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This article introduces the knowledge about "what are the syntax differences between python 2.7 and python 3.0". In the actual case operation process, many people will encounter such difficulties. Next, let Xiaobian lead you to learn how to deal with these situations! I hope you can read carefully and learn something!

I. Basic grammar

1. Python3 default encoding

Python2's default encoding is ASSCII, which is one of the reasons why Python2 often encounters coding problems. As for why ASSCII is used as the default encoding, the reason is that Unicode was not available when Python was born. Python 3 uses UTF-8 as the default encoding, so you no longer need to write # coding= utf-8 at the top of your file.

2. Iterator

Many of the built-in functions and methods that returned list objects in Python2 have been changed to return iterator-like objects in Python 3, because iterators 'lazy loading makes it more efficient to manipulate large data. The range and xrange functions in Python2 are merged into range, and if they are compatible with both 2 and 3, you can do this:

In addition, dictionary objects 'dict.keys() and dict.values() methods no longer return lists, but rather an iterator-like "view" object. The higher-order functions map, filter, and zip also return non-list objects. Python2 iterators must implement the next method, while Python 3 changed it to next.

Print is no longer a statement, but a function.

For example, it used to be print 'abc' and now it's print ('abc ').

But Python 2.6 + can use from future import print_function to achieve the same functionality.

4. The original 1/2 (division of two integers) results in 0, and now it is 0.5.

$ /opt/python3.6/bin/python3

Python 3.6.2 (default, Aug 4 2017, 10:27:08)

[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> 1/2

0.5

>>> 1/3

0.3333333333333333

New string formatting method format replaces %

Python3 completely obsolutes long+int

Unified to int , supports high-precision integer operations

Python2 uses raw_input , Python 3 uses input

II. Types of data

1. String

String is one of the biggest changes, and it reduces coding problems to the lowest possible level. In Python2, strings have two types, one unicode, the other str, the former represents a text string, the latter represents a byte sequence, but there is no obvious boundary between the two, developers also feel very confused, do not understand the reason for the encoding error, but in Python 3 the two made a strict distinction, respectively, str represents a string, byte represents a byte sequence, any data that needs to be written to text or network transmission only receives a byte sequence, This stops coding errors at the source.

III. Modular programming

1. Added some new modules

concurrent.futures、venv、unittest.mock、asyncio、selectors、typing

Remove some modules or functions

gopherlib, md5, contextlib.nested, inspect.getmoduleinfo, etc.

The main reasons for removing the content are 2 points:

（1）. Outdated technology that few people use anymore;

（2）. A new alternative product emerged which proved to be of little significance. In theory, it has little impact on developers.

3. Module name modification

Combine related modules into one package, such as:

For example httplib, BaseHTTPServer, CGIHTTPServer, SimpleHTTPServer, Cookie, cookelib are put into http.

urllib, urllib2, urlparse, robotparse into urllib.

SocketServer module name changed to socketserver, Queue module name changed to queue

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