Second, the core style of Python Foundation

Directory

I. Statements and syntax

Two. Variable definition and Assignment

Three. Memory management

Memory Management:

Reference count:

Simple example

Four. Python objects

Five. Identifiers

Six. Dedicated underline identifiers

Seven. Write the basic style of the module

Eight. Demonstration

I. Statements and syntax

#: Notes

\: Translate carriage return, continue on the previous line, in a long line of statements can be used to cut into multiple lines, because of its poor readability, it is not recommended to use

;: Connect two statements to one line with poor readability, not recommended

:: Separate the head and body of the code

Statements (code blocks) are indented to reflect different levels of code, with 4 spaces recommended (Do not use tab)

Python files are organized in a modular way, and writing a. py end file actually writes a module

Two. Variable definition and Assignment

A=1:1 memory variables in memory, A is a reference to a variable, Python is a dynamic language, and the variable and its type do not have to declare the type beforehand

Difference from C: a=1 no return value

Note:

The C-language variable declaration must be at the beginning of the code and be preceded by all statements

C++,java can declare variables anywhere, but must declare variable names and types

Python can also declare variables anywhere, but when a variable is defined, the interpreter determines its type based on the value to the right of the equation.

Variables must be assigned before they can be used

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Three. Memory management

Memory Management:

Variable does not need to specify type

Programmers need not care about memory management

The variable will be automatically recycled

Del ability to directly free memory objects (reduce reference count of objects)

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Reference count:

Increase reference count

object is created and its reference is assigned to a variable, reference count plus 1 (example a=1)

A reference to the same object is assigned to another variable, the reference count plus 1 (example b=a)

The object is called by the function as a parameter, and the reference count plus 1 (example int (a)

Object becomes an element in the container object, reference count plus 1 (example list_test=[' Alex ', ' Z ', a])

Reduce reference count

A as a parameter of the function call, after the function is run, all local variables including a are destroyed, the reference count minus 1

The variable is assigned to another object, and the reference count of the original object is reduced by 1 (example b=2,1 the reference to this memory object remains a)

Use Del to delete a reference to the object, minus 1 for the reference count (Example del a)

A as an element in the container list_test, is cleared and the reference count is reduced (example List_test.remove (a))

The container itself is destroyed (Example del list_test)

Note: Python memory recycling is given to a separate code, the garbage collector (with reference counters and a cyclic garbage collector), and the reference count is not cleared immediately (there may be cyclic calls) at zero

Instead of having to tangle with circular reference collection, just remember that the garbage collector helps you automatically clean up memory.

Cases:

X=1 # Creates a memory variable 1, passes a reference to the variable 1 to X, at the moment the reference count of 1 is increased to 2y=2 # of 1y=x # # to create a new memory variable 2, pass the reference to the variable 2 to Y, which points to 1 y, which is 2, so the reference count for 1 is reduced to 1del X # A reference to memory object 1 is deleted, and at the moment 1 is no longer referenced, it becomes the target of the Python interpreter's recovery.

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Four. Python objects

In Python, where the object model is used to store data, the factory function used to generate the data type is essentially a class, and the result of the new data is the instantiation of an object

There are three characteristics of the object

Identity: Memory address, can be confirmed with ID (), the same ID is the same object

Type: Can be viewed with type (), the type of the return value is also an object

Value

Five. Identifiers

Definition: A valid string collection that is allowed as a name

The name must be meaningful and readable.

The first letter must be a letter or an underscore (_)

The remaining characters can be letters and numbers or underscores

Case sensitive

Two styles: Conn_obj or Connobj

Cannot use keywords, cannot use built-in

Keyword table:

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Six. Dedicated underline identifiers

_xxx: Cannot import with from module import *

__XXX__: System-defined Name

__XXX: Private variables in class

Underscores are special for the interpreter and are symbols used by built-in identifiers, and it is not recommended that custom variables begin with an underscore

But if it's a private variable in the class, __xxx will be a good habit.

Add:

System variable __name__ will derive different values depending on how the python file is loaded

Python files are imported as modules: __name__= module name or file name

Python file executed: __name__= ' __main__ '

When we use Python to write a software, there should be only one main program that contains a lot of top-level code (that is, code without indentation).

The Python interpreter reads the top-level code immediately, and the other. py files should have only a few top-level code, all of which should be encapsulated

In a function or class

Usually at the end of the file, combine the __name__ variable and write the test code

Seven. Write the basic style of the module

1 title

2 Documentation Comments

3 Module Import

4 variable definition

5 class definition statements

6 function definition Statements

7 Main Program

8 Test Code

Eight. Demonstration

#_ *_coding:utf-8_*_

#!/usr/bin/env python

‘‘‘

This is an example module

‘‘‘

Import Sys,os

Debug=true

Class Test:

‘‘‘

Test class

‘‘‘

Pass

def main ():

‘‘‘

Test func

: return:

‘‘‘

Pass

if __name__ = = ' __main__ ':

Main ()

This article is from the "Wandering Wind" blog, please be sure to keep this source http://774148515.blog.51cto.com/10937712/1894717

Second, the core style of Python Foundation