7. Input and Output
7. Input and Output
There are several ways to present the output of a program; data can be printed in a human-readable form, or written to a file for future use. This chapter will discuss some of the possibilities.
7.1. Fancier Output Formatting
So far we’ve encountered two ways of writing values: expression statements and the print() function. (A third way is using the write() method of file objects; the standard output file can be referenced as sys.stdout. See the Library Reference for more information on this.)
Often you’ll want more control over the formatting of your output than simply printing space-separated values. 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 type has 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.
The string module contains a Template class which offers yet another way to substitute values into strings.
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 (or will force a SyntaxError if there is no equivalent syntax). For objects which don’t have a particular representation for human consumption, str() will return the same value as repr(). Many values, such as numbers or structures like lists and dictionaries, have the same representation using either function. Strings, in particular, have two distinct representations.
Some examples:
>>> 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'))"
Here are two ways to write a table of squares and cubes:
>>> 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 >>> 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
(Note that in the first example, one space between each column was added by the way print() works: it always adds spaces between its arguments.)
This example demonstrates 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(). These methods do not write anything, they just return a new string. If the input string is too long, they don’t truncate it, but return it unchanged; this will mess up your column lay-out but that’s usually better than the alternative, which would be lying about a value. (If you really want truncation you can always add a slice operation, as in x.ljust(n)[:n].)
There is another method, str.zfill(), which pads a numeric string on the left with zeros. It understands about plus and minus signs:
>>> '12'.zfill(5)
'00012'
>>> '-3.14'.zfill(7)
'-003.14'
>>> '3.14159265359'.zfill(5)
'3.14159265359'
Basic usage of the str.format() method looks like this:
>>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))
We are the knights who say "Ni!"
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 can be used to refer to the position of the object passed into the str.format() method.
>>> print('{0} and {1}'.format('spam', 'eggs'))
spam and eggs
>>> print('{1} and {0}'.format('spam', 'eggs'))
eggs and spam
If keyword arguments are used in the str.format() method, their values are referred to by using the name of the argument.
>>> print('This {food} is {adjective}.'.format(
... food='spam', adjective='absolutely horrible'))
This spam is absolutely horrible.
Positional and keyword arguments can be arbitrarily combined:
>>> print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',
other='Georg'))
The story of Bill, Manfred, and Georg.
'!a' (apply ascii()), '!s' (apply str()) and '!r' (apply repr()) can be used to convert the value before it is formatted:
>>> import math
>>> print('The value of PI is approximately {}.'.format(math.pi))
The value of PI is approximately 3.14159265359.
>>> print('The value of PI is approximately {!r}.'.format(math.pi))
The value of PI is approximately 3.141592653589793.
An optional ':' and format specifier can follow the field name. This allows greater control over how the value is formatted. The following example rounds Pi to three places after the decimal.
>>> import math
>>> print('The value of PI is approximately {0:.3f}.'.format(math.pi))
The value of PI is approximately 3.142.
Passing an integer after the ':' will cause that field to be a minimum number of characters wide. This is useful for making tables pretty.
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
... print('{0:10} ==> {1:10d}'.format(name, phone))
...
Jack ==> 4098
Dcab ==> 7678
Sjoerd ==> 4127
If you have a really long format string that you don’t want to split up, it would be nice if you could reference the variables to be formatted by name instead of by position. This can be done by simply passing the dict and using square brackets '[]' to access the keys
>>> 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
This could also be done by passing the table as keyword arguments with the ‘**’ notation.
>>> 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
This is particularly useful in combination with the built-in function vars(), which returns a dictionary containing all local variables.
For a complete overview of string formatting with str.format(), see Format String Syntax.
7.1.1. Old string formatting
The % operator can also be used for string formatting. It interprets the left argument much like a sprintf()-style format string to be applied to the right argument, and returns the string resulting from this formatting operation. For example:
>>> import math
>>> print('The value of PI is approximately %5.3f.' % math.pi)
The value of PI is approximately 3.142.
More information can be found in the printf-style String Formatting section.
7.2. Reading and Writing Files
open() returns a file object, and is most commonly used with two arguments: open(filename, mode).
>>> f = open('workfile', 'w')
The first argument is a string containing the filename. The second argument is another string containing a few characters describing the way in which the file will be used. mode can be 'r' when the file will only be read, 'w' for only writing (an existing file with the same name will be erased), and 'a' opens the file for appending; any data written to the file is automatically added to the end. 'r+' opens the file for both reading and writing. The mode argument is optional; 'r' will be assumed if it’s omitted.
Normally, files are opened in text mode, that means, you read and write strings from and to the file, which are encoded in a specific encoding (the default being UTF-8). 'b' appended to the mode opens the file in binary mode: now the data is read and written in the form of bytes objects. This mode should be used for all files that don’t contain text.
