Day06 - Python基本语法 Part 3，函式与类别

范例程序主要来自於W3Schools。

条件式

a = 10
b = 20
if a > b and a >= 0:
  print("a > b")
elif a == b:
  print("a == b")

# 若该条件内暂时没有要执行的程序码，需使用「pass」，不可留空
elif a < 0:
  pass
else:
  print("a < b")

回圈

Python中标准回圈有while和for两种，其中break、continue用法跟大部分程序语言差不多，比较特殊的是如果不符合回圈条件而离开回圈的话，会执行else区块(若有提供)：

i = 1
while i < 6:
  i += 1
  if i == 3:
    continue
  print(i)
else:
  print("Loop end. Now i is " + str(i))

但若因break跳出，则不会执行else区块：

i = 1
while i < 6:
  i += 1
  if i == 3:
    continue
  if i == 5:
    break
  print(i)
else:
  print("Loop end. Now i is " + str(i))

函式

函式的宣告与使用：

def my_function():
  print("This is my function")

my_function()

宣告有引数(argument)的函式；

def my_function(name, age):
  x = "My name is {}. My age is {}."
  print(x.format(name, age))

my_function("Alice", 20)

提供引数预设值：

def my_function(name="None", age=99):
  x = "My name is {}. My age is {}."
  print(x.format(name, age))


my_function("Alice", 20)
my_function()
my_function("Bob")
my_function(name= "Cathy", age=15)
my_function(age=0)

如需宣告引数数量可变动，则在引数前面加上「*」，呼叫函式时放入的引数将以Tuple的形式传入函式内部：

def my_function(*args):
  for x in args:
    print("Input: " + x)

my_function("Apple","Orage","Banana")

变数的值域

在function内部宣告的区域变数，其变数范围只存在於function内。
在程序本体宣告的全域变数，变数范围存在整支程序中。
当function出现与全域变数相同命名的变数，这时此变数在该function内会视为区域变数，不影响全域变数的数值。
透过「global」宣告，可在函式内部使用全域变数。

x = 300

# 宣告myfunc1，印出其中区域变数x的数值 = 1000
def myfunc1():
  x = 1000
  print("myfunc1: x = " + str(x))

# 宣告myfunc2，印出全域变数x的数值
def myfunc2():
  global x
  print("myfunc2: x = " + str(x))

myfunc1()
myfunc2()
print("global : x = " + str(x))

Class

Python是一种物件导向的程序语言，物件导向的概念已经有很多文章介绍，故以下将直接说明Class和Object相关操作。

建立Class和物件

# 宣告Class
class Book:
  name = "unknown"
  author = "unknown"

# 建立物件
book1 = Book()
print(book1.author)

初始化函式：init(arg1, arg2, arg3) 在建立物件时会被呼叫。
重要：Class内每个函式的第一个引数arg1将作为该Class的执行个体使用，类似this的用法。

class Book:
  def __init__(self, name, author):
    self.name = name
    self.author = author
  
  def print_name(test):
    print("Book name: " + test.name)


book1 = Book("Pride and Prejudice", "Jane Austen")
book1.print_name()

物件内的属性可被删除

class Book:
  def __init__(self, name, author):
    self.name = name
    self.author = author
  
  def print_name(test):
    print("Book name: " + test.name)


book1 = Book("Pride and Prejudice", "Jane Austen")
del book1.name
book1.print_name()

继承
宣告子类别Book，继承Product

# 宣告父类别
class Product:
  def __init__(self, name, unitprice, quantity):
    self.name = name
    self.unitprice = unitprice
    self.quantity = quantity

  def print_information(self):
    print("Product Name: " + self.name)
    print("Price: " + str(self.unitprice))
    print("Stock: " + str(self.quantity))

# 宣告子类别Book，继承Product
class Book(Product): 
  pass

prod1 = Product("Apple", 10, 100)
prod2 = Book("Pride and Prejudice", 18, 10)

prod1.print_information()
prod2.print_information()

覆写(Override)：

子类别使用相同的命名会覆盖父类别。
使用super().function()继承父类别的方法(Method)和属性(Property)

# 宣告父类别
class Product:
  def __init__(self, name, unitprice, quantity):
    self.name = name
    self.unitprice = unitprice
    self.quantity = quantity

  def print_information(self):
    print("Product Name: " + self.name)
    print("Price: " + str(self.unitprice))
    print("Stock: " + str(self.quantity))

# 宣告子类别Book，继承Product，并覆写__init__()和print_information()
class Book(Product): 
  def __init__(self, name, unitprice, quantity, author):
    super().__init__(name, unitprice, quantity) 
    self.author = author
  
  def print_information(self):
    super().print_information()
    print("Author: " + self.author)


prod1 = Product("Apple", 10, 100)
prod2 = Book("Pride and Prejudice", 18, 10, "Jane Austen")

prod1.print_information()
prod2.print_information()