数学建模

 

编程知识点

编程基础-Python-Errors and Classes

 

Wang Haihua

🚅 🚋😜 🚑 🚔

Errors and Exceptions

• There are many built-in Exceptions in Python.
• An error is caused by not following the porper structure (syntax) = SyntaxError
• Exceptions are errors that occur during execution. (They are also sometimes called 'logical errors')
• Even if a satement or expression is syntactically correct, it may cause an error when we attempt to execute it. This is an Exception.

print "Hello World!"   

  File "<ipython-input-1-8afc9091282f>", line 1
    print "Hello World!"
          ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print("Hello World!")?

Bugs

Bugs are usually caused by human mistakes. They don't necessarily have to end up in an error.

#bug example.

num1 = input("Enter the first integer: ")
num2 = input("Enter the second integer: ")

print(num1, "+", num2, "=", num1 + num2)

Enter the first integer: 8
Enter the second integer: 7
8 + 7 = 87

#bug example.

num1 = int(input("Enter the first integer: "))
num2 = int(input("Enter the second integer: "))

print(num1, "+", num2, "=", num1 + num2)

Enter the first integer: hey

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-19915655c439> in <module>
      1 #bug example.
      2 
----> 3 num1 = int(input("Enter the first integer: "))
      4 num2 = int(input("Enter the second integer: "))
      5 

ValueError: invalid literal for int() with base 10: 'hey'

Some Error Examples

#exception example, ValueError.

num3 = int(input("First integer: "))
num4 = int(input("Second integer: "))

print(num3, "/",num4, "=", num3/num4)

First integer: 6
Second integer: 7.4

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-2ca720ac0e5e> in <module>
      2 
      3 num3 = int(input("First integer: "))
----> 4 num4 = int(input("Second integer: "))
      5 
      6 print(num3, "/",num4, "=", num3/num4)

ValueError: invalid literal for int() with base 10: '7.4'

int(3.8)

3

# ZeroDivisionError.


num3 = int(input("First integer: "))
num4 = int(input("Second integer: "))

print(num3, "/",num4, "=", num3/num4)

First integer: 7
Second integer: 0

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-11-c273cdf866df> in <module>
      5 num4 = int(input("Second integer: "))
      6 
----> 7 print(num3, "/",num4, "=", num3/num4)

ZeroDivisionError: division by zero

Exception Handling

try:

the situations where we can get exceptions

except "Exception Name":

the operations in case of exceptions

x = "Alan Turing"

int(x)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-417db27f4b47> in <module>
      1 x = "Alan Turing"
      2 
----> 3 int(x)

ValueError: invalid literal for int() with base 10: 'Alan Turing'

try:
  int(x)

except ValueError:
  print("Please enter an integer value!!!")

Please enter an integer value!!!

num3 = input("First integer: ")
num4 = input("Second integer: ")

try:
  num3_int = int(num3)
  num4_int = int(num4)

  print(num3_int, "/",num4_int, "=", num3_int/num4_int)

except ValueError:
  print("Please enter an integer value!!!")

First integer: 6
Second integer: 0

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-8-194a4a382dd0> in <module>
      6   num4_int = int(num4)
      7 
----> 8   print(num3_int, "/",num4_int, "=", num3_int/num4_int)
      9 
     10 except ValueError:

ZeroDivisionError: division by zero

num3 = input("First integer: ")
num4 = input("Second integer: ")

try:
  num3_int = int(num3)
  num4_int = int(num4)

  print(num3_int, "/",num4_int, "=", num3_int/num4_int)

except ZeroDivisionError:
    print("Please enter the second input different than 0 value!!!")

First integer: 6
Second integer: 2
6 / 2 = 3.0

num3 = input("First integer: ")
num4 = input("Second integer: ")

try:
  num3_int = int(num3)
  num4_int = int(num4)

  print(num3_int, "/",num4_int, "=", num3_int/num4_int)

except ValueError:
  print("Please enter an integer value!!!")
except ZeroDivisionError:
  print("Please enter the second input different than 0 value!!!")
except:
  print("Unknown error...")

