Python must learn bytes every day, pythonbytes bytes

Python must learn bytes every day, pythonbytes bytes

The bytecode in Python is expressed in the form of B 'xxx. X can be represented by characters or in ASCII format \ xnn. nn consists of 256 types of characters from 00-ff (hexadecimal.

I. Basic operations

The following describes the basic operations of a byte. It can be seen that it is very similar to a string:

In [40]: B = B "abcd \ x64" In [41]: bOut [41]: B 'abcd' In [42]: type (B) out [42]: bytesIn [43]: len (B) Out [43]: 5In [44]: B [4] Out [44]: 100 #100 in hexadecimal notation \ x64

If you want to modify a byte in a byte string, you must convert it to bytearray before modifying it:

In[46]: barr = bytearray(b)In[47]: type(barr)Out[47]: bytearrayIn[48]: barr[0] = 110In[49]: barrOut[49]: bytearray(b'nbcdd')

2. Relationship between byte and character

As mentioned above, bytes are very similar to characters. In fact, they can be converted to each other. Bytes can be converted to corresponding characters in some encoding form. Bytes can be converted to characters by using the encode () method, while characters can be converted to bytes by using the decode () method:

In [50]: s = "My life is short, I use Python" In [51]: B = s. encode ('utf-8') In [52]: bOut [52]: B '\ xe4 \ xba \ xe7 \ x94 \ x9f \ xe8 \ x8b \ xa6 \ xe7 \ x9f \ xad \ xef \ xbc \ x8c \ xe6 \ x88 \ x91 \ xe7 \ x94 \ xa8Python 'In [53]: c = s. encode ('gb18030') In [54]: cOut [54]: B '\ xc8 \ xcb \ xc9 \ xfa \ xbf \ xe0 \ xb6 \ xcc \ xa3 \ xac \ xce \ xd2 \ xd3 \ xc3python' In [55]: B. decode ('utf-8') Out [55]: 'Life is short, I use python' In [56]: c. decode ('gb18030') Out [56]: 'Life is short. I use Python 'In [57]: c. decode ('utf-8') Traceback (most recent call last): exec (code_obj, self. user_global_ns, self. user_ns) File "<ipython-input-57-8b50aa70bce9>", line 1, in <module> c. decode ('utf-8') UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc8 in position 0: invalid continuation byteIn [58]: B. decode ('gb18030') Out [58'

We can see that the characters and bytes parsed by different encoding methods are completely different. If different encoding methods are used for encoding and decoding, garbled characters may occur, or even conversion fails. Because each encoding method contains different types of bytes, the \ xc8 in the preceding example exceeds the maximum character of UTF-8.

Iii. Application

For the simplest example, I want to crawl the content of a webpage. Now I want to crawl the pages returned by Baidu when searching for Python. Baidu uses the UTF-8 encoding format, if the returned result is not decoded, It is a super long byte string. After correct decoding, a normal html page is displayed.

import urllib.requesturl = "http://www.baidu.com/s?ie=utf-8&wd=python"page = urllib.request.urlopen(url)mybytes = page.read()encoding = "utf-8"print(mybytes.decode(encoding))page.close()

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