Pass by assignment

Writing Functions in Python

Shayne Miel

Software Architect @ Duo Security

A surprising example

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

Digging deeper

Digging deeper

a = [1, 2, 3]

Digging deeper

a = [1, 2, 3]

b = a

Digging deeper

a = [1, 2, 3]

b = a

a.append(4)

print(b)

[1, 2, 3, 4]

Digging deeper

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]

Digging deeper

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

Pass by assignment

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

Pass by assignment

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

my_list = [1, 2, 3]

Pass by assignment

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

my_list = [1, 2, 3]

foo(my_list)

Pass by assignment

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

my_list = [1, 2, 3]

foo(my_list)

print(my_list)

[99, 2, 3]

Pass by assignment

def bar(x):
  x = x + 90

my_var = 3

Pass by assignment

def bar(x):
  x = x + 90

my_var = 3

bar(my_var)

Pass by assignment

def bar(x):
  x = x + 90

my_var = 3

bar(my_var)

my_var

3

Immutable or Mutable?

Immutable

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

Mutable

• list
• dict
• set
• bytearray
• objects
• functions
• almost everything else!

Mutable default arguments are dangerous!

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]

Let's practice!

Writing Functions in Python