指派呼叫傳遞

Python 函式寫作

Shayne Miel

Software Architect @ Duo Security

出乎意料的例子

def foo(x):
  x[0] = 99

my_list = [1, 2, 3]

foo(my_list)

print(my_list)

[99, 2, 3]

def bar(x):
  x = x + 90

my_var = 3

bar(my_var)

print(my_var)

3

再深入一點

再深入一點

a = [1, 2, 3]

再深入一點

a = [1, 2, 3]

b = a

再深入一點

a = [1, 2, 3]

b = a

a.append(4)

print(b)

[1, 2, 3, 4]

再深入一點

a = [1, 2, 3]

b = a

a.append(4)

print(b)

[1, 2, 3, 4]

b.append(5)

print(a)

[1, 2, 3, 4, 5]

再深入一點

a = [1, 2, 3]

b = a

a.append(4)

print(b)

[1, 2, 3, 4]

b.append(5)

print(a)

[1, 2, 3, 4, 5]

a = 42

指派呼叫傳遞

def foo(x):
  x[0] = 99

指派呼叫傳遞

def foo(x):
  x[0] = 99

my_list = [1, 2, 3]

指派呼叫傳遞

def foo(x):
  x[0] = 99

my_list = [1, 2, 3]

foo(my_list)

指派呼叫傳遞

def foo(x):
  x[0] = 99

my_list = [1, 2, 3]

foo(my_list)

print(my_list)

[99, 2, 3]

指派呼叫傳遞

def bar(x):
  x = x + 90

my_var = 3

指派呼叫傳遞

def bar(x):
  x = x + 90

my_var = 3

bar(my_var)

指派呼叫傳遞

def bar(x):
  x = x + 90

my_var = 3

bar(my_var)

my_var

3

不可變或可變？

不可變（Immutable）

• int
• float
• bool
• string
• bytes
• tuple
• frozenset
• None

可變（Mutable）

• list
• dict
• set
• bytearray
• 物件
• 函式
• 幾乎其他全部！

可變的預設參數很危險！

def foo(var=[]):
  var.append(1)
  return var

foo()

[1]

foo()

[1, 1]

def foo(var=None):
  if var is None:
    var = []
  var.append(1)
  return var

foo()

[1]

foo()

[1]

一起來練習吧！

Python 函式寫作