Beautiful Python

Alicia Pérez

www.stylesage.co

Accent?

We are flying!

>>> import antigravity

Strings

colors = ['red', 'blue', 'green', 'yellow']
result = ''
for s in colors:
    result += s
# result: 'redbluegreenyellow'

colors = ['red', 'blue', 'green', 'yellow']
result = ''.join(colors)
# result: 'redbluegreenyellow'

Good

Bad

Create a string from a list

Strings

colors = ['red', 'blue', 'green', 'yellow']
result = ''
for s in colors:
    result += s + ','

result = result[:-1]
# result: 'red,blue,green,yellow'

colors = ['red', 'blue', 'green', 'yellow']
result = ','.join(colors)
# result: 'red,blue,green,yellow'

Good

Bad

Create a string from a list joined by comma

Strings

my_very_big_string = """For a long time I used to go to bed early. Sometimes,
    when I had put out my candle, my eyes would close so quickly that I had not even
    time to say "Im going to sleep."""

my_very_big_string = (
    "For a long time I used to go to bed early. Sometimes, "
    "when I had put out my candle, my eyes would close so quickly "
    "that I had not even time to say: Im going to sleep."
)

Good

Bad

Multi-line

Strings

contains?

stylesage = 'Fashion meets Big Data'
if stylesage.contains('Fashion'):
    pass

Bad

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'contains'

Strings

stylesage = 'Fashion meets Big Data'
if stylesage.find("Fashion") != -1:
    pass

Good

Bad

stylesage = 'Fashion meets Big Data'
if 'Fashion' in stylesage:
    pass

~~contains~~ in

Strings

format

                               # OR
'%s %s' % ('one', 'two')              '%d %d' % (1, 2)
# output: 'one two'                   # output: '1 2'

New

Old

                               # OR
'{} {}'.format('one', 'two')          '{} {}'.format(1, 2)
# output: 'one two'                   # output: '1 2'

Strings

Basic format

                               # OR
'%s %s' % ('one', 'two')              '%d %d' % (1, 2)
# output: one two                     # output: 1 2

New

Old

                               # OR
'{} {}'.format('one', 'two')          '{} {}'.format(1, 2)
# output: one two                     # output: one two

# explicit positional index
'{1} {0}'.format('one', 'two')
# output: two one

Strings

Padding and aligning

                               # OR
'%10s' % ('test',)                    '%-10s' % ('test',)
# output: '      test'                # output: 'test      '

New

Old

                               # OR
'{:>10}'.format('test')               '{:10}'.format('test')
# output: '      test'                # output: 'test      '

Strings

Padding and aligning

                               # OR
'%10s' % ('test',)                    '%-10s' % ('test',)
# output: '      test'                # output: 'test      '

New

Old

                               # OR
'{:>10}'.format('test')               '{:10}'.format('test')
# output: '      test'                # output: 'test      '

# choose the padding character
'{:_<10}'.format('test')
# output: 'test______'

Strings

Padding and aligning

                               # OR
'%10s' % ('test',)                    '%-10s' % ('test',)
# output: '      test'                # output: 'test      '

New

Old

                               # OR
'{:>10}'.format('test')               '{:10}'.format('test')
# output: '      test'                # output: 'test      '

# choose the padding character
'{:_<10}'.format('test')
# output: 'test______'

# center align value
'{:_^10}'.format('test')
# output: '___test___'

Strings

Padding and aligning

New

# This operations are not available with old-style formatting

'{:=5d}'.format((- 23))
# output: '-  23'

'{:%Y-%m-%d %H:%M}'.format(datetime(2001, 2, 3, 4, 5))
# output: '2001-02-03 04:05'

person = {'first': 'Jean-Luc', 'last': 'Picard'}
'{p[first]} {p[last]}'.format(p=person)
# output: 'Jean-Luc Picard'

data = [4, 8, 15, 16, 23, 42]
'{d[4]} {d[5]}'.format(d=data)
# output: '23 42'

tu = (12,45,22222,103,6)
print '{0} {2} {1} {2} {3} {2} {4} {2}'.format(*tu)
# output: '12 22222 45 22222 103 22222 6 22222'

Booleans

if len(items) != 0:
    pass

if name != "":
    pass

Good

Bad

if items:
    pass

if name:
    pass

Check if variable equals a constant

False	True
False (== 0)	True (== 1)
"" (empty string)	any string but "" (" ", "anything")
0, 0.0	any number but 0 (1, 0.1, -1, 3.14)
[], (), {}, set()	any non-empty container ([0], (None,),[''])
None	almost any object that's not explicitly False

Booleans

Check if variable equals a constant

Booleans

if x >= start and x <= end:
    # do stuff

Good

Bad

if start <= x <= end:
    # do stuff

Chained comparisons

Comparisons

values = range(10)
i = 0
found = False
while not found and i < len(values):
    found = not values[i] % 2
    i += 1

Good

Bad

values = range(10)
anyPair = any([not value % 2 for value in values])

# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# [True, False, True, False, True, False, True, False, True, False]
# anyPair: True

Any

Comparisons

values = range(10)
i = 0
allPairs = True
while allPairs and i < len(values):
    allPairs = not values[i] % 2
    i += 1

Good

Bad

values = range(10)
allPairs = all([not value % 2 for value in values])

# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# [True, False, True, False, True, False, True, False, True, False]
# output: True

All

Comparisons

if x > 10:
    result = 'Yes'
else:
    result = 'No'

Good

Bad

result = 'Yes' if x > 10 else 'No'

