[python tutorial] The difference between Python2.x and 3.x versions

The difference between Python 2.x and 3.x versions

Python version 3.0 is often called Python 3000, or Py3k for short. This is a major upgrade compared to earlier versions of Python.

In order not to bring too much burden, Python 3.0 was not designed with downward compatibility in mind.

Many programs designed for earlier Python versions cannot run properly on Python 3.0.

In order to take care of existing programs, Python 2.6 is a transitional version that basically uses the syntax and libraries of Python 2.x. It also considers the migration to Python 3.0 and allows the use of some Python 3.0 syntax and functions.

New Python programs are recommended to use Python 3.0 version syntax.

Unless the execution environment cannot install Python 3.0 or the program itself uses a third-party library that does not support Python 3.0. Third-party libraries that currently do not support Python 3.0 include Twisted, py2exe, PIL, etc.

Most third-party libraries are working hard to be compatible with Python 3.0 version. Even if Python 3.0 cannot be used immediately, it is recommended to write a program that is compatible with Python 3.0 and then execute it using Python 2.6 or Python 2.7.

Main changes

The changes in Python 3.0 are mainly in the following aspects:

The print statement is gone and replaced by the print() function. Python 2.6 and Python 2.7 partially support this form of print syntax. In Python 2.6 and Python 2.7, the following three forms are equivalent:

print "fish"
print ("fish") #注意print后面有个空格
print("fish") #print()不能带有任何其它参数

However, Python 2.6 actually supports the new print() syntax:

from __future__ import print_function
print("fish", "panda", sep=', ')

The new str category represents a Unicode string, equivalent to Python 2.x version of the unicode category. The byte sequence is represented by a syntax similar to b"abc" and represented by the bytes class, which is equivalent to the str class of Python 2.x.

Now the two categories can no longer be automatically converted implicitly, so "fish"+b"panda" is an error in Python 3.x. The correct way to write it is "fish"+b"panda".decode("utf-8"). Python 2.6 can automatically recognize a sequence of bytes as a Unicode string by:

from __future__ import unicode_literals
print(repr("fish"))

The division operator "/" always returns a floating point number in Python 3.x. In Python 2.6, it will be judged whether the dividend and divisor are integers. If it is an integer, it will return an integer value, which is equivalent to integer division; if it is a floating point number, it will return a floating point value.

In order to make Python 2.6 uniformly return floating point values, you can:

from __future__ import division
print(3/2)

The syntax for catching exceptions is changed from except exc, var to except exc as var. Use the syntax except (exc1, exc2) as var to catch multiple categories of exceptions at the same time. Python 2.6 already supports both syntaxes.

New way of writing set: {1,2,3,4}. Note that {} still represents an empty dictionary (dict).

Dictionary comprehensions {expr1: expr2 for k, v in d}, this syntax is equivalent to

result={}
for k, v in d.items():
    result[expr1]=expr2
return result

Set Comprehensions {expr1 for x in stuff}. This syntax is equivalent to:

result = set()
for x in stuff:
    result.add(expr1)
return result

The octal number must be written as 0o777. The original form 0777 cannot be used; the binary number must be written as 0b111. A new bin() function is added to convert an integer into a binary string. Python 2.6 already supports both syntaxes.

dict.keys(), dict.values(), dict.items(), map(), filter(), range(), zip() no longer return lists, but iterators.

If there is no well-defined meaningful order between two objects. Comparing them using <, >, <=, >= will throw an exception. For example, 1 < "" will return True in Python 2.6, but will throw an exception in Python 3.0. Now the cmp(), instance.__cmp__() functions have been removed.

You can annotate the parameters and return values ​​of functions. This feature allows the IDE to conduct more in-depth analysis of the source code. For example, add category information to parameters:

def sendMail(from_: str, to: str, title: str, body: str) -> bool:
    pass

Combine int and long types.

Multiple modules were renamed (according to PEP8):

old name

new name

_winreg winreg

ConfigParser configparser

copy_reg copy

Queue queue

SocketServer socketserver

repr reprlib

StringIO module is now merged into the new io module. New, md5, gopherlib and other modules have been deleted. Python 2.6 already supports the new io module.

httplib, BaseHTTPServer, CGIHTTPServer, SimpleHTTPServer, Cookie, cookielib are merged into the http package.

The exec statement is canceled, leaving only the exec() function. Python 2.6 already supports the exec() function.

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