Explanation of python basic knowledge points

Section 16 MySQLdb

• ##win64-bit installation python-mysqldb1.2.5

• Installing MySQLdb under ubuntu

  sudo apt-get install python-MySQLdb

• Import MySQLdb library

  import MySQLdb

• Create database connection

  conn = MySQLdb.connect(host="localhost",user="root",passwd="123456",db="test",charset="utf8")

• connect object properties

• commit(): If the database table is modified, submit to save the current data. Of course, if this user doesn't have permission and just does it, nothing will happen.

• rollback(): If you have permission, cancel the current operation, otherwise an error will be reported.

• cursor([cursorclass]): Cursor pointer.

• Create a cursor (pointer) cursor

  cur = conn.cursor()

• cursor method of executing commands:

• execute(query, args): Execute a single sql statement. query is the sql statement itself, and args is the list of parameter values. The return value after execution is the number of affected rows.

• executemany(query, args): Execute a single sql statement, but repeatedly execute the parameters in the parameter list, and the return value is the number of affected rows

• Insert a record into the data table

  cur.execute("insert into users (username,password,email) values (%s,%s,%s)",("python","123456","python@gmail.com"))

• Insert multiple records into the data table

  cur.executemany("insert into users (username,password,email) values (%s,%s,%s)",(("google","111222","g@gmail.com"),("facebook","222333","f@face.book"),("github","333444","git@hub.com"),("docker","444555","doc@ker.com")))

• Submit database operation

  conn.commit()

• Query data

  cur.execute("select * from users")

• ##fetchall(self)

  :Receive all Return result rows.

• fetchmany(size=None)

  : Receive size return result rows. If the value of size is greater than the number of returned result rows, then Will return cursor.arraysize pieces of data.

• fetchone()

  : Returns one result row.

• scroll(value, mode='relative')

  : Move the pointer to a certain row. If mode='relative', it means moving the value bar from the current row. If mode='absolute', it means moving the value bar from the result set. The first line moves the value bar.<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">cur.execute(&quot;select * from users&quot;) lines = cur.fetchall() for line in lines:     print line cur.execute("select * from users where id=1") line_first = cur.fetchone()     #只返回一条 print line_first cur.execute(&quot;select * from users&quot;) print cur.fetchall()</pre><div class="contentsignin">Copy after login</div></div>

The method of cursor object to obtain data

The operation of cursor
• cur.scroll(n)

  or cur.scroll(n,"relative"): means to move up or down relative to the current position , n is a positive number, which means downward (forward), n is a negative number, which means upward (backward)

There is another way to achieve "absolute" movement, not " Relative" move: Add a parameter "absolute"

• cur.scroll(1)
  cur.scroll(-2)
  cur.scroll(2,"absolute")    #回到序号是2,但指向第三条

Update data

• cur.execute("update users set username=%s where id=2",("mypython"))
  conn.commit()

Specify the database

• conn = MySQLdb.connect("localhost","root","123456",port=3306,charset="utf8")    #创建数据库时不指定那个数据库
  conn.select_db("test")      #连接创建后再指定

Close the database

• cur.close()     #先关闭游标
  conn.close()    #再关闭数据库

Section 17 Object-oriented

Classes and objects

Process-oriented and object-oriented programming
Process-oriented programming: functional programming, C programs, etc.
Object-oriented programming: C++, Java, Python, etc.

Classes and objects: are two important concepts in object-oriented
Class: is an abstraction of things, such as: car model
Object: is an instance of a class, such as: QQ car, bus

Example description
The car model can abstract the characteristics and behavior of the car, and then instantiate a real car entity Come out

Python class definition

Python class definition
Use the class keyword to define a class, and the first letter of the class name should be capitalized
When programmers need to create a type that cannot be represented by a simple type, they need to create a class
A class combines the required variables and functions. This inclusion is also called "encapsulation"

The structure of the Python class

• class 类名：
      成员变量
      成员函数
  
  class MyClass():
      first = 123
      def fun(self):
          print "I am function"

Creation of objects
Handles are used to distinguish different objects
Properties of objects And methods correspond to member variables and member functions in the class

• if __name__ == "__main__":
      myClass = MyClass()     #创建类的一个实例

The process of creating an object is called instantiation; when an object is created, it contains three characteristics: Object handles, properties and methods.

Constructor __init__

• class Person:
      def __init__(self, name, lang, website):
          self.name = name
          self.lang = lang
          self.website = website

self is a very magical parameter
self points to an instance of the class. When the instance calls a method, self points to the instance of the called method

Subclass, parent class and inheritance

• # 抽象形状类
  class Shape:
      # 类的属性
      edge = 0
      # 构造函数
      def __init__(self, edge):
          self.edge = edge
      # 类的方法
      def getEdge(self):
          return self.edge
      # 抽象方法  
      def getArea(self):
          pass
  
  #三角形类，继承抽象形状类
  class Triangle(Shape):
      width = 0
      height = 0
      # 构造函数
      def __init__(self, width, height):
          #调用父类构造函数
          Shape.__init__(self, 3)
          self.width = width
          self.height = height
      #重写方法
      def getArea(self):
          return self.width * self.height / 2
  
  #四边形类，继承抽象形状类       
  class Rectangle(Shape):
      width = 0
      height = 0
      # 构造函数
      def __init__(self, width, height):
          #调用父类构造函数
          Shape.__init__(self, 4)
          self.width = width
          self.height = height
      #重写方法
      def getArea(self):
          return self.width * self.height
  
  triangle = Triangle(4,5);
  print triangle.getEdge()
  print triangle.getArea()
  
  rectangle = Rectangle(4,5);
  print rectangle.getEdge()
  print rectangle.getArea()

Python supports multiple inheritance, but it is not recommended

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