In text mode, the default when reading is to convert platform-specific line endings (\n on Unix, \r\n on Windows) to just \n. When writing in text mode, the default is to convert occurrences of \n back to platform-specific line endings. This behind-the-scenes modification to file data is fine for text files, but will corrupt binary data like that in JPEG or EXE files. Be very careful to use binary mode when reading and writing such files.
7.2.1. Methods of File Objects
The rest of the examples in this section will assume that a file object called f has already been created.
To read a file’s contents, call f.read(size), which reads some quantity of data and returns it as a string or bytes object. size is an optional numeric argument. When size is omitted or negative, the entire contents of the file will be read and returned; it’s your problem if the file is twice as large as your machine’s memory. Otherwise, at most size bytes are read and returned. If the end of the file has been reached, f.read() will return an empty string ('').
>>> f.read()
'This is the entire file.\n'
>>> f.read()
''
f.readline() reads a single line from the file; a newline character (\n) is left at the end of the string, and is only omitted on the last line of the file if the file doesn’t end in a newline. This makes the return value unambiguous; if f.readline() returns an empty string, the end of the file has been reached, while a blank line is represented by '\n', a string containing only a single newline.
>>> f.readline()
'This is the first line of the file.\n'
>>> f.readline()
'Second line of the file\n'
>>> f.readline()
''
For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:
>>> for line in f:
... print(line, end='')
...
This is the first line of the file.
Second line of the file
If you want to read all the lines of a file in a list you can also use list(f) or f.readlines().
f.write(string) writes the contents of string to the file, returning the number of characters written.
>>> f.write('This is a test\n')
15
To write something other than a string, it needs to be converted to a string first:
>>> value = ('the answer', 42)
>>> s = str(value)
>>> f.write(s)
18
f.tell() returns an integer giving the file object’s current position in the file represented as number of bytes from the beginning of the file when in binary mode and an opaque number when in text mode.
To change the file object’s position, use f.seek(offset, from_what). The position is computed from adding offset to a reference point; the reference point is selected by the from_what argument. A from_what value of 0 measures from the beginning of the file, 1 uses the current file position, and 2 uses the end of the file as the reference point. from_what can be omitted and defaults to 0, using the beginning of the file as the reference point.
>>> 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'
In text files (those opened without a b in the mode string), only seeks relative to the beginning of the file are allowed (the exception being seeking to the very file end with seek(0, 2)) and the only valid offset values are those returned from the f.tell(), or zero. Any other offset value produces undefined behaviour.
When you’re done with a file, call f.close() to close it and free up any system resources taken up by the open file. After calling f.close(), attempts to use the file object will automatically fail.
>>> f.close()
>>> f.read()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ValueError: I/O operation on closed file
It is good practice to use the with keyword when dealing with file objects. This has the advantage that the file is properly closed after its suite finishes, even if an exception is raised on the way. It is also much shorter than writing equivalent try-finally blocks:
>>> with open('workfile', 'r') as f:
... read_data = f.read()
>>> f.closed
True
File objects have some additional methods, such as isatty() and truncate() which are less frequently used; consult the Library Reference for a complete guide to file objects.
7.2.2. Saving structured data with json
Strings can easily be written to and read from a file. Numbers take a bit more effort, since the read() method only returns strings, which will have to be passed to a function like int(), which takes a string like '123' and returns its numeric value 123. When you want to save more complex data types like nested lists and dictionaries, parsing and serializing by hand becomes complicated.
Rather than having users constantly writing and debugging code to save complicated data types to files, Python allows you to use the popular data interchange format called JSON (JavaScript Object Notation). The standard module called json can take Python data hierarchies, and convert them to string representations; this process is called serializing. Reconstructing the data from the string representation is called deserializing. Between serializing and deserializing, the string representing the object may have been stored in a file or data, or sent over a network connection to some distant machine.
Note
The JSON format is commonly used by modern applications to allow for data exchange. Many programmers are already familiar with it, which makes it a good choice for interoperability.
If you have an object x, you can view its JSON string representation with a simple line of code:
>>> json.dumps([1, 'simple', 'list'])
'[1, "simple", "list"]'
Another variant of the dumps() function, called dump(), simply serializes the object to a text file. So if f is a text file object opened for writing, we can do this:
json.dump(x, f)
To decode the object again, if f is a text file object which has been opened for reading:
x = json.load(f)
This simple serialization technique can handle lists and dictionaries, but serializing arbitrary class instances in JSON requires a bit of extra effort. The reference for the json module contains an explanation of this.
