Elegant Solutions For Everyday Python ProblemsNina Zakharenko@nnjabit.ly/elegant-pythonℹ There are links in these slides. Follow along ^

Whatiselegant code?@nnja

"Perfection is achieved, not when there is nothing more to add, but when there is nothing

le! to take away." — Antoine de Saint-Exupery

@nnja

Beauty isin the eye ofthe beholder

@nnja

how do we do it?code is elegant when you pick the best tool for the job

Resources for converting from Python 2 -> 3

@nnja

magicmethods

image source

You're used to implementing __str__ and __repr__ --but there's a whole other world of powerful magic methods!

By implementing a few straightforward methods,you can make your objects behave like built-ins such as:

— numbers— lists— dictionaries— and more...

@nnja

class Money:

currency_rates = { '$': 1, '€': 0.88, }

def __init__(self, symbol, amount): self.symbol = symbol self.amount = amount

def __repr__(self): return '%s%.2f' % (self.symbol, self.amount)

def convert(self, other): """ Converts amount in other currency to amount in this currency. """ new_amount = ( other.amount / self.currency_rates[other.symbol] * self.currency_rates[self.symbol]) return Money(self.symbol, new_amount)

@nnja

>>> soda_cost = Money('$', 5.25)

>>> pizza_cost = Money('€', 7.99)

>>> soda_cost $5.25

>>> pizza_cost €7.99

@nnja

class Money:

def __add__(self, other): """ Implements addition of two Money instances using '+' """ new_amount = self.amount + self.convert(other).amount return Money(self.symbol, new_amount)

def __sub__(self, other): """ Implements subtraction of two Money instances using '-' """ new_amount = self.amount - self.convert(other).amount return Money(self.symbol, new_amount)

@nnja

>>> soda_cost = Money('$', 5.25)

>>> pizza_cost = Money('€', 7.99)

>>> soda_cost + pizza_cost $14.33

More on Magic Methods: Dive into Python3 - Special Method Names

>>> soda_cost = Money('$', 5.25)

>>> pizza_cost = Money('€', 7.99)

>>> soda_cost + pizza_cost $14.33

>>> pizza_cost + soda_cost €12.61

More on Magic Methods: Dive into Python3 - Special Method Names

>>> soda_cost = Money('$', 5.25)

>>> pizza_cost = Money('€', 7.99)

>>> soda_cost + pizza_cost $14.33

>>> pizza_cost + soda_cost €12.61

>>> pizza_cost - soda_cost €3.37

More on Magic Methods: Dive into Python3 - Special Method Names

some magic methods map to built-in functions

class Alphabet: letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def __len__(self): return len(self.letters)

>>> my_alphabet = Alphabet()>>> len(my_alphabet) 26

@nnja

Making classes iterable— In order to be iterable, a class needs to implement

__iter__()— __iter__() must return an iterator— In order to be an iterator a class needs to implement

__next__() which must raise StopIteration when exhausted

or next() in python2

Great explanation of iterable vs. iterator vs. generator

example scenario

We have a Server instance running services on different ports.

Some services are active, some are inactive.

When we loop over our the Server instance, we only want to loop over active services.

@nnja

class IterableServer:

services = ( {'active': False, 'protocol': 'ftp', 'port': 21}, {'active': True, 'protocol': 'ssh', 'port': 22}, {'active': True, 'protocol': 'http', 'port': 21}, ) def __init__(self): self.current_pos = 0 def __iter__(self): # can return self, because __next__ implemented return self def __next__(self): while self.current_pos < len(self.services): service = self.services[self.current_pos] self.current_pos += 1 if service['active']: return service['protocol'], service['port'] raise StopIteration next = __next__ # optional python2 compatibility

@nnja

>>> for protocol, port in IterableServer(): print('service %s is running on port %d' % (protocol, port))

service ssh is running on port 22service http is running on port 21

... not bad@nnja

tip: use a generatorwhen your iterator doesn't need to maintain a lot of state

@nnja

class Server:

services = ( {'active': False, 'protocol': 'ftp', 'port': 21}, {'active': True, 'protocol': 'ssh', 'port': 22}, {'active': True, 'protocol': 'http', 'port': 21}, )

def __iter__(self): for service in self.services: if service['active']: yield service['protocol'], service['port']

