(Some of the) Things I wish I knew before starting using Python for Data Science Miguel Cabrera mfcabrera@gmail.com

Background C/Java Experience Python at the University Mostly Numpy/Scikit-Learn Not Pythonic

From This

To This

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Integration Time You have to integrate your code into existing code base. You have to make your code maintainable and reusable. Sometimes your code deal with semi-structure and textual data.

The Things

Autovivification

One way Straight out of Wikipedia: 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 e f t r e e ( ) : r e t u r n d e f a u l t d i c t ( t r e e ) c o m m o n _ n a m e = t r e e ( ) c o m m o n _ n a m e [ ' M a m m a l i a ' ] [ ' P r i m a t e s ' ] [ ' H o m o ' ] [ ' H . s a p i e n s ' ] = ' h u m a n b e i n g ' r e t u r n c o m m o n _ n a m e d e f a u l t d i c t ( < f u n c t i o n t r e e a t 0 x 1 0 0 6 0 7 c 8 0 > , { ' M a m m a l i a ' : d e f a u l t d i c t ( < f u n c t i o n t r e e a t 0 x 1 0 0 6 0 7 c 8 0 > , { ' P r i m a t e s ' : d e f a u l t d i c t ( < f u n c t i o n t r e e a t 0 x 1 0 0 6 0 7 c 8 0 > , { ' H o m o ' : d e f a u l t d i c t ( < f u n c t i o n t r e e a t 0 x 1 0 0 6 0 7 c 8 0 > , { ' H . s a p i e n s ' : ' h u m a n b e i n g ' } ) } ) } ) } )

Another Way This on Stackoverflow shows an alternative (maybe clearer) way: question c l a s s V i v i d i c t ( d i c t ) : d e f _ _ m i s s i n g _ _ ( s e l f , k e y ) : v a l u e = s e l f [ k e y ] = t y p e ( s e l f ) ( ) r e t u r n v a l u e c o m m o n _ n a m e = V i v i d i c t ( ) c o m m o n _ n a m e [ ' M a m m a l i a ' ] [ ' P r i m a t e s ' ] [ ' H o m o ' ] [ ' H . s a p i e n s ' ] = ' h u m a n b e i n g ' r e t u r n c o m m o n _ n a m e Mammalia : (Primates : (Homo : (H. sapiens : human being)))

What for? We have this: id-1 a 20 10 id-2 a 50 2 id-1 b -1 -5 id-3 c 10 30 id-2 d -1 -2 And let's say we would like to end up with something like: { " i d - 1 " : { " a " : { " s c o r e _ 1 " : 2 0 , " s c o r e _ 2 " : 1 0 } } { " b " : { " s c o r e _ 1 " : - 1 , " s c o r e _ 2 " : - 5 } } }

With a ViviDict i m p o r t p p r i n t c l a s s V i v i d i c t ( d i c t ) : d e f _ _ m i s s i n g _ _ ( s e l f , k e y ) : v a l u e = s e l f [ k e y ] = t y p e ( s e l f ) ( ) r e t u r n v a l u e z o m b i e = V i v i d i c t ( ) f o r r o w i n t a b l e : z o m b i e [ r o w [ 0 ] ] [ r o w [ 1 ] ] [ ' s c o r e _ 1 ' ] = r o w [ 2 ] z o m b i e [ r o w [ 0 ] ] [ r o w [ 1 ] ] [ ' s c o r e _ 2 ' ] = r o w [ 3 ] p p r i n t . p p r i n t ( z o m b i e ) { ' i d - 1 ' : { ' a ' : { ' s c o r e _ 1 ' : 2 0 , ' s c o r e _ 2 ' : 1 0 } , ' b ' : { ' s c o r e _ 1 ' : - 1 , ' s c o r e _ 2 ' : - 5 } } , ' i d - 2 ' : { ' a ' : { ' s c o r e _ 1 ' : 5 0 , ' s c o r e _ 2 ' : 2 } , ' d ' : { ' s c o r e _ 1 ' : - 1 , ' s c o r e _ 2 ' : - 2 } } , ' i d - 3 ' : { ' c ' : { ' s c o r e _ 1 ' : 1 0 , ' s c o r e _ 2 ' : 3 0 } } }

Iterators and Iterables

What? source: http://nvie.com/posts/iterators-vs-generators/

Example: A Generator g e n e r a t o r = ( w o r d + ' ! ' f o r w o r d i n ' h i t m e b a b y o n e m o r e t i m e ' . s p l i t ( ) ) t r y : l e n ( g e n e r a t o r ) e x c e p t T y p e E r r o r : p r i n t ( " G e n e r a t o r s h a s n o l e n g t h ! " ) f o r w i n g e n e r a t o r : p r i n t w G e n e r a t o r s h a s n o l e n g t h ! h i t ! m e ! b a b y ! o n e ! m o r e ! t i m e !

