論證傳遞和可變性

首先，一些術語：

• argument（ 實際引數）： 傳遞給函式的實際變數;
• parameter（ 形式引數）： 函式中使用的接收變數。

在 Python 中，引數通過*賦值*傳遞 （與其他語言相反，其中引數可以通過值/引用/指標傳遞）。

• 變數引數將改變引數（如果引數的型別是可變的）。

  def foo(x):        # here x is the parameter
      x[0] = 9       # This mutates the list labelled by both x and y
      print(x)
  
  y = [4, 5, 6]
  foo(y)             # call foo with y as argument
  # Out: [9, 5, 6]   # list labelled by x has been mutated
  print(y)           
  # Out: [9, 5, 6]   # list labelled by y has been mutated too
  

• 重新分配引數不會重新分配引數。

  def foo(x):        # here x is the parameter, when we call foo(y) we assign y to x
      x[0] = 9       # This mutates the list labelled by both x and y
      x = [1, 2, 3]  # x is now labeling a different list (y is unaffected)
      x[2] = 8       # This mutates x's list, not y's list
  
  y = [4, 5, 6]      # y is the argument, x is the parameter
  foo(y)             # Pretend that we wrote "x = y", then go to line 1
  y
  # Out: [9, 5, 6]
  

在 Python 中，我們真的不給變數賦值，而不是我們結合** （即分配，附加）變數（被視為名稱 ）的物件。**

• 不可變： 整數，字串，元組等。所有操作都會複製。
• 可變： 列表，詞典，集合等。操作可能會也可能不會發生變異。

x = [3, 1, 9]
y = x
x.append(5)    # Mutates the list labelled by x and y, both x and y are bound to [3, 1, 9]
x.sort()       # Mutates the list labelled by x and y (in-place sorting)
x = x + [4]    # Does not mutate the list (makes a copy for x only, not y)
z = x          # z is x ([1, 3, 9, 4])
x += [6]       # Mutates the list labelled by both x and z (uses the extend function).
x = sorted(x)  # Does not mutate the list (makes a copy for x only).
x
# Out: [1, 3, 4, 5, 6, 9]
y
# Out: [1, 3, 5, 9]
z
# Out: [1, 3, 5, 9, 4, 6]