@nnja

Why does this work?use single parenthesis ( ) to create a generator comprehension^ technically, a generator expression but I like this term better, and so does Ned Batchelder

>>> my_gen = (num for num in range(1))>>> my_gen <generator object <genexpr> at 0x107581bf8>

@nnja

An iterator must implement __next__()

>>> next(my_gen) # remember __len__() mapped to built-in len() 0

and raise StopIteration when there are no more elements

>>> next(my_gen)... StopIteration Traceback (most recent call last)

For more tools for working with iterators, check out itertools

To make your object behave like a dict:

behavior method

key in my_dict my_dict.__contains__(key)

my_dict[key] my_dict.__getitem__(key)

my_dict[key] = value my_dict.__setitem__(key, value)

del my_dict[key] my_dict.__delitem__(key)

read: emulating container types@nnja

Advanced example: Headers in werkzeug

Headers is a dict-like object, with some special behaviors.

— Ordered, and can store the same key multiple times— Representation: formatted headers suitable for HTTP

transmission— Instead of raising a KeyError like a standard dictionary

when a key is not present, raises 400 BAD REQUEST

Read the code@nnja

✨Method✨ ✨Magic✨@nnja

alias methods

class Word:

def __init__(self, word): self.word = word

def __repr__(self): return self.word

def __add__(self, other_word): return Word('%s %s' % (self.word, other_word))

# Add an alias from method __add__ to the method concat concat = __add__

@nnja

When we add an alias from __add__ to concat because methods are just objects

>>> # remember, concat = __add__>>> first_name = Word('Max')>>> last_name = Word('Smith')

>>> first_name + last_name Max Smith

>>> first_name.concat(last_name) Max Smith

>>> Word.__add__ == Word.concat True@nnja

getattr(object, name, default)

>>> class Dog: sound = 'Bark' def speak(self): print(self.sound + '!', self.sound + '!')

>>> my_dog = Dog()>>> my_dog.speak() Bark! Bark!

>>> getattr(my_dog, 'speak') <bound method Dog.speak of <__main__.Dog object at 0x10b145f28>>

>>> speak_method = getattr(my_dog, 'speak')>>> speak_method() Bark! Bark!

read the docs

example: command line tool with dynamic commands

class Operations: def say_hi(self, name): print('Hello,', name)

def say_bye(self, name): print ('Goodbye,', name)

def default(self, arg): print ('This operation is not supported.')

if __name__ == '__main__': operations = Operations() # let's assume error handling command, argument = input('> ').split() getattr(operations, command, operations.default)(argument)

read the docs

Output

› python getattr.py

> say_hi Nina

Hello, Nina

> blah blah

This operation is not supported.

✨additional reading - inverse of getattr() is setattr()

functool.partial(func, *args, **kwargs)

>>> from functools import partial>>> basetwo = partial(int, base=2)>>> basetwo # functools.partial(<class 'int'>, base=2)>>> basetwo('10010') 18

— Return a new partial object which behaves like func called with args & kwargs

— if more args are passed in, they are appended to args— if more keyword arguments are passed in, they extend

and override kwargsread the doc

library I !: github.com/jpaugh/agithub

agithub is a (badly named) REST API client with transparent syntax which facilitates rapid prototyping — on any REST API!

Implemented in 400 lines.Add support for any REST API in ~30 lines of code.

agithub knows everything it needs to about protocol (REST, HTTP, TCP), but assumes nothing about your upstream API.

@nnja

define endpoint url & other connection properties

class GitHub(API):

def __init__(self, token=None, *args, **kwargs):

props = ConnectionProperties(

api_url = kwargs.pop('api_url', 'api.github.com'))

self.setClient(Client(*args, **kwargs))

self.setConnectionProperties(props)

then, start using the API!

>>> gh = GitHub('token')

>>> status, data = gh.user.repos.get(visibility='public', sort='created')

>>> # ^ Maps to GET /user/repos

>>> data

... ['tweeter', 'snipey', '...']

github.com/jpaugh/agithub

black magic!but, how?... !

