Introduction to advanced python

Introduction to  
Advanced Python
Charles-Axel Dein - June 2014, revised June 2016

License
• This presentation is shared under Creative Commons 
Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
• More information about the license can be found here
• Please give proper attribution to the original author  
(Charles-Axel Dein)

Checkout my Github for  
the source of this presentation  
and more resources:
github.com/charlax/python-education

What’s the goal of this
presentation?

Goal 1:
Discover cool & underused  
Python features

Goal 2:
Improve your code’s
readability

Write code that explains to
a human what we want  
the computer to do.
– D. Knuth

Do not use those patterns
just when you feel like it

Readability can be
achieved through
conciseness…

… but it can also be hindered  
by overuse of magic

Let’s talk about  
a concrete example

Before
def toast(bread):
if bread == "brioche":
raise ValueError("Are you crazy?")
toaster = Toaster()
if toaster.has_bread("bagel"):
raise ToasterNotEmptyError("Toaster already has some bagel")
try:
toaster.set_thermostat(5)
for slot in [toaster.slot1, toaster.slot2]:
slot.insert(bread)
except BreadBurnedError:
print("zut")
finally:
toaster.eject()

Let’s make it more
expressive!

def toast(bread):
if bread == "brioche":
toaster = Toaster()
if toaster.has_bread("bagel"):
raise ToasterNotEmptyError(
"Toaster already has some bagel")
try:
for slot in [toaster.slot1, toaster.slot2]:
slot.insert(bread)
except BreadBurnedError:
print("zut")
finally:
toaster.eject()
@nobrioche
def toast(bread):
toaster = Toaster()
if "bagel" in toaster:
raise ToasterNotEmptyError( 
"Toaster already has some bagel")
with handle_burn():
for slot in toaster:
slot.insert(bread)

It almost reads like
English.
(only with a French accent)

Note:
this is just an example!

In practice you’d want to be
a bit more explicit about
things.

Note:
examples were run with  
Python 3.4

Attach additional
responsibilities to an
object

In Python , almost
everything is an object
In [1]: def toast(): pass
In [2]: toast
Out[2]: <function __main__.toast>
In [3]: type(toast)
Out[3]: function

Python’s decorators (@)
are just syntactic sugar

def function():
pass
function = decorate(function)
# is exactly equivalent to:
@decorate
def function():
pass

AfterBefore
def toast(bread):
if bread == 'brioche':
...
def burn(bread):
...
def no_brioche(function):
def wrapped(bread):
return function(bread)
return wrapped
@no_brioche
def toast(bread): pass
@no_brioche
def burn(bread): pass

Cachingimport functools
@functools.lru_cache(maxsize=32)
def get_pep(num):
"""Retrieve text of 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'
pep = get_pep(8)
pep = get_pep(8)

Other use cases
• Annotating/registering (e.g. Flask’s app.route)
• Wrapping in a context (e.g. mock’s mock.patch)

To go further
• Parametrized decorators
• Class decorators
• ContextDecorator

Originally meant to provide
reusable  
try… finally blocks

with open("/etc/resolv.conf") as f:
print(f.read())
# Is **roughly** equivalent to:
try:
f = open("/etc/resolv.conf")
print(f.read())
finally:
f.close()

Context managers wrap
execution in a context

class Timer(object):
def __enter__(self):
self.start = time.clock()
return self
def __exit__(self, exception_type, exception_value, traceback):
self.end = time.clock()
self.duration = self.end - self.start
self.duration_ms = self.duration * 1000
with Timer() as timer:
time.sleep(.1)
assert 0.01 < timer.duration_ms < 0.3

AfterBefore
try:
os.remove('whatever.tmp')
except FileNotFoundError:
pass
from contextlib import suppress
with suppress(FileNotFoundError):
os.remove('somefile.tmp')

redis pipeline
with redis_client.pipeline() as pipe:
pipe.set('toaster:1', 'brioche')
bread = pipe.get('toaster:2')
pipe.set('toaster:3', bread)

SQL transaction from contextlib import contextmanager
@contextmanager
def session_scope():
"""Provide a transactional scope around a series of operations."""
session = Session()
try:
yield session
session.commit()
except:
session.rollback()
raise
finally:
session.close()
def run_my_program():
with session_scope() as session:
ThingOne().go(session)
ThingTwo().go(session)

Other use cases
• Acquiring/releasing a lock
• Mocking

To go further
• PEP 343
• contextlib module
• @contextlib.contextmanager
• contextlib.ExitStack
• Single use, reusable and reentrant context managers

Iterators: what are they?
• An object that implements the iterator protocol
• __iter__() returns the iterator (usually self)
• __next__() returns the next item or raises StopIteration
• This method will be called for each iteration until it raises
StopIteration

