Python Tutorial 学习笔记5 --IO

There are two ways to format your output; the first way is to do all the string handling yourself; using string slicing and concatenation operations you can create any layout you can
imagine. The string types have some methods that perform useful operations for padding strings to a given column width; these will be discussed shortly. The second way is to use the 

str.format()

 method.

One question remains, of course: how do you convert values to strings? Luckily, Python has ways to convert any value to a string:
pass it to the

repr()

 or 

str()

 functions. The 

str()

 function
is meant to return representations of values which are fairly human-readable, while 

repr()

 is
meant to generate representations which can be read by the interpreter.

例1：



例2：the 

str.rjust()

 method
of string objects, which right-justifies a string in a field of a given width by padding it with spaces on the left.There are similar methods 

str.ljust()

 and 

str.center()

.



例3：There is another method, 

str.zfill()

,
which pads a numeric string on the left with zeros.



例4：The brackets and characters within them (called format fields) are replaced with the objects
passed into the 

str.format()

 method.
A number in the brackets refers to the position of the object passed into the

str.format()

 method.



例5：

'!s'

 (apply 

str()

)
and 

'!r'

 (apply 

repr()

)
can be used to convert the value before it is formatted.



例6：Passing an integer after the 

':'

 will
cause that field to be a minimum number of characters wide. This is useful for making tables pretty.



NOTES：This could also be done by passing the table as keyword arguments with the ‘**’ notation.

例7：Old string formatting. The 

%

 operator
can also be used for string formatting.



例8：

open()

 returns
a file object, and is most commonly used with two arguments:

open(filename, mode)

.











例9：It is good practice to use the 

with

 keyword
when dealing with file objects.It is also much shorter than writing equivalent 

try

-

finally

 blocks.

 


例10：data exchange with json



例11：If f is a file object opened for writing, we can use dumps() serializes the object to a file.