BEAUTIFUL PYTHON IDIOMS PARIS.PY MEETUP - 2013/06/12 

WHO AM I? olivier hervieu software python developer at tinyclues my twitter (junk) account: @ _ o l i v i e r _ 

WHY THIS PRESENTATION? inspired by Raymond Hettinger's conference at Pycon inspired by Ned Batchelder's conference at Pycon 90 minutes of conferences in 10! 

PART 1: THE GOOD 

LOOPING OVER NUMBERS bad: even worse: better: f o r i i n r a n g e ( 6 ) : p r i n t i * * 2 f o r i i n [ 0 , 1 , 2 , 3 , 4 , 5 ] : p r i n t i * * 2 f o r i i n x r a n g e ( 6 ) : p r i n t i * * 2 

LOOPING OVER COLLECTIONS evil things: f o r i i n x r a n g e ( l e n ( c o l o r s ) ) : p r i n t c o l o r s [ i ] f o r i i n x r a n g e ( l e n ( c o l o r s ) - 1 , - 1 , - 1 ) : p r i n t c o l o r s [ i ] c o l o r s = [ ' b l u e ' , ' r e d ' , ' y e l l o w ' , ' p u r p l e ' ] f o r i i n x r a n g e ( l e n ( c o l o r s ) ) : p r i n t i , c o l o r s [ i ] 

No content

please consider (and use!) the following: f o r c o l o r i n c o l o r s : p r i n t c o l o r f o r c o l o r i n r e v e r s e d ( c o l o r s ) : p r i n t c o l o r c o l o r s = [ ' b l u e ' , ' r e d ' , ' y e l l o w ' , ' p u r p l e ' ] f o r i , c o l o r i n e n u m e r a t e ( c o l o r s ) : p r i n t i , c o l o r 

LOOPING OVER TWO (OR MORE) COLLECTIONS dumb version: and what you really want to do: the generator way: see also: i z i p _ l o n g e s t in i t e r t o o l s f i r s t n a m e s = [ ' A l i c e ' , ' B o b ' , ' J o h n ' ] l a s t n a m e s = [ ' D u p o n t ' , ' S l e i g h ' , ' D o e ' ] a g e s = [ 2 4 , 6 3 , 3 3 ] z i p p e d _ d a t a = [ ] n = m i n ( l e n ( f i r s t n a m e s ) , l e n ( l a s t n a m e s ) , l e n ( a g e s ) ) f o r i i n r a n g e ( n ) : z i p p e d _ d a t a . a p p e n d ( ( f i r s t n a m e s [ i ] , l a s t n a m e s [ i ] , a g e s [ i ] ) ) z i p ( f i r s t n a m e s , l a s t n a m e s , a g e s ) i z i p ( f i s t n a m e s , l a s t n a m e s , a g e s ) 

CREATING DICTIONARIES FROM PAIRS f i r s t n a m e s = [ ' A l i c e ' , ' B o b ' , ' J o h n ' ] a g e s = [ 2 4 , 6 3 , 3 3 ] d = d i c t ( i z i p ( f i r s t n a m e s , a g e s ) ) # { ' A l i c e ' : 2 4 , ' B o b ' : 6 3 , ' J o h n ' : 3 3 } d = d i c t ( e n u m e r a t e ( f i r s t n a m e s ) ) # { 0 : ' A l i c e ' , 1 : ' B o b ' , 2 : ' J o h n ' } 

MASTERING LIST/SET/DICT COMPREHENSION or is equivalent to: s u m = 0 f o r i i n x r a n g e ( 1 0 ) : s u m + = i * * 2 p r i n t s u m p o w _ 2 = [ ] f o r i i n x r a n g e ( 1 0 ) : p o w _ 2 . a p p e n d ( i * * 2 ) p r i n t s u m ( p o w _ 2 ) p r i n t s u m ( [ i * * 2 f o r i i n x r a n g e ( 1 0 ) ] ) p r i n t s u m ( i * * 2 f o r i i n x r a n g e ( 1 0 ) ) 

c o l o r s = [ ' b ' , ' r ' , ' g ' , ' g ' , ' r ' , ' r ' , ' r ' , N o n e ] u n i q u e _ c o l o r s = { c o l o r f o r c o l o r i n c o l o r s i f c o l o r } p o w _ 2 _ m a p = { k : k * * 2 f o r k i n x r a n g e ( 1 0 0 0 0 ) } 