What does it have to do with Data Science? Data Streaming through Lazy Evaluation Excellent discussion: http://rare-technologies.com/data-streaming-in-python-generators-iterators- iterables/

Something more useful c l a s s H d f s L i n e S e n t e n c e ( o b j e c t ) : d e f _ _ i t e r _ _ ( s e l f ) : s t r e a m = s e l f . s o u r c e . o p e n ( ' r ' ) f o r l i n e i n s t r e a m : c i d , s = l i n e . s p l i t ( ' \ t ' ) s = u " " . j o i n ( c o d e c s . d e c o d e ( w o r d , ' u t f - 8 ' , ' r e p l a c e ' ) f o r w o r d i n s . s p l i t ( ) ) s = s . s p l i t ( ) y i e l d s

NamedTuples

Why Many Python developers write code around the d i c t class or tuples You never know what to expect Code becomes hard to read From http://stackoverflow.com/questions/2970608/what-are-named-tuples-in-python p t 1 = ( 1 . 0 , 5 . 0 ) p t 2 = ( 2 . 5 , 1 . 5 ) f r o m m a t h i m p o r t s q r t l i n e _ l e n g t h = s q r t ( ( p t 1 [ 0 ] - p t 2 [ 0 ] ) * * 2 + ( p t 1 [ 1 ] - p t 2 [ 1 ] ) * * 2 )

Enter NamedTuples Named tuples assign meaning to each position in a tuple and allow for more readable, self-documenting code. They can be used wherever regular tuples are used, and they add the ability to access fields by name instead of position index. f r o m c o l l e c t i o n s i m p o r t n a m e d t u p l e P o i n t = n a m e d t u p l e ( ' P o i n t ' , ' x y ' ) p t 1 = P o i n t ( 1 . 0 , 5 . 0 ) p t 2 = P o i n t ( 2 . 5 , 1 . 5 ) f r o m m a t h i m p o r t s q r t l i n e _ l e n g t h = s q r t ( ( p t 1 . x - p t 2 . x ) * * 2 + ( p t 1 . y - p t 2 . y ) * * 2 )

NamedTuples provide cool methods Some of them: Name Description _ a s d i c t Return a new OrderedDict which maps field names to their values _ m a k e ( i t e r a b l e ) Class method that makes a new instance from an existing sequence or iterable.

You can extend a NamedTuple _ H o t e l B a s e = n a m e d t u p l e ( ' H o t e l D e s c r i p t o r ' , [ ' c l u s t e r _ i d ' , ' t r u s t _ s c o r e ' , ' r e v i e w s _ c o u n t ' , ' c a t e g o r y _ s c o r e s ' , ' i n t e n s i t y _ f a c t o r s ' ] , ) c l a s s H o t e l D e s c r i p t o r ( _ H o t e l B a s e ) : d e f c o m p u t e _ p r i o r ( s e l f ) : i f n o t s e l f . t r u s t _ s c o r e o r n o t s e l f . r e v i e w s _ c o u n t : r a i s e N o t E n o u g h D a t a F o r R a n k i n g ( " C a n n o t c o m p u t e p r i o r w i t h o u t t y s c o r e a n d r e v i e w s " ) r e t u r n _ c o m p u t e _ p r i o r ( s e l f . t r u s t _ s c o r e , s e l f . r e v i e w s _ c o u n t ) ( . . . )

Conclusion (Aspiring) Data Scientists / Engineers should learn: Standard library (i.e. the c o l l e c t i o n s module in particular) Iterables and Iterators Object oriented practices Documenting your code How to package Exposing your models (i.e. via an API)

Questions?

Created by Miguel Cabrera.