python3.md

• Python3 Feature
  • Feature 1: Advanced unpacking
  • Feature 2: Keyword only arguments
  • Feature 5: Everything is an iterator
  • Feature 6: No more comparison of everything to everything
  • Feature 7: yield from
  • Feature 8: asyncio
  • Feature 9: Standard library additions
  • Feature 10: Fun
  • Feature 11: Unicode and bytes
  • Feature 12: Matrix Multiplication
  • Feature 13: Pathlib

Python3 Feature

Feature 1: Advanced unpacking

>>> a, b, *rest = range(10)
>>> rest
[2, 3, 4, 5, 6, 7, 8, 9]
>>>
>>> a, *rest, b = range(10)
>>> b
9
>>> rest
[1, 2, 3, 4, 5, 6, 7, 8]
>>>
>>> *rest, b = range(10)
>>> rest
[0, 1, 2, 3, 4, 5, 6, 7, 8]
>>> b
9

Get the first and last lines of a file

>>> with open("using_python_to_profit") as f:
...     first, *_, last = f.readlines()

Refactor your functions

def f(a, b, *args):
    stuff

=>

def f(*args):
    a, b, *args = args
    stuff

Feature 2: Keyword only arguments

def f(a, b, *args, option=True):
    ...

• option comes after *args.
• The only way to access option is to explicitly call f(a, b, option=True)
• You can write just a * if you don't want to collect *args.

def f(a, b, *, option=True):
    ...

---

def sum(a, b, biteme=False):
    if biteme:
        shutil.rmtree('/')3
    else:
        return a + b

>>> sum(1,2,3) 
... 

Instead write

def sum(a, b, * ,  biteme=False):
    if biteme:
        shutil.rmtree('/')3
    else:
        return a + b

>>> sum(1, 2, 3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sum() takes 2 positional arguments but 3 were given

Feature 5: Everything is an iterator

Feature 6: No more comparison of everything to everything

Feature 7: yield from

• Pretty great if you use generators
• Instead of writing

for i in gen():
    yield i

• Just write

yield from gen()

• Easily refactor generators into subgenerators.

---

• Makes it easier to turn everything into a generator. See "Feature 5: Everything is an iterator" above for why you should do this.
• Instead of accumulating a list, just yield or yield from.
• for an example, we will create a list with dup numbers , like [0,0,1,1,2,2,3,3,...]

# bad
def dup(n):
    A = []
    for i in range(n):
        A.extend([i, i])
    return A

# good
def dup(n):
    for i in range(n):
        yield i
        yield i

# better
def dup(n):
    for i in range(n):
        yield from [i, i]

Feature 8: asyncio

Feature 9: Standard library additions

• faulthandler
• ipaddress

>>> ipaddress.ip_address('192.168.0.1')
IPv4Address('192.168.0.1')
>>> ipaddress.ip_address('2001:db8::')
IPv6Address('2001:db8::')

• functools.lru_cache
  • A LRU cache decorator for your functions.

@lru_cache(maxsize=32)
def get_pep(num):
    ... 

>>> get_pep.cache_info()
CacheInfo(hits=3, misses=8, maxsize=32, currsize=8)

• enum
  • Finally, an enumerated type in the standard library.

>>> from enum import Enum
>>> class Color(Enum):
...     red = 1
...     green = 2
...     blue = 3
...

Feature 10: Fun

Function annotations

def f(a: stuff, b: stuff = 2) -> result:
    ...

• Annotations can be arbitrary Python objects.
• Python doesn't do anything with the annotations other than put them in an __annotations__ dictionary.

Feature 11: Unicode and bytes

• In Python 2, str acts like bytes of data.
• There is also unicode type to represent Unicode strings.
• In Python 3, str is a string.
• bytes are bytes.
• There is no unicode. str strings are Unicode.

Feature 12: Matrix Multiplication

np.dot(a, b)

#in python 3

a @ b

Feature 13: Pathlib

filepath = os.path.join(directory, "test_file.txt")

# in python 3
filepath = directory / "test_file.txt"