@nnja

class API: def __getattr__(self, key): return IncompleteRequest(self.client).__getattr__(key) __getitem__ = __getattr__

class IncompleteRequest: def __getattr__(self, key): if key in self.client.http_methods: htmlMethod = getattr(self.client, key) return partial(htmlMethod, url=self.url) else: self.url += '/' + str(key) return self __getitem__ = __getattr__

class Client: http_methods = ('get') # ...

def get(self, url, headers={}, **params): return self.request('GET', url, None, headers)

github.com/jpaugh/agithub source: base.py

class API: def __getattr__(self, key): return IncompleteRequest(self.client).__getattr__(key) __getitem__ = __getattr__

class IncompleteRequest: def __getattr__(self, key): if key in self.client.http_methods: htmlMethod = getattr(self.client, key) return partial(htmlMethod, url=self.url) else: self.url += '/' + str(key) return self __getitem__ = __getattr__

class Client: http_methods = ('get') # ...

def get(self, url, headers={}, **params): return self.request('GET', url, None, headers)

github.com/jpaugh/agithub source: base.py

class API: def __getattr__(self, key): return IncompleteRequest(self.client).__getattr__(key) __getitem__ = __getattr__

class IncompleteRequest: def __getattr__(self, key): if key in self.client.http_methods: htmlMethod = getattr(self.client, key) return partial(htmlMethod, url=self.url) else: self.url += '/' + str(key) return self __getitem__ = __getattr__

class Client: http_methods = ('get') # ...

def get(self, url, headers={}, **params): return self.request('GET', url, None, headers)

github.com/jpaugh/agithub source: base.py

given a non-existant path:

>>> status, data = this.path.doesnt.exist.get()>>> status... 404

& because __getitem__ is aliased to __getattr__:

>>> owner, repo = 'nnja', 'tweeter'>>> status, data = gh.repos[owner][repo].pulls.get()>>> # ^ Maps to GET /repos/nnja/tweeter/pulls>>> data.... # {....}

... !github.com/jpaugh/agithub

lambdasmall (usually one line) function

@nnja

Use lambda + dict for a switch-like statement

def math(term, num):

def default(num):

return 'Operation not supported'

return {

'double': lambda n: n * 2,

'triple': lambda n: n * 3,

'quadruple': lambda n: n * 4,

'square': lambda n: n ** 2,

}.get(term, default)(num)

>>> math('square', 2)

4

>>> math('exponent', 5)

'Operation not supported'

@nnja

lambda for filter, map, reduce is discouraged☝ terms borrowed from functional programming

use a list or generator comprehension instead

>>> nums = range(10)

>>> map(lambda x: x**2, nums)>>> [x**2 for x in nums]>>> (x**2 for x in nums) # generator exp takes advantage of lazy evaluation

>>> filter(lambda x: x % 2 != 0, nums)>>> [x for x in nums if x % 2 != 0]>>> (x for x in nums if x % 2 != 0)

ℹ in many cases, reduce can be replaced with a built-in like sum or a for loop ℹ differences in python2: filter, map return a list, not an iterable and reduce is a built-in.

Additional reading - Guido's thoughts on Lambda

Context Managers& new in python 3: async context managers

When should I use one?Need to perform an action before and/or after an operation.

Common scenarios:

— Closing a resource after you're done with it (file, network connection)

— Resource management

@nnja

Example Problem: Feature Flags

Turn features of your application on and off easily.

Uses of feature flags:

— A/B Testing— Rolling Releases — Show Beta version to users opted-in to Beta Testing

Program

More on Feature Flags

Simple Example - FeatureFlags Class

class FeatureFlags: """ Example class which stores Feature Flags and their state. """

SHOW_BETA = 'Show Beta version of Home Page'

flags = { SHOW_BETA: True }

@classmethod def is_on(cls, name): return cls.flags[name]

@classmethod def toggle(cls, name, on): cls.flags[name] = on

feature_flags = FeatureFlags()

@nnja

How do we temporarily turn features on and off when testing flags?

