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<!DOCTYPE html>
<html>
<head>
<title>10 awesome features of Python that you can't use because you refuse to upgrade to Python 3</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
<style type="text/css">
/* Slideshow styles */
</style>
</head>
<body>
<textarea id="source">
class: center, middle
# 10 awesome features of Python that you can't use because you refuse to upgrade to Python 3
.footnote[[There is also a pdf version of these slides](http://asmeurer.github.io/python3-presentation/python3-presentation.pdf)]
---
class: center, middle
# 10 awesome features of Python that you can't use because you refuse to upgrade to Python 3
# or
# Turning it up to 13!
---
# Prelude
- Last month (March) APUG: only three people use Python 3 (including me)
- Lots of new features of Python 3.
- Some have been backported to Python 2.7. (like dictionary/set comprehensions
or set literals, `__future__.print_function`)
- But there's more than that.
- New features that you *can't* use unless you are in Python 3.
- New syntax. New interpreter behavior. Standard library fixes.
- And it's more than bytes/unicode...
---
layout: true
# Feature 1: Advanced unpacking
---
- You can already do this:
```py
>>> a, b = range(2)
>>> a
0
>>> b
1
```
--
- Now you can do this:
```py
>>> a, b, *rest = range(10)
>>> a
0
>>> b
1
>>> rest
[2, 3, 4, 5, 6, 7, 8, 9]
```
--
- `*rest` can go anywhere:
```py
>>> a, *rest, b = range(10)
>>> a
0
>>> b
9
>>> rest
[1, 2, 3, 4, 5, 6, 7, 8]
```
```py
>>> *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
```py
>>> with open("using_python_to_profit") as f:
... first, *_, last = f.readlines() # Warning: this puts the whole file contents in memory!
>>> first
'Step 1: Use Python 3\n'
>>> last
'Step 10: Profit!\n'
```
---
layout: true
# Feature 2: Keyword only arguments
---
```py
def f(a, b, *args, option=True):
...
```
--
- `option` comes *after* `*args`.
--
- The only way to access it is to explicitly call `f(a, b, option=True)`
--
- You can write just a `*` if you don't want to collect `*args`.
```py
def f(a, b, *, option=True):
...
```
---
- No more, "Oops, I accidentally passed too many arguments to the function, and
one of them was swallowed by a keyword argument".
```py
def sum(a, b, biteme=False):
if biteme:
shutil.rmtree('/')
else:
return a + b
```
```py
>>> sum(1, 2)
3
```
```py
>>> sum(1, 2, 3)
```
--
.center[<img src="bomb.jpg" width="500">]
---
- Instead write
```py
def sum(a, b, *, biteme=False):
if biteme:
shutil.rmtree('/')
else:
return a + b
```
```py
>>> 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
```
--
.center[]
---
- Or, "I reordered the keyword arguments of a function, but something was
implicitly passing in arguments expecting the order"
- Example:
```py
def maxall(iterable, key=None):
"""
A list of all max items from the iterable
"""
key = key or (lambda x: x)
m = max(iterable, key=key)
return [i for i in iterable if key(i) == key(m)]
```
```py
>>> maxall(['a', 'ab', 'bc'], len)
['ab', 'bc']
```
---
- The `max` builtin supports `max(a, b, c)`. We should allow that too.
```py
def maxall(*args, key=None):
"""
A list of all max items from the iterable
"""
if len(args) == 1:
iterable = args[0]
else:
iterable = args
key = key or (lambda x: x)
m = max(iterable, key=key)
return [i for i in iterable if key(i) == key(m)]
```
- We just broke any code that passed in the key as a second argument without
using the keyword.
```py
>>> maxall(['a', 'ab', 'ac'], len)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 10, in maxall
TypeError: unorderable types: builtin_function_or_method() > list()
```
- (Actually in Python 2 it would just return `['a', 'ab', 'ac']`, see feature 6).
- By the way, `max` shows that this is already possible in Python 2, but only
if you write your function in C.
- Obviously, we should have used `maxall(iterable, *, key=None)` to begin
with.
---
- You can make your APIs "future change proof".