See also
pickle - the pickle module
Contrary to JSON, pickle is a protocol which allows the serialization of arbitrarily complex Python objects. As such, it is specific to Python and cannot be used to communicate with applications written in other languages. It is also insecure by default: deserializing pickle data coming from an untrusted source can execute arbitrary code, if the data was crafted by a skilled attacker.
7. Input and Output的更多相关文章
- [20160704]Addition program that use JOptionPane for input and output
//Addition program that use JOptionPane for input and output. import javax.swing.JOptionPane; public ...
- Python Tutorial 学习(七)--Input and Output
7. Input and Output Python里面有多种方式展示程序的输出.或是用便于人阅读的方式打印出来,或是存储到文件中以便将来使用.... 本章将对这些方法予以讨论. 两种将其他类型的值转 ...
- [Python] Print input and output in table
Print the input and output in a table using prettyTable. from prettytable import PrettyTable import ...
- Input and Output File
Notes from C++ Primer File State Condition state is used to manage stream state, which indicates if ...
- [20171128]rman Input or output Memory Buffers.txt
[20171128]rman Input or output Memory Buffers.txt --//做一个简单测试rman 的Input or output Memory Buffers. 1 ...
- Angular4学习笔记(六)- Input和Output
概述 Angular中的输入输出是通过注解@Input和@Output来标识,它位于组件控制器的属性上方. 输入输出针对的对象是父子组件. 演示 Input 新建项目connInComponents: ...
- Python - 3. Input and Output
from:http://interactivepython.org/courselib/static/pythonds/Introduction/InputandOutput.html Input a ...
- Java中的IO流,Input和Output的用法,字节流和字符流的区别
Java中的IO流:就是内存与设备之间的输入和输出操作就成为IO操作,也就是IO流.内存中的数据持久化到设备上-------->输出(Output).把 硬盘上的数据读取到内存中,这种操作 成为 ...
- Angular2中Input和Output
@Input @Input是用来定义模块的输入的,用来让父模块往子模块传递内容: @Output 子模块自定义一些event传递给父模块用@Output. 对于angular2中的Input和Outp ...
- angular 的 @Input、@Output 的一个用法
angular 使用 @input.@Output 来进行父子组件之间数据的传递. 如下: 父元素: <child-root parent_value="this is parent ...
随机推荐
- RSA 签名、验证、加密、解密帮助类
import java.io.IOException; import java.security.InvalidKeyException; import java.security.KeyFactor ...
- 使用GridSearchCV进行网格搜索微调模型
import numpy as np import pandas as pd from sklearn.feature_extraction.text import TfidfVectorizer f ...
- JMeter 使用 http长连接的方法
前言 如果需要在JMeter通过http长连接发送请求,首先需要选择了Use KeepAlive 长连接协议,虽然默认是勾选的,但也需要确认一下. 除了选择了Use KeepAlive 长连接协议,还 ...
- accept 和 content-Type区别
accept表示 客服端(浏览器)支持的类型,也是希望服务器响应发送回来的的数据类型. 例如:Accept:text/xml; ,也就是希望服务器响应发送回来的是xml文本格式的内容 区别: 1.Ac ...
- win10查看激活到期时间
我们知道Windows系统需要激活后才可以使用全部功能,那么你的Windows10激活了吗?如何查看激活时间呢?是不是永久激活的?带着这些问题,下面我们就一个一个逐一查看一下吧. 工具/原料 Wi ...
- win7蓝屏死机0x0000003B错误蓝屏故障解决
win7蓝屏死机0x0000003B错误蓝屏故障解决 刚才一个朋友问我:电脑蓝屏了怎么办. 我问他要了电脑的截图,自己看了错误代码:0x0000003B 搜索资料,查询了一番.都是说电脑中病毒或者是系 ...
- Echartjs axis.getAxesOnZeroOf is not a function
该问题已经解决,下面是解决思路! 问题描述: axis.getAxesOnZeroOf is not a function 使用echart 出现报这句错误,请求解决方案! 问题原因: 我给坐标设置了 ...
- python标准库之glob介绍
python标准库之glob介绍 glob 文件名模式匹配,不用遍历整个目录判断每个文件是不是符合. 1.通配符 星号(*)匹配零个或多个字符 import glob for name in glob ...
- 基于OpenCV制作道路车辆计数应用程序
基于OpenCV制作道路车辆计数应用程序 发展前景 随着科学技术的进步和工业的发展,城市中交通量激增,原始的交通方式已不能满足要求:同时,由于工业发展为城市交通提供的各种交通工具越来越多,从而加速了城 ...
- 一行代码让3D翻转后的文本恢复清晰
FlashPlayer10提供的3D功能有一个相当蛋疼的问题:只要设置过rotationX.rotationY或者rotationZ属性,显示对象里面的文字(尤其是设备字体,位图文本)就会一直处于模糊 ...