Want:

with feature_flag(FeatureFlags.SHOW_BETA): assert '/beta' == get_homepage_url()

@nnja

Using Magic Methods __enter__ and __exit__

class feature_flag: """ Implementing a Context Manager using Magic Methods """

def __init__(self, name, on=True): self.name = name self.on = on self.old_value = feature_flags.is_on(name)

def __enter__(self): feature_flags.toggle(self.name, self.on)

def __exit__(self, *args): feature_flags.toggle(self.name, self.old_value)

See: contextlib.contextmanager

The be!er way: using the contextmanager decorator

from contextlib import contextmanager

@contextmanagerdef feature_flag(name, on=True): old_value = feature_flags.is_on(name) feature_flags.toggle(name, on) yield feature_flags.toggle(name, old_value)

See: contextlib.contextmanager

The be!er way: using the contextmanager decorator

from contextlib import contextmanager

@contextmanagerdef feature_flag(name, on=True): """ The easier way to create Context Managers """ old_value = feature_flags.is_on(name) feature_flags.toggle(name, on) # behavior of __enter__() yield feature_flags.toggle(name, old_value) # behavior of __exit__()

See: contextlib.contextmanager

either implementation

def get_homepage_url(): """ Method that returns the path of the home page we want to display. """ if feature_flags.is_on(FeatureFlags.SHOW_BETA): return '/beta' else: return '/homepage'

def test_homepage_url_with_context_manager():

with feature_flag(FeatureFlags.SHOW_BETA): # saw the beta homepage... assert get_homepage_url() == '/beta'

with feature_flag(FeatureFlags.SHOW_BETA, on=False): # saw the standard homepage... assert get_homepage_url() == '/homepage'

@nnja

either implementation

def get_homepage_url(): """ Method that returns the path of the home page we want to display. """ if feature_flags.is_on(FeatureFlags.SHOW_BETA): return '/beta' else: return '/homepage'

def test_homepage_url_with_context_manager():

with feature_flag(FeatureFlags.SHOW_BETA): assert get_homepage_url() == '/beta' print('seeing the beta homepage...')

with feature_flag(FeatureFlags.SHOW_BETA, on=False): assert get_homepage_url() == '/homepage' print('seeing the standard homepage...')

@nnja

DecoratorsThe simple explanation:

Syntactic sugar that allows modification of an underlying function.

@nnja

Recap— Wrap a function in another function.— Do something:

— before the call— after the call— with provided arguments— modify the return value or arguments

@nnja

def say_after(hello_function):

def say_nice_to_meet_you(name):

hello_function(name)

print('It was nice to meet you!')

return say_nice_to_meet_you

def hello(name):

print('Hello', name)

>>> say_after(hello)('Nina')

Hello Nina It was nice to meet you!

— say_after(hello) returns the function say_nice_to_meet_you

— then we call say_nice_to_meet_you('Nina')@nnja

def say_after(hello_function):

def say_nice_to_meet_you(name):

hello_function(name)

print('It was nice to meet you!')

return say_nice_to_meet_you

@say_after

def hello(name):

print('Hello', name)

>>> hello('Nina')

Hello Nina It was nice to meet you!

— calling the decorated function hello(name)— is the same as calling an undecorated hello with

say_after(hello)('Nina')

@nnja

closure example

def multiply_by(num): def do_multiplication(x): return x * num return do_multiplication

multiply_by_five = multiply_by(5)

>>> multiply_by_five(4) 20

@nnja

decorators that take arguments

def greeting(argument): def greeting_decorator(greet_function): def greet(name): greet_function(name) print('It was %s to meet you!' % argument) return greet return greeting_decorator

@greeting('bad')def aloha(name): print ('Aloha', name)

@nnja

decorators that take arguments

def say_this_after(argument):

def say_after(hello_function):

def say_after_meeting(name):

hello_function(name)

print('It was %s to meet you' % argument)

return say_after_meeting

return say_after

@say_this_after('bad')

def hello(name):

print('Hello', name)

Is the same as calling this on an undecorated function:

say_after_bad = say_this_after('bad')(hello)say_after_bad('Nina')

@nnja

losing context with a decorator !

def say_bye(func): def wrapper(name): func() print('Bye', name) return wrapper

@say_byedef my_name():""" Say my name""" print('Nina')

>>> my_name.__name__ 'wrapper'>>>> my_name.__doc__ # ... empty

@nnja

solution: use wraps, or wrapt library! !

from contextlib import wrapsdef say_adios(func): @wraps(func) # pass in which function to wrap def wrapper(): func() print('Adios!') return wrapper

@say_adiosdef say_max():""" Says the name Max""" print('Max')

>>> say_max.__name__ 'say_max'>>> say_max.__doc__ ' Says the name Max'

@nnja

Example use: StatsD

— Collects statistics such as counters and timers over UDP

— Pluggable backend services like Graphite, Grafana— Lets us make pretty graphs!