- Stupid example:
```py
def extendto(value, shorter, longer):
"""
Extend list `shorter` to the length of list `longer` with `value`
"""
if len(shorter) > len(longer):
raise ValueError('The `shorter` list is longer than the `longer` list')
shorter.extend([value]*(len(longer) - len(shorter)))
```
```py
>>> a = [1, 2]
>>> b = [1, 2, 3, 4, 5]
>>> extendto(10, a, b)
>>> a
[1, 2, 10, 10, 10]
```
--
- Hmm, maybe it makes more sense for `longer` to come before `shorter`...
- Too bad, you'll break the code.
---
- In Python 3, you can use
```py
def extendto(value, *, shorter=None, longer=None):
"""
Extend list `shorter` to the length of list `longer` with `value`
"""
if shorter is None or longer is None:
raise TypeError('`shorter` and `longer` must be specified')
if len(shorter) > len(longer):
raise ValueError('The `shorter` list is longer than the `longer` list')
shorter.extend([value]*(len(longer) - len(shorter)))
```
- Now, `a` and `b` *have* to be passed in as `extendto(10, shorter=a,
longer=b)`.
--
- Or if you prefer, `extendto(10, longer=b, shorter=a)`.
---
- Add new keyword arguments without breaking API.
- Python 3 did this in the standard library.
--
- For example, functions in `os` have `follow_symlinks` option.
--
- So you can just use `os.stat(file, follow_symlinks=False)` instead of `os.lstat`.
--
- In case that sounds more verbose, it lets you do
```py
s = os.stat(file, follow_symlinks=some_condition)
```
instead of
```py
if some_condition:
s = os.stat(file)
else:
s = os.lstat(file)
```
--
- But `os.stat(file, some_condition)` doesn't work.
- Keeps you from thinking it's a two-argument function.
---
- In Python 2, you have to use `**kwargs` and do the handling yourself.
--
- Lots of ugly `option = kwargs.pop(True)` at the top of your functions.
--
- No longer self documenting.
--
- If you somehow are writing for a Python 3 only codebase, I highly recommend
making all your keyword arguments keyword only, especially keyword arguments
that represent "options".
---
layout: true
# Feature 3: Chained exceptions
---
- **Situation:** you catch an exception with `except`, do something, and then
raise a different exception.
```py
def mycopy(source, dest):
try:
shutil.copy2(source, dest)
except OSError: # We don't have permissions. More on this later
raise NotImplementedError("automatic sudo injection")
```
- **Problem:** You lose the original traceback
```py
>>> mycopy('noway', 'noway2')
>>> mycopy(1, 2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 5, in mycopy
NotImplementedError: automatic sudo injection
```
--
- What happened with the `OSError`?
---
- Python 3 shows you the whole chain of exceptions:
```py
mycopy('noway', 'noway2')
Traceback (most recent call last):
File "<stdin>", line 3, in mycopy
File "/Users/aaronmeurer/anaconda3/lib/python3.3/shutil.py", line 243, in copy2
copyfile(src, dst, follow_symlinks=follow_symlinks)
File "/Users/aaronmeurer/anaconda3/lib/python3.3/shutil.py", line 109, in copyfile
with open(src, 'rb') as fsrc:
PermissionError: [Errno 13] Permission denied: 'noway'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 5, in mycopy
NotImplementedError: automatic sudo injection
```
--
- You can also do this manually using `raise from`
```py
raise exception from e
```
```py
>>> raise NotImplementedError from OSError
OSError
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NotImplementedError
```
---
layout: true
# Feature 4: Fine grained `OSError` subclasses
---
- The code I just showed you is wrong.
- It catches `OSError` and assumes it is a permission error.
- But `OSError` can be a lot of things (file not found, is a directory, is not a
directory, broken pipe, ...)
- You really have to do
```py
import errno
def mycopy(source, dest):
try:
shutil.copy2(source, dest)
except OSError as e:
if e.errno in [errno.EPERM, errno.EACCES]:
raise NotImplementedError("automatic sudo injection")
else:
raise
```
--
- Wow. That sucks.
--
.center[<img src="rage.png" width="200">]
---
- Python 3 fixes this by adding a ton of [new exceptions](https://docs.python.org/3.4/library/exceptions.html#os-exceptions).