READING FILES the < 2 . 6 way: you should use: f = o p e n ( ' d a t a . t x t ' ) t r y : d a t a = f . r e a d ( ) f i n a l l y : f . c l o s e ( ) w i t h o p e n ( ' d a t a . t x t ' ) a s f : d a t a = f . r e a d ( ) 

PART 2: THE BEAUTIFUL 

CONTEXT MANAGER probably my favorite use case scenario: avoiding the t r y / e x c e p t / p a s s pattern t r y : o s . r e m o v e ( ' s o m e f i l e . t m p ' ) e x c e p t O S E r r o r : p a s s f r o m c o n t e x t l i b i m p o r t c o n t e x t m a n a g e r @ c o n t e x t m a n a g e r d e f i g n o r e d ( * e x c e p t i o n s ) : t r y : y i e l d e x c e p t e x c e p t i o n s : p a s s w i t h i g n o r e d ( O S E r r o r ) : o s . r e m o v e ( ' s o m e f i l e . t m p ' ) 

I M P O R T I N F I N I T Y i t e r t o o l s . c o u n t ( ) i t e r t o o l s . c y c l e ( ) 

POWERFUL METHODS common methods come with handful parameters: t a l l _ b u i l d i n g s = { " E m p i r e S t a t e " : 3 8 1 , " S e a r s T o w e r " : 4 4 2 , " B u r j K h a l i f a " : 8 2 8 , " T a i p e i 1 0 1 " : 5 0 9 , } p r i n t m a x ( t a l l _ b u i l d i n g s . v a l u e s ( ) ) p r i n t m a x ( t a l l _ b u i l d i n g s . i t e m s ( ) , k e y = l a m b d a b : b [ 1 ] ) # ( ' B u r j K h a l i f a ' , 8 2 8 ) p r i n t m a x ( t a l l _ b u i l d i n g s , k e y = t a l l _ b u i l d i n g s . g e t ) # ' B u r j K h a l i f a ' 

c o l o r s = [ ' r e d ' , ' g r e e n ' , ' b l u e ' , ' y e l l o w ' ] f o r c o l o r i n s o r t e d ( c o l o r s ) : p r i n t c o l o r f o r c o l o r i n s o r t e d ( c o l o r s , r e v e r s e = T r u e ) : p r i n t c o l o r d e f c o m p a r e _ l e n g t h ( c 1 , c 2 ) : i f l e n ( c 1 ) < l e n ( c 2 ) : r e t u r n - 1 i f l e n ( c 1 ) > l e n ( c 2 ) : r e t u r n 1 r e t u r n 0 p r i n t s o r t e d ( c o l o r s , c m p = c o m p a r e _ l e n g t h ) p r i n t s o r t e d ( c o l o r s , k e y = l e n ) 

FOR/ELSE STATEMENTS use the e l s e statement to detect if a b r e a k has been encountered: d e f f i n d ( s e q , t a r g e t ) : f o u n d = F a l s e f o r i , v a l u e i n e n u m e r a t e ( s e q ) : i f v a l u e = = t g t : f o u n d = T r u e b r e a k i f n o t f o u n d : r e t u r n - 1 r e t u r n i d e f f i n d ( s e q , t a r g e t ) : f o r i , v a l u e i n e n u m e r a t e ( s e q ) : i f v a l u e = = t g t : b r e a k e l s e : r e t u r n - 1 r e t u r n i 

COUNTING WITH DICTIONNARIES (OR KNOW YOUR STANDARD LIBRARY) 

naive implementation: c o l o r s = [ ' r e d ' , ' g r e e n ' , ' r e d ' , ' b l u e ' , ' g r e e n ' , ' r e d ' ] d = { } f o r c o l o r i n c o l o r s : i f c o l o r n o t i n d : d [ c o l o r ] = 0 d [ c o l o r ] + = 1 

slightly better: d = { } f o r c o l o r i n c o l o r s : d [ c o l o r ] = d . g e t ( c o l o r , 0 ) + 1 

in limbo: f r o m c o l l e c t i o n s i m p o r t d e f a u l t d i c t d = d e f a u l t d i c t ( i n t ) f o r c o l o r i n c o l o r s : d [ c o l o r ] + = 1 

python guru: f r o m c o l l e c t i o n s i m p o r t C o u n t e r C o u n t e r ( c o l o r s ) 

GENERATORS d e f e v e n s ( s t r e a m ) : f o r n i n s t r e a m : i f n % 2 = = 0 : y i e l d n f o r n i n e v e n s ( x r a n g e ( 1 0 0 0 0 ) ) : d o _ s o m e t h i n g ( n ) 

