Be more pythonic

Pyladies Berlin, 2015

by Mei Li Triantafyllidi

Zen of python

PEP8

No tabs

4 spaces indentation

Yes: spam(ham[1], {eggs: 2})
No:  spam( ham[ 1 ], { eggs: 2 } )

Whitespaces

Yes: x, y = y, x
No:  x , y = y , x

Yes: counter = counter + 1
No:  counter=counter+1

Yes: result = add(x+1, 3)

Yes: result = add(x + 1, 3)

Yes: import os
     import sys

No:  import sys, os

No: from subprocess import *

Yes: from subprocess import Popen, PIPE

Import statements

Naming conventions

lower_case_under for variables and functions and methods
WordCap for classes
ALL_CAPS for constants

Tools

pep8
pylint
pyflakes

most IDEs have a plugin

Pythonic code patterns

IPython notebook

Swap variables

No:

tmp = b
b = a
a = tmp

Yes:

a, b = b, a

Swap variables more


tmp_x = x + dx * t
tmp_y = y + dy * t
tmp_dx = influence(m, x, y, dx, dy, partial='x')
tmp_dy = influence(m, x, y, dx, dy, partial='y')
x = tmp_x
y = tmp_y
dx = tmp_dx
dy = tmp_dy

Yes


x, y, dx, dy = (x + dx * t,
                y + dy * t,
                influence(m, x, y, dx, dy, partial='x'),
                influence(m, x, y, dx, dy, partial='y'))

Unpacking tuples

person_info = ("Mary", 27)

## Don't do
name = person_info[0]
age = person_info[1]


## Do
name, age = person_info

Iteration

## Don't do
for i in [0, 1, 2, 3, 4]:
    print i

## Don't do
for i in range(5):
    print i

## Do
for i in xrange(5):
    print i

## Python 3 - only range

Iteration over list

colors = ['red', 'green', 'blue', 'yellow']

# Don't do
for i in range(len(colors)):
    print colors[i]

# Do
for color in colors:
    print color

Iteration over list with index

colors = ['red', 'green', 'blue', 'yellow']

## Don't do
for i in range(len(colors)):
    print i, '-->', colors[i]

## Do
for i, color in enumerate(colors):
    print i, '-->', color

List comprehension


## Don't do
names = []
for kid in kids:
    names.append(kid["name"])

## Do
names = [kid["name"] for kid in kids]


## Don't do
baby_names = []
for kid in kids:
    if kid["age"] < 1:
        baby_names.append(kid["name"])

## Do
names = [kid["name"] for kid in kids if kid["age"] > 1]

Use builtins - sum


## Get sum of all numbers until 100
## Don't do
total = 0
for num in range(101):
    total += num

## Do
total = sum(range(101))

## Get sum of all squares of numbers until 100
## Don't do
total = 0
for num in range(101):
    total += num**2

## Do
total = sum([n**2 for n in range(101)])

Generators


## Don't do 
total = sum([n**2 for n in range(101)])

## Do
total = sum(n**2 for n in range(101))

Concatenate list strings

## Don't do
s = names[0]
for name in names[1:]:
    s += ', ' + name

## Do
s = ', '.join(names)

Sort list

names = ["Mary", "John", "Peter", "Helen"]


for name in sorted(names):
    print name

## sort by string length
for name in sorted(names, key=len):
    print name

## Sorting descending
for name in sorted(names, reverse=True):
    print name

Counting with dictionaries

colors = ['red', 'green', 'red', 'blue', 'green', 'red']

# Don't do
d = {}
for color in colors:
    if color not in d:
        d[color] = 0
    d[color] += 1

# {'blue': 1, 'green': 2, 'red': 3}

# Do
d = {}
for color in colors:
    d[color] = d.get(color, 0) + 1

Grouping with dictionaries

colors = ['gray', 'black', 'yellow', 'blue', 'green', 'red']

# Don't do
d = {}
for color in colors:
    first_char = color[0]
    if first_char not in d:
        d[first_char] = []
    d[first_char].append(color)

# {'r': ['red'], 'b': ['black', 'blue'], 
#  'g': ['gray', 'green']}

# Do
d = defaultdict(list)
for color in colors:
    first_char = color[0]
    d[first_char].append(color)

Looping over dictionaries

city_music = {"berlin": "techno", 
              "seville": "flamenco", 
              "new york": "jazz"}

# only keys
for city in city_music:
    print city

# to change the dictionary while looping - Do
for city in city_music.keys():
    if city not in europe:
        del city_music[city]

Looping over dictionaries more

city_music = {"berlin": "techno", 
              "seville": "flamenco", 
              "new york": "jazz"}

for city in city_music:
    print city, 'listens to', city_music[city] 

# more readable
for city, music in city_music.items():
    print city, 'listens to', music 

# more readable and faster
for city, music in city_music.iteritems():
    print city, 'listens to', music

Reading files

# Do not do 
fp = open("notes.txt")
text = fp.read()
fp.close()

# Do 
with open("notes.txt") as fp:
    text = fp.read()

# read file line by line
with open("notes.txt") as fp:
    for line in fp:
        print line

what's next

decorators
property instead of getter, setter
staticmethod, classmethod

resources

Sum up

Beautiful is better than ugly
Readability counts
PEP8
read builtins, str methods, collections

... so read read read code

Let's practice

http://exercism.io/

Be more pythonic

By Mei Li Triantafyllidi