>>> import dis
>>> def over_list():
... for i in None: pass
...
>>> dis.dis(over_list)
2 0 SETUP_LOOP 14 (to 17)
3 LOAD_CONST 0 (None)
6 GET_ITER
>> 7 FOR_ITER 6 (to 16)
10 STORE_FAST 0 (i)
13 JUMP_ABSOLUTE 7
>> 16 POP_BLOCK
>> 17 LOAD_CONST 0 (None)
20 RETURN_VALUE

>>> class Toasters(object):
... """Loop through toasters."""
... def __init__(self):
... self.index = 0
...
... def __next__(self):
... resource = get_api_resource("/toasters/" + str(self.index))
... self.index += 1
... if resource:
... return resource
... raise StopIteration
...
... def __iter__(self):
... return self
>>> for resource in Toasters():
... print(resource)
found /toasters/0
found /toasters/1

Generators
>>> def get_toasters():
... while True:
... resource = get_api_resource("/toasters/" + str(index))
... if not resource:
... break
... yield resource
>>> for resource in get_resources():
... print(resource)
found /toasters/0
found /toasters/1

Use cases of iterators
• Lazy evaluation of results one (batch) at time
• Lower memory footprint
• Compute just what you needed - can break in the middle (e.g.
transparently page queries)
• Unbounded sets of results

To go further
• Other uses of yield (e.g. coroutines)
• Exception handling

Special methods
• Method that starts and ends with “__”
• Allow operator overloading, in particular

Examples
• __eq__, __lt__, …: rich comparison (==, >…)
• __len__: called with len()
• __add__, __sub__, …: numeric types emulation (+, -…)
• Attribute lookup, assignment and deletion
• Evaluation, assignment and deletion of self[key]

Use case: operator
overloading

@functools.total_ordering
class Card(object):
_order = (2, 3, 4, 5, 6, 7, 8, 9, 10, 'J', 'Q', 'K', 'A')
def __init__(self, rank, suite):
assert rank in self._order
self.rank = rank
self.suite = suite
def __lt__(self, other):
return self._order.index(self.rank) < self._order.index(other.rank)
def __eq__(self, other):
return self.rank == other.rank
ace_of_spades = Card('A', 'spades')
eight_of_hearts = Card(8, 'hearts')
assert ace_of_spades < eight_of_hearts

Why should they be used?
• Greater encapsulation of the logic,  
allowing objects to be manipulated without external function/methods
• Allow uses of builtins that all Python developers know (len, repr, …)
• Allow objects to be manipulated via operators (low cognitive burden)
• Allow use of certain keywords and Python features
• with (__enter__, __exit__)
• in (__contains__)

AfterBefore
if is_greater(ace_of_spades,
eight_of_hearts):
pass
if (ace_of_spades.rank
> eight_of_hearts.rank):
pass
if ace_of_spades > eight_of_hearts:
pass

To go further
• Google “A Guide to Python's Magic Methods”
• Check how SQLAlchemy uses it to allow  
session.query(Toaster.bread == ‘croissant’)

collections
• namedtuple()
• Counter
• OrderedDict
• defaultdict
>>> # Find the ten most common words in Hamlet
>>> import re
>>> words = re.findall(r'w+', open('hamlet.txt').read().lower())
>>> Counter(words).most_common(10)
[('the', 1143), ('and', 966), ('to', 762), ('of', 669), ('i', 631),
('you', 554), ('a', 546), ('my', 514), ('hamlet', 471), ('in', 451)]

random
• randrange(start, stop[, step])
• randint(a, b)
• choice(seq)
• shufﬂe(x[, random])
• sample(population, k)

functools
• @lru_cache(maxsize=128, typed=False)
• partial(func, *args, **keywords)
• reduce(function, iterable[, initializer])

operator
• operator.attrgetter(attr)
• operator.itemgetter(item)
import operator as op
class User(object): pass
class Toaster(object): pass
toaster = Toaster()
toaster.slot, toaster.color = 1, 'red'
user = User()
user.toaster = toaster
f = op.attrgetter('toaster.slot', 'toaster.color')
assert f(user) == (1, 'red')

import cProfile
import glob
cProfile.run("glob.glob('*')")

164 function calls (161 primitive calls) in 0.000 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.000 0.000 0.000 0.000 <string>:1(<module>)
1 0.000 0.000 0.000 0.000 fnmatch.py:38(_compile_pattern)
1 0.000 0.000 0.000 0.000 fnmatch.py:48(filter)
1 0.000 0.000 0.000 0.000 fnmatch.py:74(translate)

Other topics
• Memory allocation trace
• Metaclasses
• Descriptors and properties
• List, dict, set comprehensions
• Other modules: itertools, csv, argparse, operator, etc.

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