按赋值传递

Python 函数编写

Shayne Miel

Software Architect @ Duo Security

一个出人意料的示例

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

深入理解

深入理解

a = [1, 2, 3]

深入理解

a = [1, 2, 3]

b = a

深入理解

a = [1, 2, 3]

b = a

a.append(4)

print(b)

[1, 2, 3, 4]

深入理解

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]

深入理解

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

按赋值传递

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

按赋值传递

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

my_list = [1, 2, 3]

按赋值传递

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

my_list = [1, 2, 3]

foo(my_list)

按赋值传递

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

my_list = [1, 2, 3]

foo(my_list)

print(my_list)

[99, 2, 3]

按赋值传递

def bar(x):
  x = x + 90

my_var = 3

按赋值传递

def bar(x):
  x = x + 90

my_var = 3

bar(my_var)

按赋值传递

def bar(x):
  x = x + 90

my_var = 3

bar(my_var)

my_var

3

不可变还是可变？

不可变（Immutable）

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

可变（Mutable）

• list
• dict
• set
• bytearray
• 对象
• 函数
• 几乎其他所有！

可变的默认参数很危险！

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]

Passons à la pratique !

Python 函数编写