446 lines
9.1 KiB
Python
446 lines
9.1 KiB
Python
# Transforming Code into Beautiful, Idiomatic Python
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# Raymond Hettinger
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# @raymondh
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# converted to plain text by sneak@datavibe.net, some notes by me
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# Looping over a range of numbers
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for i in [0, 1, 2, 3, 4, 5]:
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print i**2
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# better:
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for i in range(6):
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print i**2
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# best:
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for i in xrange(6):
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print i**2
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# Looping over a collection
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colors = ['red', 'green', 'blue', 'yellow']
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# yuck:
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for i in range(len(colors)):
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print colors[i]
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# iterate:
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for color in colors:
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print color
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# Looping backwards
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colors = ['red', 'green', 'blue', 'yellow']
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# yuck:
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for i in range(len(colors)-1, -1, -1):
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print colors[i]
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# pythonic:
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for color in reversed(colors):
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print color
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# Looping over a collection and indicies
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colors = ['red', 'green', 'blue', 'yellow']
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for i in range(len(colors)):
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print i, '-->', colors[i]
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# when you need the index:
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for i, color in enumerate(colors):
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print i, '-->', color
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# Looping over two collections
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names = ['raymond', 'rachel', 'matthew']
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colors = ['red', 'green', 'blue', 'yellow']
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n = min(len(names), len(colors))
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for i in range(n):
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print names[i], '-->', colors[i]
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for name, color in zip(names, colors):
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print name, '-->', color
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# iterator uses the least memory:
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for name, color in izip(names, colors):
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print name, '-->', color
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# Looping in sorted order
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colors = ['red', 'green', 'blue', 'yellow']
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for color in sorted(colors):
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print color
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for color in sorted(colors, reverse=True):
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print color
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# Custom sort order
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colors = ['red', 'green', 'blue', 'yellow']
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def compare_length(c1, c2):
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if len(c1) < len(c2): return -1
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if len(c1) > len(c2): return 1
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return 0
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print sorted(colors, cmp=compare_length)
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# no sort function needed! (think SQL)
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print sorted(colors, key=len)
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# Call a function until a sentinel value
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# old:
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blocks = []
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while True:
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block = f.read(32)
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if block == '':
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break
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blocks.append(block)
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# better: (iter takes a sentinel second arg)
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blocks = []
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for block in iter(partial(f.read, 32), ''):
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blocks.append(block)
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# Distinguishing multiple exit points in loops
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def find(seq, target):
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found = False
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for i, value in enumerate(seq):
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if value == tgt:
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found = True
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break
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if not found:
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return -1
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return i
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# for has an 'else' for finishing without breaks:
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def find(seq, target):
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for i, value in enumerate(seq):
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if value == tgt:
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break
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else:
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return -1
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return i
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# Looping over dictionary keys
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d = {'matthew': 'blue', 'rachel': 'green', 'raymond': 'red'}
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for k in d:
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print k
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# this lets you modify:
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for k in d.keys():
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if k.startswith('r'):
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del d[k]
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# best:
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d = {k : d[k] for k in d if not k.startswith('r')}
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# Looping over a dictionary keys and values
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for k in d:
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print k, '-->', d[k]
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for k, v in d.items():
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print k, '-->', v
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# least memory:
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for k, v in d.iteritems():
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print k, '-->', v
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# Construct a dictionary from pairs
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names = ['raymond', 'rachel', 'matthew']
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colors = ['red', 'green', 'blue']
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# dict() takes an iterator:
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d = dict(izip(names, colors))
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#{'matthew': 'blue', 'rachel': 'green', 'raymond': 'red'}
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d = dict(enumerate(names))
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#{0: 'raymond', 1: 'rachel', 2: 'matthew'}
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# Counting with dictionaries
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colors = ['red', 'green', 'red', 'blue', 'green', 'red']
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d = {}
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for color in colors:
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if color not in d:
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d[color] = 0
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d[color] += 1
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#{'blue': 1, 'green': 2, 'red': 3}
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# with default value:
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d = {}
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for color in colors:
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d[color] = d.get(color, 0) + 1
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# or with a defaultdict:
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d = defaultdict(int)
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for color in colors:
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d[color] += 1
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# Grouping with dictionaries -- Part I
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names = ['raymond', 'rachel', 'matthew', 'roger',
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'betty', 'melissa', 'judith', 'charlie']
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d = {}
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for name in names:
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key = len(name)
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if key not in d:
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d[key] = []
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d[key].append(name)
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#{5: ['roger', 'betty'], 6: ['rachel', 'judith'],
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# 7: ['raymond', 'matthew', 'melissa', 'charlie']}
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# Grouping with dictionaries -- Part II
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# ok, but setdefault is sort of inelegant:
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d = {}
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for name in names:
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key = len(name)
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d.setdefault(key, []).append(name)
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# best:
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d = defaultdict(list)
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for name in names:
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key = len(name)
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d[key].append(name)
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# Is a dictionary popitem() atomic?