NAMEDTUPLE c l a s s B o o k O r d e r ( o b j e c t ) d e f _ _ i n i t _ _ ( s e l f , n a m e , q u a n t i t y ) : s e l f . n a m e = n a m e s e l f . q u a n t i t y = q u a n t i t y f r o m c o l l e c t i o n i m p o r t n a m e d t u p l e B o o k O r d e r = n a m e d t u p l e ( ' B o o k O r d e r ' , [ ' n a m e ' , ' q u a n t i t y ' ] ) 

PART 3: THE AWESOME 

POWERFUL ITERATION CONTINUE A FUNCTION UNTIL A SENTINEL VALUE IS REACHED everybody write this: write this instead: b l o c k s = [ ] w h i l e T r u e : b l o c k = f . r e a d ( 3 2 ) i f b l o c k = = ' ' : b r e a k b l o c k s . a p p e n d ( b l o c k ) b l o c k s = [ ] f o r b l o c k i n i t e r ( p a r t i a l ( f . r e a d , 3 2 ) , ' ' ) : b l o c k s . a p p e n d ( b l o c k ) 

FIND ELEMENTS n e x t ( ( i f o r i i n x r a n g e ( 1 0 0 0 ) i f i > 1 0 a n d i % 2 ) , 0 ) 

ABSTRACTING ITERATION thanks to generators, simplify your code: f = o p e n ( " m y _ c o n f i g . i n i " ) f o r l i n e i n f : l i n e = l i n e . s t r i p ( ) i f l i n e . s t a r t s w i t h ( ' # ' ) : # A c o m m e n t l i n e , s k i p i t . c o n t i n u e i f n o t l i n e : # A b l a n k l i n e , s k i p i t . c o n t i n u e # A n i n t e r e s t i n g l i n e . d o _ s o m e t h i n g ( l i n e ) 

d e f i n t e r e s t i n g _ l i n e s ( f ) : f o r l i n e i n f : l i n e = l i n e . s t r i p ( ) i f l i n e . s t a r t s w i t h ( ' # ' ) : c o n t i n u e i f n o t l i n e : c o n t i n u e y i e l d l i n e w i t h o p e n ( " m y _ c o n f i g . i n i " ) a s f : f o r l i n e i n i n t e r e s t i n g _ l i n e s ( f ) : d o _ s o m e t h i n g ( l i n e ) 

USING DECORATORS the web cache example: powered re-usability with decorators! d e f w e b _ l o o k u p ( u r l , s a v e d = { } ) : i f u r l i n s a v e d : r e t u r n s a v e d [ u r l ] p a g e = u r l l i b . u r l o p e n ( u r l ) . r e a d ( ) s a v e d [ u r l ] = p a g e r e t u r n p a g e @ c a c h e d e f w e b _ l o o k u p ( u r l ) : r e t u r n u r l l i b . u r l o p e n ( u r l ) . r e a d ( ) d e f c a c h e ( f u n c ) : s a v e d = { } @ w r a p s ( f u n c ) d e f n e w f u n c ( * a r g s ) : i f a r g s i n s a v e d : r e t u r n n e w f u n c ( * a r g s ) r e s u l t = f u n c ( * a r g s ) s a v e d [ a r g s ] = r e s u l t r e t u r n r e s u l t r e t u r n n e w f u n c 

PART 4: GOODIES AND ADVICE 

EXPLICIT IS BETTER THAN IMPLICIT is better than: and: is better than: f i r s n a m e , l a s t n a m e , a g e = [ " J o h n " , " D o e " , 3 3 ] w h o a m i = [ " J o h n " , " D o e " , 3 3 ] f i r s t a n a m e = w h o m a i [ 0 ] l a s t n a m e = w h o a m i [ 1 ] a g e = w h o a m i [ 2 ] t w i t t e r _ s e a r c h ( ' @ o b a m a ' , r e t w e e t s = F a l s e , n u m t w e e t s = 2 0 , p o p u l a r = T r u e ) t w i t t e r _ s e a r c h ( ' @ o b a m a ' , F a l s e , 2 0 , T r u e ) 

UNLIMITED DEFAULT DICT i n f i n i t e _ d e f a u l t d i c t = l a m b d a : d e f a u l t d i c t ( i n f i n i t e _ d e f a u l t d i c t ) 

MATRIX TRANSPOSITION WITH ZIP a = [ [ 1 , 2 , 3 ] , [ 4 , 5 , 6 ] , [ 7 , 8 , 9 ] ] t r _ a = [ l i s t ( i ) f o r i i n z i p ( * a ) ] 

THANK YOU!