First integer: 5
Second integer: 0
Please enter the second input different than 0 value!!!

3.4/0

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-10-71f13771214b> in <module>
----> 1 3.4/0

ZeroDivisionError: float division by zero

An except clause may name multiple exceptions

num3 = input("First integer: ")
num4 = input("Second integer: ")

try:
  num3_int = int(num3)
  num4_int = int(num4)

  print(num3_int, "/",num4_int, "=", num3_int/num4_int)

except (ValueError, ZeroDivisionError):
  print("Error!!!")

First integer: 2.5
Second integer: 3
Error!!!

int("3.5")

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-14-f6c874437bf0> in <module>
----> 1 int("3.5")

ValueError: invalid literal for int() with base 10: '3.5'

See the error message

#try/except/as


num3 = input("First integer: ")
num4 = input("Second integer: ")

try:
  num3_int = int(num3)
  num4_int = int(num4)

  print(num3_int, "/",num4_int, "=", num3_int/num4_int)

except ValueError as error:
  print("Error!!!")
  print("Error message: ", error)

First integer: 5
Second integer: 5.2
Error!!!
Error message:  invalid literal for int() with base 10: '5.2'

#exception handling in loop structure


while True:
    num1 = input("First number: (Press q for quit the program): ")

    if num1 == "q":
        break

    num2 = input("Second number: ")

    try:
        num1_int = int(num1)
        num2_int = int(num2)
        print(num1_int, "/", num2_int, "=", num1_int / num2_int)
    except (ValueError, ZeroDivisionError):
        print("Error!")
        print("Please try again!")

First number: (Press q for quit the program): 9
Second number: 9
9 / 9 = 1.0
First number: (Press q for quit the program): q

#try/except/as


num3 = input("First integer: ")
num4 = input("Second integer: ")

try:
  num3_int = int(num3)
  num4_int = int(num4)

  print(num3_int, "/",num4_int, "=", num3_int/num4_int)

except (ValueError, ZeroDivisionError) as error:
  print("Error!!!")
  print("Error message: ", error)

First integer: 9
Second integer: 0
Error!!!
Error message:  division by zero

Raising Exceptions

"""
exception handling in functions
using raise command
"""


def reverse(s):
    if (type(s) != str):
        raise ValueError("Please enter a String type.")
    else:
        return s[::-1]

reverse("python")

'nohtyp'

reverse(12)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-25-9be0aeb641b6> in <module>
----> 1 reverse(12)

<ipython-input-21-fbb7f7fb3af7> in reverse(s)
      7 def reverse(s):
      8     if (type(s) != str):
----> 9         raise ValueError("Please enter a String type.")
     10     else:
     11         return s[::-1]

ValueError: Please enter a String type.

Intro to Object Oriented Programming

Python is an object oriented programming language. And almost everything in Python is an object.

An object has properties an methods.

A Class is how we construct an object (like its blueprint)

class Car():
    pass

def hello():
    pass

A Class with properties

class Car():
    colour = "yellow"
    brand = "xyz"
    door = 4
    model = 1987

car1 = Car()

print(car1)

<__main__.Car object at 0x00000239E2897A00>

print(car1.model)
print(car1.brand)
print(car1.door)
print(car1.colour)

1987
xyz
4
yellow

car2 = Car()

car2.colour = "blue"
print(car2.colour)
print(car1.colour)

blue
yellow

#We can also call attributes over the class without creating an object.

Car.model

1987

Car.brand

'xyz'

dir(car1)

['__class__',
 '__delattr__',
 '__dict__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__gt__',
 '__hash__',
 '__init__',
 '__init_subclass__',
 '__le__',
 '__lt__',
 '__module__',
 '__ne__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 '__weakref__',
 'brand',
 'colour',
 'door',
 'model']

init ()

All classes have a function called init ()

We use this function to assign values to object properties. For this, we use a special method: init ()

The init () method called automatically every time the class is used to create a new object.