—

@nnja

How can we increment a statsd counter a!er making a method call?

@nnja

from contextlib import wraps def statsd_incr(function): @wraps(function) # so that original func knows it's name and docstring def wrapper(*args, **kwargs): key = function.__name__ statsd.incr(key) function(*args, **kwargs) return wrapper

@statsd_incrdef important_operation(): print('Doing important thing...')

>>> important_operation() # statsd incremented: important_operation 'Doing important thing...'

@nnja

Other common usecases— logging— timing— validation— rate limiting— mocking/patching

@nnja

Advanced use: modifying arguments, validation

def require_auth(self, require_user_account=False): def wrapper(f): @wraps(f) def decorated(*args, **kwargs): user = request.user # assumes user in the request if require_user_account: if not user or user.is_anonymous: raise AuthenticationException args = (user,) + args return f(*args, **kwargs) return decorated return wrapper

@nnja

in api code

@require_auth(require_user_account=True)def change_profile(account): # account always belongs to an authenticated user account.update(...)

@nnja

ContextDecorators ContextManagers+ Decorators combined.

@nnja

As of python 3.2 ContextDecorators are in the standard library. They're the best of both worlds!

By using ContextDecorator you can easily write classes that can be used both as decorators with @ and context managers with the with statement.

ContextDecorator is used by contextmanager(), so you get this functionality ✨ automatically ✨.

Alternatively, you can write a class that extends from ContextDecorator or uses ContextDecorator as a mixin, and implements __enter__, __exit__ and __call__

If you use python2, a backport package is available here: contextlib2

@nnja

Remember @contextmanager from earlier?

from contextlib import contextmanager

@contextmanagerdef feature_flag(name, on=True): old_value = feature_flags.is_on(name) feature_flags.toggle(name, on) yield feature_flags.toggle(name, old_value)

@nnja

use it as a context manager

def get_homepage_url(): beta_flag_on = feature_flags.is_on(FeatureFlags.SHOW_BETA) return '/beta' if beta_flag_on else '/homepage'

with feature_flag(FeatureFlags.SHOW_BETA): assert get_homepage_url() == '/beta'

or use as a decorator

@feature_flag(FeatureFlags.SHOW_BETA, on=False)def get_profile_page(): beta_flag_on = feature_flags.is_on(FeatureFlags.SHOW_BETA) return 'beta.html' if beta_flag_on else 'profile.html'

assert get_profile_page() == 'profile.html'

@nnja

library I !: freezegun lets your python tests ❇ travel through time! ❇

from freezegun import freeze_time

# use it as a Context Managerdef test(): with freeze_time("2012-01-14"): assert datetime.datetime.now() == datetime.datetime(2012, 1, 14) assert datetime.datetime.now() != datetime.datetime(2012, 1, 14)

# or a decorator@freeze_time("2012-01-14")def test(): assert datetime.datetime.now() == datetime.datetime(2012, 1, 14)

read the source sometime, it's mind-bending!

@nnja

MixinsClass wants new behavior,but already inherits from a different class.

