>>> s = 'Hello, world.' 
   >>> str(s) 
   'Hello, world.' 
   >>> repr(s) 
   "'Hello, world.'" 
   >>> str(1/7) 
   '0.14285714285714285' 
   >>> x = 10 * 3.25 
   >>> y = 200 * 200 
   >>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...' 
   >>> print(s) 
   The value of x is 32.5, and y is 40000... 
   >>> # The repr() of a string adds string quotes and backslashes: 
   ... hello = 'hello, world\n' 
   >>> hellos = repr(hello) 
   >>> print(hellos) 
   'hello, world\n' 
   >>> # The argument to repr() may be any Python object: 
   ... repr((x, y, ('spam', 'eggs'))) 
   "(32.5, 40000, ('spam', 'eggs'))" 
 

7.1.1. Formatted String Literals

7.1.1. 格式化字符串文字

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

   >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678} 
   >>> for name, phone in table.items(): 
   ...     print(f'{name:10} ==> {phone:10d}') 
   ... 
   Sjoerd     ==>       4127 
   Jack       ==>       4098 
   Dcab       ==>       7678 
 

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

7.1.2. The String format() Method

7.1.2. 字符串的 format() 方法

   >>> print('We are the {} who say "{}!"'.format('knights', 'Ni')) 
   We are the knights who say "Ni!" 
 

   >>> print('{0} and {1}'.format('spam', 'eggs')) 
   spam and eggs 
   >>> print('{1} and {0}'.format('spam', 'eggs')) 
   eggs and spam 
 

   >>> print('This {food} is {adjective}.'.format( 
   ...       food='spam', adjective='absolutely horrible')) 
   This spam is absolutely horrible. 
 

   >>> 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: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; ' 
   ...       'Dcab: {0[Dcab]:d}'.format(table)) 
   Jack: 4098; Sjoerd: 4127; Dcab: 8637678 
 

   >>> 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 
 

   >>> for x in range(1, 11): 
   ...     print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x)) 
   ... 
    1   1    1 
    2   4    8 
    3   9   27 
    4  16   64 
    5  25  125 
    6  36  216 
    7  49  343 
    8  64  512 
    9  81  729 
   10 100 1000 
 

7.1.3. Manual String Formatting

7.1.3. 手动格式化字符串

   >>> for x in range(1, 11): 
   ...     print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ') 
   ...     # Note use of 'end' on previous line 
   ...     print(repr(x*x*x).rjust(4)) 
   ... 
    1   1    1 
    2   4    8 
    3   9   27 
    4  16   64 
    5  25  125 
    6  36  216 
    7  49  343 
    8  64  512 
    9  81  729 
   10 100 1000 
 

   >>> '12'.zfill(5) 
   '00012' 
   >>> '-3.14'.zfill(7) 
   '-003.14' 
   >>> '3.14159265359'.zfill(5) 
   '3.14159265359' 
 
 

7.1.4. Old string formatting

7.1.4. 旧的字符串格式化方法

   >>> import math 
   >>> print('The value of pi is approximately %5.3f.' % math.pi) 
   The value of pi is approximately 3.142. 
 

7.2. Reading and Writing Files

7.2. 读写文件

   >>> f = open('workfile', 'w') 
 

   >>> with open('workfile') as f: 
   ...     read_data = f.read() 
 
   >>> # We can check that the file has been automatically closed. 
   >>> f.closed 
   True 
 

   >>> f.close() 
   >>> f.read() 
   Traceback (most recent call last): 
     File "", line 1, in  
   ValueError: I/O operation on closed file. 
 
 

7.2.1. Methods of File Objects

7.2.1. 文件对象的方法

   >>> f.read() 
   'This is the entire file.\n' 
   >>> f.read() 
   '' 
 

   >>> f.readline() 
   'This is the first line of the file.\n' 
   >>> f.readline() 
   'Second line of the file\n' 
   >>> f.readline() 
   '' 
 

   >>> for line in f: 
   ...     print(line, end='') 
   ... 
   This is the first line of the file. 
   Second line of the file 
 

   >>> f.write('This is a test\n') 
   15 
 

   >>> value = ('the answer', 42) 
   >>> s = str(value)  # convert the tuple to string 
   >>> f.write(s) 
   18 
 

   >>> f = open('workfile', 'rb+') 
   >>> f.write(b'0123456789abcdef') 
   16 
   >>> f.seek(5)      # Go to the 6th byte in the file 
   5 
   >>> f.read(1) 
   b'5' 
   >>> f.seek(-3, 2)  # Go to the 3rd byte before the end 
   13 
   >>> f.read(1) 
   b'd' 
 

7.2.2. Saving structured data with "json"

7.2.2. 使用 "json" 保存结构化数据

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

   json.dump(x, f) 
 

   x = json.load(f)