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d = {'matthew': 'blue', 'rachel': 'green', 'raymond':
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'red'}
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while d:
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key, value = d.popitem()
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print key, '-->', value
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# yes, threadsafe
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# Linking dictionaries
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defaults = {'color': 'red', 'user': 'guest'}
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parser = argparse.ArgumentParser()
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parser.add_argument('-u', '--user')
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parser.add_argument('-c', '--color')
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namespace = parser.parse_args([])
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command_line_args = {k:v for
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k, v in vars(namespace).items() if v}
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d = defaults.copy()
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d.update(os.environ)
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d.update(command_line_args)
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# faster, more memory-efficient:
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d = ChainMap(command_line_args, os.environ, defaults)
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# Improving Clarity
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# Clarify function calls with keyword arguments
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# confusing:
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twitter_search('@obama', False, 20, True)
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# clear:
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twitter_search('@obama', retweets=False, numtweets=20, popular=True)
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# Clarify multiple return values with named tuples
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doctest.testmod()
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# (0, 4) # confusing
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doctest.testmod()
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# TestResults(failed=0, attempted=4) # clear
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# create with:
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TestResults = namedtuple('TestResults', ['failed', 'attempted'])
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# is still tuple, interface works exactly the same
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# Unpacking sequences
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p = 'Raymond', 'Hettinger', 0x30, 'python@example.com'
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# ugly:
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fname = p[0]
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lname = p[1]
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age = p[2]
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email = p[3]
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# better:
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fname, lname, age, email = p
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# Updating multiple state variables
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def fibonacci(n):
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x = 0
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y = 1
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for i in range(n):
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print x
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t = y
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y = x + y
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x = t
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def fibonacci(n):
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x, y = 0, 1
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for i in range(n):
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print x
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x, y = y, x+y
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# Tuple packing and unpacking
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# given influence():
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# bad and easily bug-ridden:
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tmp_x = x + dx * t
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tmp_y = y + dy * t
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tmp_dx = influence(m, x, y, dx, dy, partial='x')
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tmp_dy = influence(m, x, y, dx, dy, partial='y')
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x = tmp_x
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y = tmp_y
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dx = tmp_dx
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dy = tmp_dy
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# good:
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x, y, dx, dy = (x + dx * t,
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y + dy * t,
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influence(m, x, y, dx, dy, partial='x'),
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influence(m, x, y, dx, dy, partial='y'))
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# Concatenating strings
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names = ['raymond', 'rachel', 'matthew', 'roger',
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'betty', 'melissa', 'judith', 'charlie']
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s = names[0]
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for name in names[1:]:
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s += ', ' + name
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print s
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print ', '.join(names)
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# Updating sequences
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names = ['raymond', 'rachel', 'matthew', 'roger',
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'betty', 'melissa', 'judith', 'charlie']
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# slow slow slow:
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del names[0]
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names.pop(0)
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names.insert(0, 'mark')
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# double-ended queue:
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names = deque(['raymond', 'rachel', 'matthew', 'roger',
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'betty', 'melissa', 'judith', 'charlie'])
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# much faster:
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del names[0]
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names.popleft()
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names.appendleft('mark')
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# Using decorators to factor-out administrative logic
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def web_lookup(url, saved={}):
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if url in saved:
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return saved[url]
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page = urllib.urlopen(url).read()
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saved[url] = page
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return page
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@cache
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def web_lookup(url):
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return urllib.urlopen(url).read()
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# Caching decorator
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def cache(func):
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saved = {}
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@wraps(func)
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def newfunc(*args):
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if args in saved:
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return newfunc(*args)
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result = func(*args)
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saved[args] = result
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return result
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return newfunc
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# Factor-out temporary contexts
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old_context = getcontext().copy()
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getcontext().prec = 50
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print Decimal(355) / Decimal(113)
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setcontext(old_context)
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# better:
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with localcontext(Context(prec=50)):
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print Decimal(355) / Decimal(113)
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# How to open and close files
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f = open('data.txt')
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try:
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data = f.read()
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finally:
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f.close()
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with open('data.txt') as f:
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data = f.read()
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# How to use locks
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# Make a lock
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lock = threading.Lock()
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# Old-way to use a lock
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lock.acquire()
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try:
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print 'Critical section 1'
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print 'Critical section 2'
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finally:
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lock.release()
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# New-way to use a lock
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with lock:
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print 'Critical section 1'
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print 'Critical section 2'
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# Factor-out temporary contexts
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try:
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os.remove('somefile.tmp')
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except OSError:
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pass
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# better:
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with ignored(OSError):
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os.remove('somefile.tmp')
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# Context manager: ignored()
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@contextmanager
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def ignored(*exceptions):
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try:
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yield
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except exceptions:
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pass
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# Factor-out temporary contexts
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with open('help.txt', 'w') as f:
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oldstdout = sys.stdout
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sys.stdout = f
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try:
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help(pow)
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finally:
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sys.stdout = oldstdout
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with open('help.txt', 'w') as f:
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with redirect_stdout(f):
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help(pow)
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# Context manager: redirect_stdout()
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@contextmanager
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def redirect_stdout(fileobj):
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oldstdout = sys.stdout
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sys.stdout = fileobj
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try:
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yield fieldobj
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finally:
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sys.stdout = oldstdout
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# List Comprehensions and Generator Expressions
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# old:
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result = []
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for i in range(10):
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s = i ** 2
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result.append(s)
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print sum(result)
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# better:
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print sum([i**2 for i in xrange(10)])
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# best:
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print sum(i**2 for i in xrange(10))
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