@nnja

Simple Example: Timestamp Mixin for sqlalchemy models

class Timestamp(object): """Adds `created` and `updated` columns to a model. """ created = sa.Column(sa.DateTime, default=datetime.utcnow) updated = sa.Column(sa.DateTime, default=datetime.utcnow)

class SomeModel(Base, Timestamp): __tablename__ = 'somemodel' id = sa.Column(sa.Integer, primary_key=True)

@nnja

Intermediate Example: UserMixin in flask-security

Use it with your User database model, get useful helper methods on model instances:- is_active- get_auth_token- has_role

Mixins are not just for models- Used in many other projects, Django uses them for Views

@nnja

Clever example: ReprMixin in sqlalchemy

class ReprMixin(object): """ Provides a method to repr all fields on a sqlalchemy model class """ def __repr__(self): def column_names_to_repr(): for col in self.__table__.c: yield col.name, repr(getattr(self, col.name))

def format_key_to_value(map): for key, value in map: yield '%s=%s' % (key, value)

args = '(%s)' % ', '.join(format_key_to_value(column_names_to_repr())) return "<{}{}>".format(type(self).__name__, args)

@nnja

Using ReprMixin

class MyModel(Base, ReprMixin):

id = sa.Column(sa.Integer, primary_key=True)

name = sa.Column(sa.String)

category = sa.Column(sa.String)

nina = MyModel(name='Nina', state='Oregon')

Result:Instead of: <__main__.MyModel at 0x10587e810>

Result repr(nina) is now: <MyModel(id=1, name=Nina", state="Oregon")>

@nnja

NamedTupleUseful when you need lightweight representations of data.

Create tuple subclasses with named fields.

@nnja

Simple Example

from collections import namedtuple

CacheInfo = namedtuple( "CacheInfo", ["hits", "misses", "max_size", "curr_size"])

@nnja

Giving NamedTuples default values

RoutingRule = namedtuple( 'RoutingRule', ['prefix', 'queue_name', 'wait_time'])

(1) By specifying defaults

RoutingRule.__new__.__defaults__ = (None, None, 20)

(2) or with _replace to customize a prototype instance

default_rule = RoutingRule(None, None, 20)user_rule = default_rule._replace(prefix='user', queue_name='user-queue')

@nnja

NamedTuples can be subclassed and extended

class Person(namedtuple('Person', ['first_name', 'last_name'])): """ Stores first and last name of a Person""" __slots__ = ()

def __str__(self): return '%s %s' % (self.first_name, self.last_name)

>>> me = Person('nina', 'zakharenko')

>>> str(me) 'nina zakharenko'

>>> me Person(first_name='nina', last_name='zakharenko')

@nnja

Tip

Use __slots__ = () in your NamedTuples!

— It prevents the creation of instance dictionaries.— It lowers memory consumption.— Allows for faster access

@nnja

Signalsaka pub / sub

@nnja

library I !: github.com/jek/blinker

Blinker provides a fast dispatching system that allows any number of interested parties to subscribe to events, or "signals".

Signal receivers can subscribe to specific senders or receive signals sent by any sender.

@nnja

>>> from blinker import signal>>> started = signal('round-started')>>> def each(round):... print "Round %s!" % round...>>> started.connect(each)

>>> def round_two(round):... print "This is round two."...>>> started.connect(round_two, sender=2)

@nnja

>>> from blinker import signal>>> started = signal('round-started')>>> def each(round):... print "Round %s!" % round...>>> started.connect(each)

>>> def round_two(round):... print "This is round two."...>>> started.connect(round_two, sender=2)

>>> for round in range(1, 4):... started.send(round)...Round 1!Round 2!This is round two.Round 3!

@nnja

Blinker does so much more!Read the docs: pythonhosted.org/blinker/

Learn about:- Subscribing to Signals- Emitting Signals- Sending & Receiving data via signals- Optimizations, & more

@nnja

Uses

Flask & many flask extensions use blinker under the hood to send signals you can listen for if you install the library.

Other packages provide their own signal implementations like celery and Django.

@nnja

New tools in your toolbox:— Magic Methods & Method ❇Magic❇— Decorators— ContextManagers— ContextDecorators— Lambda— NamedTuple— Signals

@nnja

Python is awesome!Python presents us with an incredible toolbox.

A flexible language with powerful features let us program in fun and creative ways.

@nnja

Don't be a mindless Code Monkey

@nnja

Always code as if the person who ends up maintaining your code is a violent psychopath who knows where you live. !

— Martin Golding

@nnja

Instead, be an Elegant Pythonista!

@nnja

Thanks!

@nnja

nina@nnja.io

bit.ly/elegant-python

@nnja