Lecture 4: Strings and documentation

Transcription

1 Lecture 4: Strings and documentation Introduction for Linguists (LT2102) Markus Forsberg SprΣakbanken University of Gothenburg September 17, 2010

2 FAQ: How do I get the highest grade on the course? You need a pass on the assignments. The exam decides you final grade. However, the assignments may influence your grade in the following ways: if you are close to pass or pass with distinction on the exam, then your work on the assignments may tip you one way or the other; if you work hard on the assignments, you shouldn t have a problem with the exam.

3 Comments Comments are used to inactivate code: # def function(): # print hello and, more importantly, to explain code: x = x+1 # increment x by 1 NO! x = x+1 # keep track of number of entries YES! num_of_entries = num_of_entries + 1 # self-documenting name

4 Module interface A Module interface gives an overview of what a module contains without giving any implementation details. # test.py def print_name(name): print name >>> import test >>> help(test)... >>> dir(test) [..., print_name ]

5 Adding documentation to the interface We use documentation strings to add documentation to the interface. A documentation string is just ordinary strings placed on specific places in the code. # test.py """This is a test module""" def print_name(name): """Doc-string for print_name""" print name >>help(test)

6 Escape characters We use escape characters to write special characters, such as newlines, and to cancel a character s formal meaning. >>> print "tab: \t newline: \n next line" tab: newline: next line >>> print "\" is an escaped double quote" " is an escaped double quotation >>> print \ is an escaped single quote is an escaped single quotation

7 Characters in a computer There are no characters in a computer, only numbers representing characters. Character encoding: what numbers are used to represent what characters. We have to know the character encoding of a text to be able to display a text properly.

8 ASCII table In the beginning was English (7 bits = 128 characters):

9 Extended ASCII table (latin1) And then they though of other languages close by and some symbols (8 bits = 256 characters):

10 Unicode And then they decided to go for every character. Every character gets a number. However, this mapping is abstract, not the actually character encoding.

11 Unicode character encodings Encodings to represent Unicode: UTF-8, UTF-16, UTF-32 UTF-8 is popular since are the same as ASCII (English text is valid). UTF-8 has started to replace Latin1 as the default encoding.

12 Unicode in Python Python has a special kind of strings called unicode strings. >> type(u this is a unicode string ) <type unicode > We use s.decode(encoding), to decode a string s encoded with ENCODING into a Python unicode string. We use s.encode(encoding) for doing the reverse.

13 Reading a file with Cyrillic characters >>> with open( cyrillic.txt ) as f:... s = f.read() >>> print s Письмо В июне 1935 года я на полгода вернулся в Англию с моего ранчо в Южной Америке. Жизнь... >>> print type(s), length:, len(s) <type str > length: 637 >>> print s.upper() # doesn t work Письмо В июне 1935 года я на полгода вернулся в Англию с моего ранчо в Южной Америке. Жизнь в Америке сложилась для нас непросто. Вместе со всеми мы страдали от последствий мирового кризиса. В Англии у меня был ряд дел, которые, как мне представлялось, требовали для своего разрешения моего личного присутствия. Моя жена осталась управлять нашим ранчо.

14 Reading a file with Cyrillic characters (cont.) >>> s_unicode = s.decode( UTF-8 ) >>> print type(s_unicode), length:, len(s_unicode) <type unicode > length: 353 >>> print s_unicode.upper() # works! ПИСЬМО В ИЮНЕ 1935 ГОДА Я НА ПОЛГОДА ВЕРНУЛСЯ В АНГЛИЮ С МОЕГО РАНЧО В ЮЖНОЙ АМЕРИКЕ. ЖИЗНЬ В АМЕРИКЕ СЛОЖИЛАСЬ ДЛЯ НАС НЕПРОСТО. ВМЕСТЕ СО ВСЕМИ МЫ СТРАДАЛИ ОТ ПОСЛЕДСТВИЙ МИРОВОГО КРИЗИСА. В АНГЛИИ У МЕНЯ БЫЛ РЯД ДЕЛ, КОТОРЫЕ, КАК МНЕ ПРЕДСТАВЛЯЛОСЬ, ТРЕБОВАЛИ ДЛЯ СВОЕГО РАЗРЕШЕНИЯ МОЕГО ЛИЧНОГО ПРИСУТСТВИЯ. МОЯ ЖЕНА ОСТАЛАСЬ УПРАВЛЯТЬ НАШИМ РАНЧО. >>> with open( cyrillic_upper.txt,mode= w ) as f:... f.write(s_unicode.upper())... Traceback (most recent call last): File "<stdin>", line 2, in <module> UnicodeEncodeError: ascii codec can t encode characters in position 0-5: ordinal not in range(128)

15 Reading a file with Cyrillic characters (cont.) >>> s_unicode_utf8 = s_unicode.upper().encode( UTF-8 ) >>> with open( cyrillic_upper.txt,mode= w ) as f:... f.write(s_unicode_utf8)... >>> $ cat rus_upper.txt ПИСЬМО В ИЮНЕ 1935 ГОДА Я НА ПОЛГОДА ВЕРНУЛСЯ В АНГЛИЮ С МОЕГО РАНЧО В ЮЖНОЙ АМЕРИКЕ. ЖИЗНЬ В АМЕРИКЕ СЛОЖИЛАСЬ ДЛЯ НАС НЕПРОСТО. ВМЕСТЕ СО ВСЕМИ МЫ СТРАДАЛИ ОТ ПОСЛЕДСТВИЙ МИРОВОГО КРИЗИСА. В АНГЛИИ У МЕНЯ БЫЛ РЯД ДЕЛ, КОТОРЫЕ, КАК МНЕ ПРЕДСТАВЛЯЛОСЬ, ТРЕБОВАЛИ ДЛЯ СВОЕГО РАЗРЕШЕНИЯ МОЕГО ЛИЧНОГО ПРИСУТСТВИЯ. МОЯ ЖЕНА ОСТАЛАСЬ УПРАВЛЯТЬ НАШИМ РАНЧО.

