Beautiful Python SyPy  April  2014 Xuanyi  Chew

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What is beauty? •  Tacit  knowledge •  Beauty  is  subjective(?) •  We  all  know  it,  but  cannot  explain  it. •  But  science  can!

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Cosmos:  A  Space  Time  Odyssey,  FOX

Beauty Is Quantifiable •  Sensible •  Simple •  Standard •  Smart

Beautiful Python Code is: •  Sensible •  Simple •  Standard •  Smart

CHECK FOR PALINDROME

Check for Palindrome is_palindrome = lambda s: s == s[::-1]! !

Check for Palindrome is_palindrome = lambda s: s == s[::-1]! ! ✔ Sensible ✔ Simple ✖ Standard ✔ Smart

Check for Palindrome def is_palindrome(s):! return s == s[::-1]!

WRITE A SPELL CHECK

Write a Spell Check import re, collections! ! def words(text): return re.findall('[a-z]+', text.lower()) ! ! def train(features):! model = collections.defaultdict(lambda: 1)! for f in features:! model[f] += 1! return model! ! NWORDS = train(words(file('big.txt').read()))! ! alphabet = 'abcdefghijklmnopqrstuvwxyz'! ! def edits1(word):! splits = [(word[:i], word[i:]) for i in range(len(word) + 1)]! deletes = [a + b[1:] for a, b in splits if b]! transposes = [a + b[1] + b[0] + b[2:] for a, b in splits if len(b)>1]! replaces = [a + c + b[1:] for a, b in splits for c in alphabet if b]! inserts = [a + c + b for a, b in splits for c in alphabet]! return set(deletes + transposes + replaces + inserts)! ! def known_edits2(word):! return set(e2 for e1 in edits1(word) for e2 in edits1(e1) if e2 in NWORDS)! ! def known(words): return set(w for w in words if w in NWORDS)! ! def correct(word):! candidates = known([word]) or known(edits1(word)) or known_edits2(word) or [word]! return max(candidates, key=NWORDS.get)!

Write a Spell Check import re, collections! ! def words(text): return re.findall('[a-z]+', text.lower()) ! ! def train(features):! model = collections.defaultdict(lambda: 1)! for f in features:! model[f] += 1! return model! ! NWORDS = train(words(file('big.txt').read()))! ! alphabet = 'abcdefghijklmnopqrstuvwxyz'! ! def edits1(word):! splits = [(word[:i], word[i:]) for i in range(len(word) + 1)]! deletes = [a + b[1:] for a, b in splits if b]! transposes = [a + b[1] + b[0] + b[2:] for a, b in splits if len(b)>1]! replaces = [a + c + b[1:] for a, b in splits for c in alphabet if b]! inserts = [a + c + b for a, b in splits for c in alphabet]! return set(deletes + transposes + replaces + inserts)! ! def known_edits2(word):! return set(e2 for e1 in edits1(word) for e2 in edits1(e1) if e2 in NWORDS)! ! def known(words): return set(w for w in words if w in NWORDS)! ! def correct(word):! candidates = known([word]) or known(edits1(word)) or known_edits2(word) or [word]! return max(candidates, key=NWORDS.get)! ✔ Sensible ✔ Simple ✔ Standard ✔ Smart

A SIEVE OF ERATOSTHENES

A Sieve Of Erathosthenes from itertools import compress! ! def vprimes(maximum=10**6):! maxidx = maximum//2! sieve = [True] * maxidx! j = 0! for i in range(1, int(maxidx**0.5)+1):! j += 4*i! if sieve[i]:! step = 2*i + 1! sieve[j::step] = [False] * -(-(maxidx-j) // step)! primes = list(compress(range(1, maximum, 2), sieve))! primes[0] = 2! return primes!

A Sieve Of Erathosthenes ! def rwh_primes2(n):! correction = (n%6>1)! n = {0:n,1:n-1,2:n+4,3:n+3,4:n+2,5:n+1}[n%6]! sieve = [True] * (n/3)! sieve[0] = False! for i in xrange(int(n**0.5)/3+1):! if sieve[i]:! k = 3*i+1|1! sieve[((k*k)/3) ::2*k] = [False]*((n/6-(k*k)/6-1)/k+1)! sieve[(k*k+4*k-2*k*(i&1))/3::2*k] = [False]*((n/6-(k*k +4*k-2*k*(i&1))/6-1)/k+1)! return [2,3] + [3*i+1|1 for i in xrange(1,n/3-correction) if sieve[i]]! !

Non-Toy Examples There  can  be  beautiful  modules  and  projects   too – requests  by  Kenneth  ReiT – flask  by  Armin  Ronacher – Minecraft  in  Python  by  Michael  Fogleman   (900+  lines!  Eat  this,  Notch!)

How to Write Beautiful Python •  Use  correct  data  structures •  Write  obvious  code •  Be  clever  in  algorithm,  not  in  code •  PEP-­‐‑8  is  a  good  guide •  PEP-­‐‑20  is  a  GREAT  guide.