Lecture 23

Mutable Data�

reminder/Announcement

• Mic
• HW10 for Extra credit: due Tuesday of Final week.
• Pytests
• Sets
• Abstraction barriers

Mutability

• Immutable – the value of the object cannot be changed:
• integers, floats, booleans
• strings, tuples
• Mutable – the value of the object can be changed:
• Lists
• Dictionaries

Mutability

• Immutable – the value of the object cannot be changed:
• integers, floats, booleans
• strings, tuples
• Mutable – the value of the object can be changed:
• Lists
• Dictionaries

Mutability

• Immutable – the value of the object cannot be changed:
• integers, floats, booleans
• strings, tuples
• Mutable – the value of the object can be changed:
• Lists
• Dictionaries

Observation:Demo

• The same object can change the value during the course of computation.
• All names that refer to the same object are affected by a mutation.

Mutation Can Happen Within a Function Call

A function can change the value of any object in its scope.

>>> four = [1, 2, 3, 4]

Mutation Can Happen Within a Function Call

A function can change the value of any object in its scope

>>> four = [1, 2, 3, 4]

>>> len(four)

4

Mutation Can Happen Within a Function Call

A function can change the value of any object in its scope

>>> four = [1, 2, 3, 4]

>>> len(four)

4

>>> mystery(four)

Mutation Can Happen Within a Function Call

A function can change the value of any object in its scope

>>> four = [1, 2, 3, 4]

>>> len(four)

4

>>> mystery(four)

>>> len(four)

2

Mutation Can Happen Within a Function Call

A function can change the value of any object in its scope

>>> four = [1, 2, 3, 4]

>>> len(four)

4

>>> mystery(four)

>>> len(four)

2

def mystery(s):

s.pop()

s.pop()

Mutation Can Happen Within a Function Call. demo

A function can change the value of any object in its scope

>>> four = [1, 2, 3, 4]

>>> len(four)

4

>>> mystery(four)

>>> len(four)

2

def mystery(s):

s.pop()

s.pop()

​

​

def mystery(s):

s[2:] = []

Mutation Can Happen Within a Function Call. demo

A function can change the value of any object in its scope

>>> four = [1, 2, 3, 4]

>>> len(four)

4

>>> another_mystery() # no arguments

>>> len(four)

2

def mystery(s):

s.pop()

s.pop()

​

​

​

Mutation Can Happen Within a Function Call. demo

A function can change the value of any object in its scope

>>> four = [1, 2, 3, 4]

>>> len(four)

4

>>> another_mystery() # no arguments

>>> len(four)

2

def mystery(s):

s.pop()

s.pop()

​

​

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def another_mystery():

four.pop()

four.pop()

How about tuples?

>>> four = (1, 2, 3, 4)

>>> len(four)

4

>>> mystery(four)

>>> four

What is four?

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A: (1, 2, 3, 4)

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B: (1, 2)

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C: ( )

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D: Something else

def mystery(s):

s[2:] = ( )

From value to storage …

• A variable assigned a compound value (object) is a reference (memory address) to that object.
• Mutable object can be changed but the variable(s) still refer to it

Explanation (board)

From value to storage …

• A variable assigned a compound value (object) is a reference to that object.
• Mutable object can be changed but the variable(s) still refer to it

2

x = [1, 2, 3]

y = 6

x [1] = y

x [1]

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From value to storage …

• A variable assigned a compound value (object) is a reference to that object.
• Mutable object can be changed but the variable(s) still refer to it

Parameter passing: Output?

def test(x):

x = x + 1

y = 10

test(y)

print(y)

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A: 10

B: 11

C: None

D: Error

E: I do not know

Parameter passing: Output?

def test(x):

x[0] = x[0] + 1

y = [1, 2, 3]

test(y)

print(y)

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A: [1, 2, 3]

B: [2, 2, 3]

C: None

D: Error

E: I do not know

What is the output?

var = ([1, 2], [3, 4])

copy = var

var[0][1] = "changed"

var = ([1, "changed"], [3, 4])

print(copy is var)

