strings.txt
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Strings - literals and formatting

 Strings are values that represent text: 'Hello, world!'

 'This string contains "quotes"'

 "This string also contains 'quotes'"  # single or double quotes both work

 """
 Triple quotes delimit block quotes that can contain linebreaks
 and 'quotes' and "quotes" - often used for docstrings 
 """

 'Special characters are escaped with backslash\nLike newline, for example'

 String formatting expressions often appear in print statements, can go anywhere
  format string % expressions  # evaluate expressions, put results in format string
  '%s %s\n' % (23, 'Skidoo')   # format string with % placeholders, then values
  '%s %s\n' % (x, s)           # can be used to format variables at runtime
  '%6.2f%8.3f' % (42,42)       # can do C-style numeric format with columns etc.

 Block quotes with format make a simple templating engine:
  Create whole page with % placeholders, then % (...) provides values to fill in



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String type

 str is a collection type - values have parts that can be individually accessed

 string parts are characters - not a separate type, just strings of length 1

 str is a sequence type - access characters by integer indices s[0], s[1], ...

 Zero-indexed, s[0] is first.  Can index backward from end, s[-1] is last etc.

 Slice notation for segments of a sequence: s[i:j] i inclusive, j exclusive

 Slice shortcuts: s[:n] means s[0:n], first n, s[-n:] means last n

 sequences are iterables, for ... iterates over each element in turn

   for c in s:
     ... c ...      # loop body processes each chacter in turn

 This is NOT necessary:    If you need the index also, use enumerate:

   for i in len(s):           for i, c in enumerate(s): # multiple assignment
     ... s[i] ...               ... i ... c ...
 

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Strings are immutable

 Immutable means values cannot be changed

   c    = s[i]  # OK - can read a character from a string

   s[i] = c     # ERROR - can NOT set a character in a string!

 We always have to create a new string.  String operators return new values.

   s  = 'Hello '     # What is s now?

   s + 'world'       # Now?
      
   t  =  s + 'world' # Assign t to capture the results of the expression

   s += 'world'      # What is s now?   What happened?  
 





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String operators and library

 s, t, n = 'supercalifragilistic', 'expealidocious', 3  # sample data

 s + t, s * n  # concatentation, replication

 s == t        # test equality (not identity)

 s < t, s <= t, s > t  # test order (alphabetic)

 len(s) # length

 'ist' in s  # test substring occurence

 s.find('ist')   # search for substring, return index or -1 if not found

 s.find('ist', n)  # search for substring starting at index n, etc.

 s.split('i') # break at 'i', return list of strings (often break at ' ' or '\n')

 'i'.join(ss) # join list of strings into one with separator char (often ' ', '\n')

 ... and many, many more ...


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Regular expressions

  Regular expressions are a mini-language for describing string patterns
   Can search or match patterns that have many possible matches, not just one
   Complicated, use only when the Python string library is not powerful enough
   
  Motivation: I'd like to search for any....
    IP address - 128.95.181.12  10.0.0.1  etc.
    email address - jon@uw.edu  jon@u.washington.edu etc.
    URL - http://staff.washington.edu/jon/index.html  file:///Users/jon/index.html

  A regular expression is an ordinary Python string that contains metacharacters:
   a  etc. matches only itself (most characters are not metacharacters)
   .  match any character
   [abc] match any one of these characters (defines a character class)
   * match zero or more more repetitions of the previous character class
   ?, +  match zero or one, one or more repetitions 
   {n,m} match at least n but at most m repetitions of the previous class
   .. etc ...

 Use Python re library functions to match sample text against the regular expr.

 Demo: Python re library with BioPython






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Unicode

 Ordinary Python 2.x str type can only contain 8-bit bytes (ASCII, English ...)

 Cannot represent foreign alphabets, accents or other diacritical marks, math, ...

 Unicode - "universal code" - can represent all this and more

 Python 2.6 does support Unicode strings with unicode type, u prefix: u'....' 
   u'...' strings can include bytes encoded like this: u'...\x...\x..'

 It's a long story -  see the HOW-TO at python.org, other refs.

 Python 3 str type is like Python 2.x unicode type, encode/decode is streamlined