- You can just do
```py
def mycopy(source, dest):
try:
shutil.copy2(source, dest)
except PermissionError:
raise NotImplementedError("automatic sudo injection")
```
- (Don't worry, `PermissionError` subclasses from `OSError` and still has
`.errno`. Old code will still work).
---
layout: true
# Feature 5: Everything is an iterator
---
- This is the hardest one to sell.
- Iterators exist in Python 2 as well.
- But you have to use them. Don't write `range` or `zip` or `dict.values` or
....
---
- If you do...
--
```py
def naivesum(N):
"""
Naively sum the first N integers
"""
A = 0
for i in range(N + 1):
A += i
return A
```
--
```py
In [3]: timeit naivesum(1000000)
10 loops, best of 3: 61.4 ms per loop
```
--
```py
In [4]: timeit naivesum(10000000)
1 loops, best of 3: 622 ms per loop
```
--
```py
In [5]: timeit naivesum(100000000)
```
--
.center[<img src="startupdisk.png" width="500">]
---
.center[]
---
- Instead write some variant (`xrange`, `itertools.izip`,
`dict.itervalues`, ...).
- Inconsistent API anyone?
---
- In Python 3, `range`, `zip`, `map`, `dict.values`, etc. all return memory-efficient iterables.
- If you want a list, just wrap the result with `list`.
- Explicit is better than implicit.
- Harder to write code that accidentally uses too much memory, because the
input was bigger than you expected.
---
layout: true
# Feature 6: No more comparison of everything to everything
---
- In Python 2, you can do
```py
>>> max(['one', 2]) # One *is* the loneliest number
'one'
```
--
- Hurray. I just disproved math!
.center[]
---
- It's because in Python 2, you can `<` compare anything to anything.
```py
>>> 'abc' > 123
True
>>> None > all
False
```
--
- In Python 3, you can't do this:
```py
>>> 'one' > 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unorderable types: str() > int()
```
- This avoids subtle bugs, e.g., from not coercing all types from int to str
or visa versa.
- Especially when you use `>` implicitly, like with `max` or `sorted`.
- In Python 2:
```py
>>> sorted(['1', 2, '3'])
[2, '1', '3']
```
---
layout: true
# Feature 7: yield from
---
- Pretty great if you use generators
- Instead of writing
```py
for i in gen():
yield i
```
Just write
```py
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`.
- **Bad**
```py
def dup(n):
A = []
for i in range(n):
A.extend([i, i])
return A
```
**Good**
```py
def dup(n):
for i in range(n):
yield i
yield i
```
**Better**
```py
def dup(n):
for i in range(n):
yield from [i, i]
```
---
In case you don't know, generators are awesome because:
- Only one value is computed at a time. Low memory impact (see `range` example
above).
- Can break in the middle. Don't have to compute everything just to find out
you needed none of it. Compute just what you need. If you often *don't* need
it all, you can gain a lot of performance here.
- If you need a list, just call `list()` on the generator iterator. If you need slicing, use the efficient `itertools.islice()`.
- Function state is "saved" between yields.
- This leads to interesting possibilities, à la coroutines...
---
layout: true
# Feature 8: asyncio
---
- Uses new coroutine features and saved state of generators to do
asynchronous IO.
```py
# Taken from Guido's slides from “Tulip: Async I/O for Python 3” by Guido
# van Rossum, at LinkedIn, Mountain View, Jan 23, 2014
@coroutine
def fetch(host, port):
r,w = yield from open_connection(host,port)
w.write(b'GET /HTTP/1.0\r\n\r\n ')
while (yield from r.readline()).decode('latin-1').strip():
pass
body=yield from r.read()
return body
@coroutine
def start():
data = yield from fetch('python.org', 80)
print(data.decode('utf-8'))
```
--
- Not going to lie to you. I still don't get this.
--
- It's OK, though. Even David Beazley had a hard time with it:
.center[<img src="dabeaz.png" width="500">]
---
layout: true
# Feature 9: Standard library additions
---
## `faulthandler`
- Display (limited) tracebacks, even when Python dies the hard way.
- Won't work with `kill -9`, but does work with, e.g., segfaults.