A : Error

B: None

C: ([1, “changed”], [3, 4])

D: True

E: False

What is the output?

def func(lst):

new_list = lst

new_list[0] = ‘m’

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param = “string”

func(param)

print(param)

A : Error

B: None

C: string

D: mtring

E: None of the above

mic

What is the output?

def func(lst):

new_list = lst

new_list[0] = 'changed'

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param = [1, 2, 3, 4, 5]

func(param)

print(param)

A : Error

B: None

C: [1, 2, 3, 4, 5]

D: [“changed”, 2, 3, 4, 5]

E: None of the above

Examples

x = [1, 2 , 3]

y = x

print (y)

x[1] = 11

print (y)

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x = [1, 2]

y = [x, x, x]

print (y)

x[1] = 3

print (y)

y[2] = [1, 3]

print (y[0] == y[2])

print (y[0] is y[2])

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x = [1, 2 , 3]

y = x

print (y)

x[1] = 11

print (y)

x = [1, 2]

y = [x, x, x]

print (y)

x[1] = 3

print (y)

y[2] = [1, 3]

print (y[0] == y[2])

print (y[0] is y[2])

Copies, ‘is’ and ‘==‘

>>> alist = [1, 2, 3, 4]

>>> alist == [1, 2, 3, 4] # Equal values?

True

>>> alist is [1, 2, 3, 4] # same object?

False

>>> blist = alist # assignment refers

>>> alist is blist # to same object. Shallow copy

True

>>> blist = list(alist) # type constructors copy

>>> blist is alist

False

Copies, ‘is’ and ‘==‘

>>> alist = [1, 2, 3, 4]

>>> alist == [1, 2, 3, 4] # Equal values?

True

False

>>> blist = alist[ : ] #integers

>>> blist is alist

>>> blist

[1, 2, 3, 4]

​

Copies, ‘is’ and ‘==‘

>>> alist = [[1], [2], [3], [4]]

>>> blist = alist[:]

>>> blist[0][0] = 10

>>> alist is blist

False

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But try to print these lists.

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Real deep copy

import copy

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list_a = [[1, 2], [3, 4], [5, 6]]

list_b = copy.deepcopy(list_a)

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# Modify an element in the copied list

list_b[0][0] = 10

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print(list_a) # Output: [[1, 2], [3, 4], [5, 6]]

print(list_b) # Output: [[10, 2], [3, 4], [5, 6]]

Identity Operators: is is not

Identity Operators

Identity Operators

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Identical objects are always equal values

In-place algorithm

• Problem to solve: Given a list of integers. Change every even number to an odd one, so that your function mutates the list.

In-place algorithm

• Problem to solve: Given a list of integers. Change every even number to an odd one, so that your function mutates the list.

def to_odd(lst):

output = []

for i in lst:

if i%2==0:

output.append(i+1)

else:

output.append(i)

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return output

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Not in place!��Another list of the O(n) space is used.

In-place algorithm

• Problem to solve: Given a list of integers. Change every even number to an odd one, so that your function mutates the list.

def to_odd(lst):

for i in range(len(lst)):

if lst[i]%2==0:

lst[i] = lst[i] + 1

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In place!��No other space besides a given list was used.

def to_odd(lst):

output = []

for i in lst:

if i%2==0:

output.append(i+1)

else:

output.append(i)

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return output

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>>> lst = [1,2,3]

>>> to_odd(lst)

>>> print(lst)

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[1, 2, 3] #values are NOT changed

def to_odd(lst):

for i in range(len(lst)):

if lst[i]%2==0:

lst[i] = lst[i] + 1

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>>> lst = [1,2,3]

>>> to_odd(lst)

>>> print(lst)

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[1, 3, 3] #values are changed

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Mutable Default Arguments are Dangerous

• A default argument value is part of a function value, not generated by a call

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Mutable Default Arguments are Dangerous

• A default argument value is part of a function value, not generated by a call

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What will happen if I call f()again?

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A: 1

B: 2

C: 3

D: 0

E: Error

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