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comprehnesions, decorators

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`@@ -1,3 +1,54 @@`
	`1`	`+# Construct a dictionary from pairs`
	`2`	`+`
	`3`	`+# izip returns iterables`
	`4`	`+# xrange is better than range`
	`5`	`+# Loop over dict keys and values`
	`6`	`+`
	`7`	`+# creates many items in mem`
	`8`	`+fork,vind.items()`
	`9`	`+`
	`10`	`+# better in space`
	`11`	`+fork,vind.iteritems()`
	`12`	`+`
	`13`	`+names= []`
	`14`	`+colors= []`
	`15`	`+# faster than zip`
	`16`	`+d=dict(izip(names,colors))`
	`17`	`+`
	`18`	`+# counting with dicts`
	`19`	`+# ok:`
	`20`	`+d= {}`
	`21`	`+forcolorincolors`
	`22`	`+ifcolornotind:`
	`23`	`+d[color]=0`
	`24`	`+d[color]+=1`
	`25`	`+`
	`26`	`+# best:`
	`27`	`+forcolorincolors:`
	`28`	`+d[color]=d.get(color,0)+1`
	`29`	`+`
	`30`	`+# grouping with dicts`
	`31`	`+# ok:`
	`32`	`+names= ['fred','jim']`
	`33`	`+d= {}`
	`34`	`+fornameinnames:`
	`35`	`+key=len(name)`
	`36`	`+ifkeynotind:`
	`37`	`+d[key]= []`
	`38`	`+d[key].append(name)`
	`39`	`+`
	`40`	`+# better:`
	`41`	`+d= {}`
	`42`	`+fornameinnames:`
	`43`	`+key=len(name)`
	`44`	`+d.setdefault(key, []).append(name)`
	`45`	`+`
	`46`	`+# most best:`
	`47`	`+d=defaultdict(list)`
	`48`	`+fornameinnames:`
	`49`	`+key=len(name)`
	`50`	`+d[key].append(name)`
	`51`	`+`
`1`	`52`	`# to convert a list of tuples to a dictionary using the first value as the key.`
`2`	`53`	`# Looked like this (making a database of comic book files):`
`3`	`54`

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	`1`	`+# using decorators to factor-out administrative logic`
	`2`	`+defweb_lookup(url,saved={}):`
	`3`	`+ifurlinsaved:`
	`4`	`+returnsaved[url]`
	`5`	`+page=urllib.urlopen(url).read()`
	`6`	`+saved[url]=page`
	`7`	`+returnpage`
	`8`	`+`
	`9`	`+# better:`
	`10`	`+@cache# new one is lrucache`
	`11`	`+defweblookup(url):`
	`12`	`+returnurllib.urlopen(url).read()`
	`13`	`+`
	`14`	`+# caching decorator - your utils libraries should be full of these`
	`15`	`+defcache(func):`
	`16`	`+saved= {}`
	`17`	`+@wraps(func)`
	`18`	`+defnewfunc(*args):`
	`19`	`+ifargsinsaved:`
	`20`	`+returnnewfunc(*args)`
	`21`	`+result=func(*args)`
	`22`	`+saved[args]=result`
	`23`	`+returnresult`
	`24`	`+returnnewfunc`

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	`1`	`+matrix= [[1,2,3], [4,5,6]]`
	`2`	`+`
	`3`	`+# for loop`
	`4`	`+negative_matrix= []`
	`5`	`+forrowinmatrix:`
	`6`	`+negative_matrix.append([-nforninrow])`
	`7`	`+`
	`8`	`+# list comprehension`
	`9`	`+negative_evens_matrix= [`
	`10`	`+ [-nforninrow]# or negate(n)`
	`11`	`+forrowinmatrix`
	`12`	`+ifn%2==0`
	`13`	`+]`
	`14`	`+`
	`15`	`+fromitertoolsimportizip`
	`16`	`+list(zip(*matrix))# returns tuples`
	`17`	`+print([list(col)forcolinizip(*matrix)])`

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