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Python Tutorial

5 Variables

5.1 overview

5.1.1 建立variable

  • python和R一樣，都不用宣告資料類型，就可以assign variable了：
x = 4       # x is of type int
x = "Sally" # x is now of type str
print(x)
#> Sally
  • 如果想要和C/Java一樣，assign variable時同時宣告資料類型，那要用casting
x = str(3)    # x will be '3'
y = int(3)    # y will be 3
z = float(3)  # z will be 3.0

5.1.2 用type()來確認variable資料類型 

x = 5
y = "John"
print(type(x))
#> <class 'int'>
print(type(y))
#> <class 'str'>

5.2 variable names

5.2.1 規則

  • python中對variable name的要求是：

    • 只能是由alpha-numeric + underscores所組成(A-z, 0-9, _)
    • 開頭不能是數字，只能英文或底線
    • case-sensitive，大小寫有差拉
# 合法的variable name  
myvar = "John"
my_var = "John"
_my_var = "John"
myVar = "John"
MYVAR = "John"
myvar2 = "John"

# 不合法的variable name
2myvar = "John"
my-var = "John"
my var = "John"

5.2.2 多個字的variable name

  • 如果variable name想由多個字組成，哪有三種習慣性寫法：

    • Snake Case: 像蛇一樣一節一節的，用底線隔開，例如 my_variable_name
    • Pascal Case: 每個字的開頭都給他大寫，例如 MyVariableName
    • Camel Case: 像駱駝一樣，頭低低，後面駝峰都高起，所以首字小寫，之後都大寫，例如 myVariableName

5.3 一次assign多個variables

5.3.1 多個value，assign給多個variable 

x, y, z = "Orange", "Banana", "Cherry"
print(x)
#> Orange
print(y)
#> Banana
print(z)
#> Cherry

5.3.2 同一個value，assign給多個variable 

x = y = z = "Orange"
print(x)
#> Orange
print(y)
#> Orange
print(z)
#> Orange

5.3.3 unpack a collection

  • 在python中，collection包含list, tuple, dictionary, set
  • 那我可以把這些collection中的各個element，assign給各個variable，例如：
fruits = ["apple", "banana", "cherry"]
x, y, z = fruits
print(x)
#> apple
print(y)
#> banana
print(z)
#> cherry
  • 而這招最常用的還是用在tuple上：
fruits = ("apple", "banana", "cherry")
x, y, z = fruits
print(x)
#> apple
print(y)
#> banana
print(z)
#> cherry

5.4 Assignment Operators

  • Python有許多assignment operators，整理如下：
Operator Example Same As
= x = 5 x = 5
+= x += 3 x = x + 3
-= x -= 3 x = x - 3
*= x *= 3 x = x * 3
/= x /= 3 x = x / 3
%= x %= 3 x = x % 3
// x //= 3 x = x // 3
**= x **= 3 x = x ** 3
  • 備註：
    • x % 3的意思是，我要取餘數
    • x // 3的意思是，我要取商
    • x ** 3的意思是，x的3次方

5.5 variable scope

5.5.1 Global Variables

  • 想把function中的variable(e.g. x這個variable)，拉到global environment，就先用global x來宣告我的x是屬於global的，之後在這個function內把value給assign到x，就會放在global environment裡：

  • 來看看沒宣告的例子：

x = "awesome"

def myfunc():
  x = "fantastic"
  print("Python is " + x)

myfunc()
#> Python is fantastic
print("Python is " + x)
#> Python is awesome
  • 再看看有宣告的例子：
x = "awesome"

def myfunc():
  global x
  x = "fantastic"
  print("Python is " + x)

myfunc()
#> Python is fantastic
print("Python is " + x)
#> Python is fantastic