By defining this method, we can create our objects with different values.

class Car2():

  def __init__(self): 
    print("init function calling...")

self is a reference to the current instance to the class and is used to any variable or method that belongs to the class.

car = Car2()

init function calling...

class Car3():

  def __init__(self, colour, brand, door):
    self.colour = colour
    self.brand = brand
    self.door = door


red_car = Car3("red", "abc",2)

yellow_car = Car3("yellow", "xyz", 4)

print(red_car.colour)
print(yellow_car.brand)

red
xyz

blue_car = Car3()

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-36-2993678ac391> in <module>
----> 1 blue_car = Car3()

TypeError: __init__() missing 3 required positional arguments: 'colour', 'brand', and 'door'

blue_car = Car3("blue","xyz")

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-37-6b3d9a135cf0> in <module>
----> 1 blue_car = Car3("blue","xyz")

TypeError: __init__() missing 1 required positional argument: 'door'

blue_car = Car3("blue", "BMW", 4)

print(blue_car.brand)
print(blue_car.colour)
print(blue_car.door)

BMW
blue
4

class Person():
  def __init__(self, personName, personAge):
    self.name = personName
    self.age = personAge


  def welcomePerson(self):
    print("Hello " + self.name)

  
  def showAge(self):
    print(self.age)

person1 = Person ("Asli", 28)

person1.welcomePerson()

Hello Asli

person1.showAge()

28

Inheritence

Inheritence is a way that allows us to define a class that inherits all the methods and properties from another class.

One will be the Parent Class

Another will be the Child Class

class Person:
    def __init__(self, name, lname, nationality):
        self.first_name = name
        self.last_name = lname
        self.nationality = nationality
    
    def print_name(self):
        print(self.first_name + " " + self.last_name)
    
    def print_nationality():
        print(self.name + " is " + self.natioanlity)

class Student(Person):
    pass

student = Student("Rob", "Bain", "Dutch")
student.print_name()

Rob Bain

class Student(Person):
    def __init__(self, school, year):
        self.school = school
        self.year = year

student = Student("Bogazici", 3)
student.first_name = "Ali"
student.last_name = "Cengiz"

student.print_name()

Ali Cengiz

class Student(Person):
    def __init__(self, name, lname, nationality, school, year):
        super().__init__(name, lname, nationality)
        self.school = school
        self.year = year
    
    def print_student_status(self):
        print(self.first_name + " is year " + str(self.year) + " at " + self.school)

new_student = Student("Mary", "Jenkins", "French", "Sorbonne", 2)
dir(new_student)

new_student.print_student_status()

Mary is year 2 at Sorbonne

Question

Create a new Person class that takes a name and age in the contructor.

Define 3 functions:

1. welcomePerson -> Prints a welcome message with the person's name
2. showAge -> Prints persons name
3. addLanguage -> accepts a language and adds it to a list of languages that the person is able to speak
4. showInfo ->

class Person3():
    def __init__(self, name, age):
        self.name = name
        self.age = age
        self.language = []
        
    def welcomePerson(self):
        print("Hello",self.name)
    
    def showAge(self):
        print(self.age)
        
    def addLanguage(self,new_lang):
        print("Adding new language")
        self.language.append(new_lang)
    def showInfo(self):
        print("{} is {} years old".format(self.name,self.age))
        print("He can speak:")
        for i in self.language:
              print(i)
        

Ferhat = Person3("Ferhat",21) 

Ferhat

<__main__.Person3 at 0x27972501548>

Ferhat.addLanguage("Spanish")

Adding new language

Ferhat.showAge()

21

Ferhat.showInfo()

Ferhat is 21 years old
He can speak:
Spanish

Ferhat.welcomePerson()

Hello Ferhat

Ferhat.addLanguage("Spanish")

Adding new language

Ferhat.showInfo()

Ferhat is 21 years old
He can speak:
Spanish
Spanish

Ferhat.addLanguage("English")

Adding new language

Ferhat.showInfo()

Ferhat is 21 years old
He can speak:
Spanish
Spanish
English

References

• https://github.com/SudePehlivanoglu/python-education-v2