Ternary operator

Lists

mylist = ['yellow', 'red', 'blue', 'green', 'black']

mylist[1:4]
# output: ['red', 'blue', 'green']

mylist[2:]
# output: ['blue', 'green', 'black']

mylist[:2]
# output: ['yellow', 'red']

mylist[-1]
# output: 'black'

mylist[1:-1]
# output: ['red', 'blue', 'green']

Manipulation

Lists

a = [3, 4, 5]
for i in range(len(a)):
    a[i] += 3

Good

Bad

a = [3, 4, 5]
a = [i + 3 for i in a]
# Or:
a = map(lambda i: i + 3, a)

List comprehensions

Lists

a = [3, 4, 5]
b = []
for i in a:
    if i > 4:
        b.append(i)

Good

Bad

a = [3, 4, 5]
b = [i for i in a if i > 4]
# Or:
b = filter(lambda x: x > 4, a)

Filter

Lists

a = [3, 4, 5]
i = 0
for item in items:
    print i, item
    i += 1

Good

Bad

a = [3, 4, 5]
for i, item in enumerate(a):
    print i, item

Enumerate

Generators

def firstn(n):
    num, nums = 0, []
    while num < n:
        nums.append(num)
        num += 1
    return nums

sum_of_first_n = sum(firstn(1000000))

Bad

def firstn(n):
    num = 0
    while num < n:
        yield num
        num += 1

sum_of_first_n = sum(firstn(1000000))

Good

Not in memory list

Generators

square = [i*i for i in xrange(1000000)]

List

square = (i*i for i in irange(1000000))

Generator

Generator vs list

Tuples

temp = a
a = b
b = temp

b, a = a, b

Good

Bad

Swap values

Tuples

l =['David', 'Pythonista', '+1-514-555-1234']
name, title, phone = l
# name: 'David'
# title: 'Pythonista'
# phone: '+1-514-555-1234'

Unpacking

Tuples

List of tuples

names = ['John', 'Eric', 'Terry']
surnames = ['Cleese', 'Idle', 'Gilliam']
people = []
for i in range(len(names)):
    people.append((names[i], surnames[i]))
print(people)
# output: [('John', 'Cleese'), ('Eric', 'Idle'), ('Terry', 'Gilliam')]

names = ['John', 'Eric', 'Terry']
surnames = ['Cleese', 'Idle', 'Gilliam']
zip(names, surnames)
# output: [('John', 'Cleese'), ('Eric', 'Idle'), ('Terry', 'Gilliam')]

Good

Bad

Dictionaries

Good

Bad

if d.has_key(key):
    print(d[key])

if key in d:
    print(d[key])

Dictionaries

Good

Bad

for key in d.keys():
    print key

for key in d:
    print key

Except

for key in d.keys():
    d[str(key)] = d[key]

Dictionaries

navs = {}
for (portfolio, equity, position) in data:
    if portfolio not in navs:
        navs[portfolio] = 0
    navs[portfolio] += position * prices[equity]

Good

Bad

for (portfolio, equity, position) in data:
    navs[portfolio] = (navs.get(portfolio, 0)
                       + position * prices[equity])

navs = {}
for (portfolio, equity, position) in data:
    if portfolio not in navs:
        navs[portfolio] = 0
    navs[portfolio] += position * prices[equity]

for (portfolio, equity, position) in data:
    navs.setdefault(portfolio, 0)
    navs[portfolio] += position * prices[equity]

Dictionaries

Good

Bad

navs = {}
for (portfolio, equity, position) in data:
    if portfolio not in navs:
        navs[portfolio] = 0
    navs[portfolio] += position * prices[equity]

navs = defaultdict(int)
for (portfolio, equity, position) in data:
    navs[portfolio] += position * prices[equity]

Dictionaries

Good

Bad

given = ['John', 'Eric', 'Terry', 'Michael']
family = ['Cleese', 'Idle', 'Gilliam', 'Palin']
pythons = {}
for i in range(len(given)):
    pythons[given[i]] = family[i]

given = ['John', 'Eric', 'Terry', 'Michael']
family = ['Cleese', 'Idle', 'Gilliam', 'Palin']
pythons = dict(zip(given, family))

Dictionaries

Good

Bad

values = range(4)
res = {}
for numb in values:
    res[numb] = chr(65 + numb)

res = {i : chr(65+i) for i in range(4)}

Dictionaries

Good

Bad

{0 : 'A', 1 : 'B', 2 : 'C', 3 : 'D'}

What!?!?!

Interactive "_"

>>> 1 + 1
2
>>> _
2
>>> import math
>>> math.pi / 3
1.0471975511965976
>>> angle = _
>>> math.cos(angle)
0.50000000000000011
>>> _
0.50000000000000011

filename = 'foobar.txt'
basename, _, ext = filename.rpartition('.')

Variable "_"

from __future__

>>> from __future__ import braces
  File "<stdin>", line 1
SyntaxError: not a chance

from __future__ import division
print 8/7  # prints 1.1428571428571428
print 8//7 # prints 1

Beautiful Python

Accent?

We are flying!

Strings

Strings

Strings

Strings

Strings

Strings

Strings

Strings

Strings

Strings

Strings

Booleans

Booleans

Booleans

Comparisons

Comparisons

Comparisons

Lists

Lists

Lists

Lists

Tuples

Tuples

Tuples

Dictionaries

Dictionaries

Dictionaries

Dictionaries

Dictionaries

Dictionaries

Dictionaries

What!?!?!

Interactive "_"

Variable "_"

Beautiful Python

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