python_io.md

Input and Output

1. Output Formatting

• str.format(): control formatting of output

• str(): return representations of values, which are human-readable

• repr(): genertate representations, readable by the interpreter

• Many values have the same representation using either function. However, strings and float point numbers have distinct representations

   s = 'Hello, world.'
   str(s)
   repr(s)
  
   str(1.0/7.0)
   repr(1.0/7.0)
  
   # repr() of a string adds string quotes and backslashes
   hellos = repr('hello, world\n')
   print hellos

• str.rjust(): right justifies a string, in a field of given width, by padding it with spaces on the left.

  • Similar methods: str.ljust(), str.center()
  • If the input is too long, they don't truncate, return unchanged

   # creating a table of squares and cubes using rjust
   for x in range(1, 11):
   	print repr(x).rjust(2), repr(x*x).rjust(3),
   	print repr(x*x*x).rjust(4)

• str.zfill(): pads a numeric string on the left with zeros

   '12'.zfill(5)
   '-3.14'.zfill(7)
   '3.1415926'.zfill(5)

• str.format() usage: print 'We are the {} who say "{}!"'.format('knights', 'Ni')

  • format field, brackets and characters within them, replaced with objects passed into str.format()
    1. positional and keyword arguments are supported
    2. !s apply str(), !r apply repr(), convert the value before formatted
    3. : and format specifier (optional), can follow field name
       • [] reference the variables by name instead of by position
       • also could be done by passing table as keyword args **

   # positional arguments
   print '{0} and {1}'.format('spam', 'eggs'}
   print '{1} and {0}'.format('spam', 'eggs'}
  
   # keyword arguments
   print 'This {food} is {adjective}.'.format(food='spam', adjective='absolutely horrible')
  
   # positional args and keyword args can be combined
   print 'The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg')
  
   # creating a table of squares and cubes using format()
   for x in range(1, 11):
   	print '{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x)
  
   import math
   print 'The value of PI is approximately {}.'.format(math.pi)
   print 'The value of PI is approximately {!r}.'.format(math.pi)
  
  
   table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
   for name, phone in table.items():
   	print '{0:10} ==> {1:10d}'.format(name, phone)
  
   # reference by name 
   print ('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; Dcab: {0[Dcab]:d}'.format(table))
  
   # pass dict as keyword args
   print 'Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)

2. File IO

• open(filename, mode) returns a file object
  1. mode 'r', read only
  2. mode 'w', write, erase existing one
  3. mode 'a', append
  4. mode 'r+', read and write

2.1 File Objects Methods

• f.read(size), reads some quantity of data and returns it as a string.
• f.readline(), reads a single line from the file
• list(f), f.readlines(), read all the lines in a file

# read lines from a file
for line in f:
	print line,

• f.write('This is a test\n'), writes to the file

• Write something other than a string, convert to a string first

   value = ('the answer', 42)
   s = str(value)
   f.write(s)

• f.tell(), returns an integer giving the file object's current position

• f.seek(offset, from_what), position computed from adding offset to a reference point

  • from_what, 0: from beginning; 1: from current position; 2: end of the file

• f.close(), close the file and free up system resource

• Good Practice, use with keyword to deal with file objects

  • file is properly closed after its suite finishes
  • shorter than writing try-finally

   with open('workfile', 'r') as f:
   	read_data = f.read()
  
   f.closed

2.2 JSON

• Serialize, take Python data hierarchies, convert them to string representations.
• Deserialize, reconstruct data from string representation

json.dumps([1, 'simple', 'list])

# f is a file object, x is an object
json.dump(x, f)

# decode the object
obj = json.load(f)