```py
import faulthandler
faulthandler.enable()
def killme():
# Taken from http://nbviewer.ipython.org/github/ipython/ipython/blob/1.x/examples/notebooks/Part%201%20-%20Running%20Code.ipynb
import sys
from ctypes import CDLL
# This will crash a Linux or Mac system; equivalent calls can be made on
# Windows
dll = 'dylib' if sys.platform == 'darwin' else 'so.6'
libc = CDLL("libc.%s" % dll)
libc.time(-1) # BOOM!!
killme()
```
```bash
$python test.py
Fatal Python error: Segmentation fault
Current thread 0x00007fff781b6310:
File "test.py", line 11 in killme
File "test.py", line 13 in <module>
Segmentation fault: 11
```
- Or `kill -6` (`SIGABRT`)
- Can also enable with `python -X faulthandler`
---
## `ipaddress`
- Exactly that. IP addresses.
```py
>>> ipaddress.ip_address('192.168.0.1')
IPv4Address('192.168.0.1')
>>> ipaddress.ip_address('2001:db8::')
IPv6Address('2001:db8::')
```
- Just another thing you don't want to roll yourself.
---
## `functools.lru_cache`
- A LRU cache decorator for your functions.
- From [docs](https://docs.python.org/3/whatsnew/3.2.html#functools).
```py
@lru_cache(maxsize=32)
def get_pep(num):
'Retrieve text of a Python Enhancement Proposal'
resource = 'http://www.python.org/dev/peps/pep-%04d/' % num
try:
with urllib.request.urlopen(resource) as s:
return s.read()
except urllib.error.HTTPError:
return 'Not Found'
>>> for n in 8, 290, 308, 320, 8, 218, 320, 279, 289, 320, 9991:
... pep = get_pep(n)
... print(n, len(pep))
>>> get_pep.cache_info()
CacheInfo(hits=3, misses=8, maxsize=32, currsize=8)
```
---
## `enum`
- Finally, an enumerated type in the standard library.
- Python 3.4 only.
```py
>>> from enum import Enum
>>> class Color(Enum):
... red = 1
... green = 2
... blue = 3
...
```
- Uses some magic that is only possible in Python 3 (due to metaclass
changes):
```py
>>> class Shape(Enum):
... square = 2
... square = 3
...
Traceback (most recent call last):
...
TypeError: Attempted to reuse key: 'square'
```
---
layout: true
# Feature 10: Fun
---
## Unicode variable names
```py
>>> résumé = "knows Python"
>>> π = math.pi
```
--
- Sorry, letter-like characters only.
- `🍺 = "beer"` does not work.
--
## Function annotations
```py
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.
```py
>>> def f(x: int) -> float:
... pass
...
>>> f.__annotations__
{'return': <class 'float'>, 'x': <class 'int'>}
```
- But it leaves open the possibility for library authors to do fun things.
- Example, IPython 2.0 widgets.
- Run IPython notebook (in Python 3) from IPython git checkout and open http://127.0.0.1:8888/notebooks/examples/Interactive%20Widgets/Image%20Processing.ipynb
---
layout: true
# 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.
---
layout: true
# Feature 12: Matrix Multiplication
---
In Python 3.5, you are able to replace
```py
>>> a = np.array([[1, 0], [0, 1]])
>>> b = np.array([[4, 1], [2, 2]])
>>> np.dot(a, b)
array([[4, 1],
[2, 2]])
```
with
```py
>>> a = np.array([[1, 0], [0, 1]])
>>> b = np.array([[4, 1], [2, 2]])
>>> a @ b
array([[4, 1],
[2, 2]])
```
- Any object can override `__matmul__` to use `@`.
---
layout: true
# Feature 13: Pathlib
---
- In Python 2, path handling is verbose
```py
import os
directory = "/etc"
filepath = os.path.join(directory, "test_file.txt")
if os.path.exists(filepath):
stuff
```
- In Python 3, it is much more simpler
```py
from pathlib import Path
directory = Path("/etc")
filepath = directory / "test_file.txt"
if filepath.exists():
stuff
```
---
layout: true
class: center, middle
---
# Discuss
.center[]
---
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