[Python] Ch7. Input and Output

7.1 Formatted String Literals

• Formatted string literals f:

name = 'John'
print(f'Hello my name is {name}.')
# Hello my name is John.

print(f'The value of pi is approximately {math.pi:.3f}.')
# The value of pi is approximately 3.142.

• Convert values before formatting:
  • !a for ascii()
  • !s for str()
  • !r for repr()

print(f'My hovercraft is full of {animals!r}.')
# My hovercraft is full of 'eels'.

• Expand an expression using =:

name = 'John'
area = 'HK'
print(f'He is {name=} {area=}')
# He is name='John' area='HK'

7.2 String format() Method

• str.format():

name = 'John'
area = 'HK'
print('Hello my name is {0} and I am currently in {1}'.format(name, area))
# Hello my name is John and I am currently in HK.

print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg'))
# The story of Bill, Manfred, and Georg.

table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))
# Jack: 4098; Sjoerd: 4127; Dcab: 8637678

7.3 Old String Formatting

• Use modulo %:

name = 'John'
print('Hello my name is %s' % name)
# Hello my name is John.

7.4 Reading and Writing Files

• open(filename, mode, encoding=None) returns a file obejct:

  • 1st arg: filename
  • 2nd arg: mode to be used
    1. r for read
    2. w for write
    3. +r both read and write
  • 3rd arg: Default is encoding=utf-8

• with keyword:

  • Automatcially closes the file after its suite finishes, even though exception is raised.
  • Shorter than using try-finally blocks.
  • If not using with, use f.close() to close file and free up system resources.

7.5 Methods of File Objects

A trusting little leaf of green,
A bold audacious frost;
A rendezvous, a kiss or two,
And youth for ever lost.

1. f.read(size):

   • Reads data and return it as string or bytes object.
   • size is an optional argument.

   f = open('A Fallen Leaf.txt', 'r')
   
   print(f.read())
   
   f.close()
   
   # A trusting little leaf of green,
   # A bold audacious frost;
   # A rendezvous, a kiss or two,
   # And youth for ever lost.
   

2. f.readline():

   • Reads a single line from the file.

   f = open('A Fallen Leaf.txt', 'r')
   
   print(f.readline())
   
   f.close()
   
   # A trusting little leaf of green,
   

3. Use a for loop:

   • Memory efficient, fast, and more simple code.

   f = open('A Fallen Leaf.txt', 'r')
   
   for line in f:
       print(line, end='')
   
   f.close()
   
   # A trusting little leaf of green,
   # A bold audacious frost;
   # A rendezvous, a kiss or two,
   # And youth for ever lost.
   

4. f.readlines():

   • Stores all the lines to a list.

   f = open('A Fallen Leaf.txt', 'r')
   
   print(f.readlines())
   
   f.close()
   
   # ['A trusting little leaf of green,\n', 'A bold audacious frost;\n', 'A rendezvous, a kiss or two,\n', 'And youth for ever lost.']
   

5. f.writes(string):

   • Writes string to file and returns the number of characters written.
   • Need to convert to str or bytes object.

   f = open('A Fallen Leaf.txt', 'r+')
   
   print(f.write('Trees are good.')) # 15
   
   f.close()
   

7.6 JSON

We can use json to serialize and deserialize data.

• Serialize:
  • Take Python data hierarchies and convert them to JSON string representation.
• Deserialize:
  • Reconstruct data from JSON string representation.

import json
x = [1, 'simple', 'list']
json.dumps(x)
# '[1, "simple", "list"]'