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Advanced Python Idioms

In this lecture on advanced Python idioms, we delve into sequences, iteration, and iterables, exploring comprehensions and functional programming techniques. We examine the characteristics of iterables, illustrating concepts with practical examples, such as using zip, enumerate, and built-in functions like map and filter. The lecture includes comprehensive exercises focused on applying these techniques, emphasizing the creation and manipulation of lists, sets, and dictionaries. Ideal for those seeking to deepen their understanding of Python's capabilities in data processing and representation.

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Advanced Python Idioms

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  1. Advanced Python Idioms BCHB5242013Lecture 9 BCHB524 - 2013 - Edwards

  2. Outline • Sequences, iteration, and iterables • Comprehensions • Functional Programming • Exercises BCHB524 - 2013 - Edwards

  3. Sequences and Iterables • Anything that supports: • Iterables we know about: • Strings, lists, sets, tuples • Dictionaries (keys(), values(), items()) • Files (line by line) • Iterable → list, Iterable → set, etc. • Pair iterable → dictionary! BCHB524 - 2013 - Edwards

  4. Iterables # sequences are iterable -> build listaList = list('abcdefabcdef')print"String abcdefabcdef as a list:\n  ",aList# sequences are iterable -> build set (no duplicates)aSet = set('abcdefabcdef')print"String abcdefabcdef as a set:\n  ",aSet# Two iterables can be paired up using the zip functionkeys = [1,2,3,4,5]values = 'abcde'aListOfPairs = zip(keys,values)print"A list of pairs:\n  ",aListOfPairs# list of pairs are iterable -> build dictaDict = dict(aListOfPairs)print"A dictionary from a list of pairs\n  ",aDict BCHB524 - 2013 - Edwards

  5. Special Iterable Functions • zip • merges two or more iterables • enumerate • iterates over (index of item, item) • sum, max, min, all, any • single value from many • map, filter • Applies function or test to each element • sorted, reversed • provides the items in sorted or reversed order BCHB524 - 2013 - Edwards

  6. Enumerate # sequences are iterable -> build listaList = list('abcdefabcdef')# enumerate get index with itemfor i,c inenumerate(aList):print i,c# exactly equivalent toi = 0for c in aList:print i,c    i += 1 BCHB524 - 2013 - Edwards

  7. Map # Numbers in a string... split into a listnumbers = '1,2,3,4,5,6'number_list = numbers.split(',')# Print the list, and manipulate its values!print number_listnumber_list[0] += 1# Fix the problem, apply the int function to each itemnumber_list = map(int,number_list)# Print the new list and check we can manipulate its values...print number_listnumber_list[0] += 1print number_list# Now to print it back outprint",".join(number_list)# Fix the problem, apply the str function to each itemnumber_list = map(str,number_list)# Print the new list and check that we can do a joinprint number_listprint",".join(number_list) BCHB524 - 2013 - Edwards

  8. Sorted, Reversed # Make a listaList = list('abcdefabcdef')print"aList:\n  ",aList# print the list sorted and reversed...print"Sorted:\n  ",sorted(aList)print"Reversed:\n  ",reversed(aList)print"Reversed in list:\n  ",list(reversed(aList)) BCHB524 - 2013 - Edwards

  9. Comprehensions • Comprehensions build lists using iteration[ expr for item in iterable ] # Make the times two table..timesTwo = [ 2*x for x inrange(10) ]# Make it another waytimesTwoToo = []for x inrange(10):    timesTwoToo.append(2*x)print"timesTwo:\n  ",timesTwoprint"timesTwoToo:\n  ",timesTwoToo BCHB524 - 2013 - Edwards

  10. Functional Programming • Python lets us treat functions as a basic immutable data-type. • We can’t change them after they are defined, but we can pass them in to functions. deff(x):return 2*xg = fprint f(1),g(2),f(3),g(4) BCHB524 - 2013 - Edwards

  11. sorted, revisited • The sorted function permits a keyword parameter: key • key is a function which returns the sort value # Initialize a new listaList = [1,-2,3,-4,5,-6]# Print the list as is and sortedprint"aList:\n  ",aListprint"Sorted:\n  ",sorted(aList)# Define absolute value sort keydefabsSortKey(x):returnabs(x)# Define negative value sort keydefnegSortKey(x):return -x# Demonstrate alternative sorting keysprint"Sorted by absolute value:\n  ",sorted(aList,key=absSortKey)print"Sorted by negative value:\n  ",sorted(aList,key=negSortKey) BCHB524 - 2013 - Edwards

  12. Lambda functions • Sometimes, the functions are so simple, we don’t want to define them formally. • Usually used with sorted… • Useful key functions: • Case insensitive:sorted(words,key=lambda s: s.lower()) • By dictionary value:sorted(dict.items(),key=lambda p: p[1]) • Also useful for map:map(lambda x: 2*x, (1,2,3,4,5,6)) BCHB524 - 2013 - Edwards

  13. Exercises • Write a reverse complement function (and package it up as a program) as compactly as possible, using the techniques introduced today. • Hint: Use a dictionary for complement, map to apply the get method, reversed, and join. • Write a program to compute and output the frequency of each nucleotide in a DNA sequence using a dictionary (see lec. 8). • Output the frequencies in most-occurrences to least-occurrences order. BCHB524 - 2013 - Edwards

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