side ** 2
@get_area.register
def _get_area_circle(shape: Circle):
return math.pi * (shape.radius ** 2)
get_area(Circle(1)), get_area(Square(1))
` (3.141592653589793, 1)`
Welcome to 2017
Python 3.7 was released about four years ago, but some of the features that first showed up in this release are cool—and underused. Add them to your toolkit if you haven't already.
via: https://opensource.com/article/21/5/python-37-features
作者：Moshe Zadka 选题：lujun9972 译者：译者ID 校对：校对者ID