16 Reading and writing UTF-8 files def read_file_utf8(filename): with open(filename) as f: s = f.read() return s.decode( UTF-8 ) def write_file_utf8(filename,unicode_str): utf8_str = unicode_str.encode( UTF-8 ) with open(filename,mode= w ) as f: f.write(utf8_str)

17 codecs module import codecs def read_file_utf8(filename): with codecs.open(filename,encoding= utf-8 ) as f: s = f.read() return s def write_file_utf8(filename,unicode_str): with codecs.open(filename,mode= w,encoding= utf-8 ) as f: f.write(unicode_str)

18 String literals # unicode_ex.py alphabet_murders = u"письмо В июне 1935 года я на полгода вернулся\ в Англию с моего ранчо в Южной Америке." >>> import unicode_ex Traceback (most recent call last): File "<stdin>", line 1, in <module> File "unicode_ex.py", line 3 SyntaxError: Non-ASCII character \xd0 in file unicode_ex.py on line 3, but no encoding declared; see for details # unicode_ex.py # -*- coding: utf-8 -*- alphabet_murders = u"письмо В июне 1935 года я на полгода вернулся\ в Англию с моего ранчо в Южной Америке."

19 Solving a problem Top-down (divide-and-conquer) Start with the top function, and analyze what needs to be done by giving subtasks function names (without defining the functions first). Iteratively continue with the subtasks until you end up with trivial subtasks that you solve right away. Bottom-up Start with writing help functions that solves problems that you know must be solved in order to solve the whole problem. Use the help functions to build new functions until you have solved the whole problem.

20 What is a good solution? A problem of some complexity has a large number of solutions, so which one is to be preferred? Readability is extremely important. Scary example of the opposite: print filter(none,map(lambda y:y*reduce( lambda x,y:x*y!=0,map(lambda x,y=y:y%x, range(2,int(pow(y,0.5)+1))),1),range(2,1000))) Using the idioms of current programming language. Reasonable efficiency. Reusability.

21 Zen of Python >>> import this The Zen of Python, by Tim Peters Beautiful is better than ugly. Explicit is better than implicit. Simple is better than complex. Complex is better than complicated. Flat is better than nested. Sparse is better than dense. Readability counts. Special cases aren t special enough to break the rules. Although practicality beats purity. Errors should never pass silently. Unless explicitly silenced. In the face of ambiguity, refuse the temptation to guess. There should be one-- and preferably only one --obvious way to do it. Although that way may not be obvious at first unless you re Dutch. Now is better than never. Although never is often better than *right* now. If the implementation is hard to explain, it s a bad idea. If the implementation is easy to explain, it may be a good idea. Namespaces are one honking great idea -- let s do more of those!

22 Top-down: Downey, exercise 9.8 This question is based on a Puzzler that was broadcast on the radio program Car Talk: Give me a word with three consecutive double letters. I ll give you a couple of words that almost qualify, but don t. For example, the word committee, c-o-m-m-i-t-t-e-e. It would be great except for the i that sneaks in there. Or Mississippi: M-i-s-s-i-s-s-i-p-p-i. If you could take out those i s it would work. But there is a word that has three consecutive pairs of letters and to the best of my knowledge this may be the only word. Of course there are probably 500 more but I can only think of one. What is the word? Write a program to find it.

23 Solution def is_triple_duplicate(word): for i in range(0,len(word)-5): if (word[i] == word[i+1] and word[i+2] == word[i+3] and word[i+4] == word[i+5]): return True return False def exercise1(): with open( words.txt ) as f: for word in f: if is_triple_duplicate(word): print *, word,

24 Bottom-up: Downey, exercise 9.8 Here s another Car Talk Puzzler: I was driving on the highway the other day and I happened to notice my odometer. Like most odometers, it shows six digits, in whole miles only. So, if my car had 300,000 miles, for example, I d see Now, what I saw that day was very interesting. I noticed that the last 4 digits were palindromic; that is, they read the same forward as backward. For example, is a palindrome, so my odometer could have read One mile later, the last 5 numbers were palindromic. For example, it could have read One mile after that, the middle 4 out of 6 numbers were palindromic. And you ready for this? One mile later, all 6 were palindromic! The question is, what was on the odometer when I first looked? Write a Python program that tests all the six-digit numbers and prints any numbers that satisfy these requirements.

25 Solution def palindrome(s): return s == s[::-1] def string_of_numbers(n): return str(n).zfill(6) def exercise2(): for n in range(0, ): last_four = string_of_numbers(n)[2:6] last_five = string_of_numbers(n+1)[1:6] middle = string_of_numbers(n+2)[1:5] all_six = string_of_numbers(n+3) if (palindrome(last_four) and palindrome(last_five) and palindrome(middle) and palindrome(all_six)): print string_of_numbers(n) print evidence: 4:%s, 5:%s, m:%s, 6:%s % (last_four, last_five,middle,all_six)