Python-- from scratch to learn object-oriented development 3

Python-- from scratch to learn object-oriented development 3

Taiyuan University of Technology Robotics Team 20 punch card day11

1, class methods and class attributes

1.1 is a special class of objects

Python in everything is an object:

• class AAA: defined category belongs to the class object
• obj1 = AAA () belonging to the instance object

• The program is running, type the same will be loaded into memory
• In Python, the class is a special object - class object
• The program runs, the class object in memory, only one , the use of a class can create a number of object instances
• In addition to the package Examples of attributes outer and methods, class objects can also have its own attributes and methods
  1. Class Properties
  2. Class Methods
• By class name . The way you can attribute access class or method calls the class

1.2 instance attributes and class attributes

• Class attribute is a class object defined attributes
• It is generally used to record relevant to this class feature
• Class attribute for not recording characteristics of the particular object

Example: In order to record the number of objects of this class

class Tool(object):

	# 使用赋值语句，定义类属性，记录创建工具对象的总数
	count = 0

	def __init__(self, name):
		self.name = name
		
		# 针对类属性做一个计数+1
		Tool.count += 1

# 创建工具对象
tool1 = Tool("斧头")
tool2 = Tool("榔头")
tool3 = Tool("铁锹")

# 知道使用 Tool 类到底创建了多少个对象?
print("现在创建了 %d 个工具" % Tool.count)

1.3 class methods and static methods

• Class property that is for the class object attribute definitions
  • Use assignment statements under the class keyword to define a class attribute
  • Class attribute for recording relevant to this class feature
• Class method is for the class object defined method
  • In the class method inside can directly access the class properties or invoke other class methods

The syntax

@classmethod
def 类方法名(cls):
	pass

• Class method requires a decorator @classmethod be identified, told the interpreter This is a class method
• Class method first parameter should be cls
  • Of which class method call, cls in the way is a reference which class
  • This parameter and the example method of the first parameter is a similar self
  • Tip a different name can be, but the habit of using cls
• By the class name. Invoke class method , when calling the method , parameters need to pass cls
• Within a method
• By cls. Attribute access class
• You can also cls. Calling other methods of the class

Examples demand

• Define a Tools
• Each tool has its own name
• Demand in - class number of the object class of the method of packaging a show_tool_count, using the output current of this class, created

@classmethod
def show_tool_count(cls):
	"""显示工具对象的总数"""
	print("工具对象的总数 %d" % cls.count)

Within the class methods, may be used directly access cls class properties or calling a class method

1.4 static method

• In the development, if desired in the class encapsulates a method that:
  • Not only do not need to access an instance property or call the instance method
  • Also you do not need to access the class properties or by calling the class method
• This time, this method can be packaged as a static method

The syntax

@staticmethod
def 静态方法名():
	pass

• Static methods needed decorator @staticmethod be identified, told the interpreter this is a static method
• By the class name. Call the static method

class Dog(object):
	
	# 狗对象计数
	dog_count = 0

	@staticmethod
	def run():

		# 不需要访问实例属性也不需要访问类属性的方法
		print("狗在跑...")
	
	def __init__(self, name):
		self.name = name

2, abnormal

• Program at run time, if the Python interpreter encounters an error, stop program execution, and prompts some wrong information, which is abnormal
• The program stops executing and an error message this action, we usually call: throw (raise) abnormal

2.1 catch the exception

• Program development, if the execution of some code can not be determined correctly , can increase the try (try) to catch the exception
• Catch exceptions simplest syntax:

try:
	尝试执行的代码
except:
	出现错误的处理

• try try , try to write the code below, uncertain whether the code can be executed normally
• except if not , write code below attempt fails

The type of error trapping

• During program execution, you may encounter different types of exceptions , and the need for different types of exceptions , respond differently , this time, we need to capture the type of error
• The syntax is as follows:

try:
	# 尝试执行的代码
	pass
except 错误类型1:
	# 针对错误类型1，对应的代码处理
	pass
except (错误类型2, 错误类型3):
	# 针对错误类型2 和 3，对应的代码处理
	pass
except Exception as result:
	print("未知错误 %s" % result)

• When the Python interpreter throws an exception , the first word in the last line of the error message, type is wrong

Capture unknown error

• In the development, to anticipate all possible errors , or have some difficulty
• If you want the program to any error occurs regardless , will not because Python interpreter throws an exception has been terminated , you can add another except

The syntax is as follows:

except Exception as result:
	print("未知错误 %s" % result)

Abnormal capture the full syntax

• In the actual development, in order to be able to deal with complex anomalies, abnormal complete syntax is as follows:

prompt:

• For the full syntax of scenarios, in the follow-up study, combined with actual cases will be better understood,
• Now there is a first impression to this grammatical structure

try:
	# 尝试执行的代码
	pass
except 错误类型1:
	# 针对错误类型1，对应的代码处理
	pass
except 错误类型2:
	# 针对错误类型2，对应的代码处理
	pass
except (错误类型3, 错误类型4):
	# 针对错误类型3 和 4，对应的代码处理
	pass
except Exception as result:
	# 打印错误信息
	print(result)
else:
	# 没有异常才会执行的代码
	pass
finally:
	# 无论是否有异常，都会执行的代码
	print("无论是否有异常，都会执行的代码")

• Code else only when there is no abnormality will be executed
• finally, whether or not there is an exception code that will be executed
• Prior to the exercise of a complete capture abnormal code is as follows

try:
	num = int(input("请输入整数："))
	result = 8 / num
	print(result)
except ValueError:
	print("请输入正确的整数")
except ZeroDivisionError:
	print("除 0 错误")
except Exception as result:
	print("未知错误 %s" % result)
else:
	print("正常执行")
finally:
	print("执行完成，但是不保证正确")

2.2 abnormal transfer

• Abnormal transfer - when the function / method execution exception occurs , it will pass the exception to the function / method call party
• If the transfer to the main program , still no exception handler , the program will be terminated

prompt

• In development, may increase in the main function abnormalities capture
• The other function calls in the main function, as long as the abnormal, will be passed to the main function abnormalities captured in
• This eliminates the need in the code to increase the number of exception traps , to ensure clean code

demand

1. The demo1 defined function () prompts the user and returns an integer
2. Defined functions demo2 () call demo1 ()
3. Demo2 call in the main ()

def demo1():
	return int(input("请输入一个整数："))

def demo2():
	return demo1()

try:
	print(demo2())
except ValueError:
	print("请输入正确的整数")
except Exception as result:
	print("未知错误 %s" % result)

2.3 throws raise an exception

• In development, in addition to code execution error Python interpreter throws than an exception
• You can also take the initiative to throw an exception application-specific business needs

Throw an exception

• Python is provided an exception class Exception
• In the development, if meet specific business needs, we are hoping to throw an exception, you can:
  1. Create an object of Exception
  2. Use the keyword raise thrown objects