Python笔记——file operation basics

• Note the difference between spllit() method and splitlines() method. The end-of-line character is normally always \n within a Python scripts.

>>> line = 'aaa\nbbb\nccc\n'
>>> line.split('\n')
['aaa', 'bbb', 'ccc', '']
>>> line.splitlines()
['aaa', 'bbb', 'ccc']

• String methods include calls for searching and replacing:

  >>> mystr = 'xxxSPAMxxx'
  >>> mystr.find('SPAM')   # return first offset
  3
  >>> mystr = 'xxaaxxaa'
  >>> mystr.replace('aa', 'SPAM') # global replacement
  'xxSPAMxxSPAM'
  

  This find call returns the offset of the first occurence of a substring, and replace does global search and replacement.

• The in membership operator can often be used as an alternative to find if all we need is a yes/no answer.

  >>> mystr = 'xxxSPAMxxx'
  >>> 'SPAM' in mystr
  True
  

• String methods also provide functions that are useful for thing such as case conversions, and a standard library module named string defines some usefule preset variables, among other things.

  >>> mystr = "SHRUBBERY"
  >>> mystr.lower() # case converters
  'shrubbery'
  >>> mystr.isalpha() # content tests
  True
  >>> mystr.isdigit()
  False
  >>> import string # case presets: for 'in', etc
  >>> string.ascii_lowercase
  'abcdefghijklmnopqrstuvwxyz'
  >>> string.whitespace
  ' \t\n\r\x0b\x0c'
  

• Methods for splitting up strings around a substring delimiter and putting them back together with a substring in between.

  >>> mystr = 'aaa,bbb,ccc'
  >>> mystr.split(',')        # split into substrings list
  ['aaa', 'bbb', 'ccc']
  >>> mystr = 'a b\nc\nd'
  >>> mystr.split()           # default delimiter: whitespace
  ['a', 'b', 'c', 'd']
  >>> delim = 'NI'
  >>> delim.join(['aaa', 'bbb', 'ccc'])   # join substrings list
  'aaaNIbbbNIccc'
  >>> ' '.join(['a', 'dead', 'parrot'])   # add a space between
  'a dead parrot'
  >>> chars = list('Lorreta')      # convert to characters list
  >>> chars
  ['L', 'o', 'r', 'r', 'e', 't', 'a']
  >>> chars.append('!')
  >>> chars
  ['L', 'o', 'r', 'r', 'e', 't', 'a', '!']
  >>> ''.join(chars)       # to string: empty delimiter
  'Lorreta!'
  

• Keep in mind that Python does not automatically convert strings to numbers, or vice versa.

  >>> int("42"), eval("42") # string to int conversions
  (42, 42)
  >>> str(42),repr(42) # into to string conversions
  ('42', '42')
  

• File Operation Basics

  open('file').read()       # read entire file into string
  open('file').read(N)  # read next N bytes into string
  open('file').readlines()  # read entire file into line strings list
  open('file').readline()       # read next line, through '\n'
  
  file = open('spam.txt', 'w')      # create file spam.txt
  file.write(('spam' * 5) + '\n')       # write text: return #characters written
  file.close()
  

• Using programs in two ways: as a script or as a library.