Mutable Functions Announcements Mutable Functions A Function with - - PowerPoint PPT Presentation

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Mutable Functions Announcements Mutable Functions A Function with - - PowerPoint PPT Presentation

Jul 07, 2023 219 likes •1.07k views

Mutable Functions Announcements Mutable Functions A Function with Behavior That Varies Over Time Let's model a bank account that has a balance of $100 4 A Function with Behavior That Varies Over Time Let's model a bank account that has a

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SLIDE 1

Mutable Functions

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SLIDE 2

Announcements

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SLIDE 3

Mutable Functions

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SLIDE 4

A Function with Behavior That Varies Over Time

Let's model a bank account that has a balance of $100

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SLIDE 5

A Function with Behavior That Varies Over Time

>>> withdraw(25) Let's model a bank account that has a balance of $100

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SLIDE 6

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 Let's model a bank account that has a balance of $100

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SLIDE 7

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 Let's model a bank account that has a balance of $100 Argument: amount to withdraw

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SLIDE 8

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance

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SLIDE 9

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance

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SLIDE 10

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance

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Second withdrawal of the same amount

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SLIDE 11

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance Different return value!

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Second withdrawal of the same amount

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SLIDE 12

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 >>> withdraw(60) 'Insufficient funds' Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance Different return value!

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Second withdrawal of the same amount

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SLIDE 13

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 >>> withdraw(60) 'Insufficient funds' >>> withdraw(15) 35 Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance Different return value!

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Second withdrawal of the same amount

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SLIDE 14

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 >>> withdraw(60) 'Insufficient funds' >>> withdraw(15) 35 Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance Different return value! Where's this balance stored?

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Second withdrawal of the same amount

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SLIDE 15

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 >>> withdraw(60) 'Insufficient funds' >>> withdraw(15) 35 >>> withdraw = make_withdraw(100) Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance Different return value! Where's this balance stored?

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Second withdrawal of the same amount

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SLIDE 16

A Function with Behavior That Varies Over Time

>>> withdraw(25) 75 >>> withdraw(25) 50 >>> withdraw(60) 'Insufficient funds' >>> withdraw(15) 35 >>> withdraw = make_withdraw(100) Let's model a bank account that has a balance of $100 Argument: amount to withdraw Return value: remaining balance Different return value! Where's this balance stored? Within the parent frame

  • f the function!

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Second withdrawal of the same amount

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SLIDE 17

Persistent Local State Using Environments

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pythontutor.com/composingprograms.html#code=def%20make_withdraw%28balance%29%3A%0A%20%20%20%20def%20withdraw%28amount%29%3A%0A%20%20%20%20%20%20%20%20nonlocal%20balance%0A%20%20%20%20%20%20%20%20if%20amount%20>%20balance%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20'Insufficient%20funds'%0A%20%20%20%20%20%20%20%20balance%20%3D%20balance%20- %20amount%0A%20%20%20%20%20%20%20%20return%20balance%0A%20%20%20%20return%20withdraw%0A%0Awithdraw%20%3D%20make_withdraw%28100%29%0Awithdraw%2825%29%0Awithdraw%2825%29%0Awithdraw%2860%29%0Awithdraw%2815%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 18

Persistent Local State Using Environments

The parent frame contains the balance, the local state of the withdraw function

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pythontutor.com/composingprograms.html#code=def%20make_withdraw%28balance%29%3A%0A%20%20%20%20def%20withdraw%28amount%29%3A%0A%20%20%20%20%20%20%20%20nonlocal%20balance%0A%20%20%20%20%20%20%20%20if%20amount%20>%20balance%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20'Insufficient%20funds'%0A%20%20%20%20%20%20%20%20balance%20%3D%20balance%20- %20amount%0A%20%20%20%20%20%20%20%20return%20balance%0A%20%20%20%20return%20withdraw%0A%0Awithdraw%20%3D%20make_withdraw%28100%29%0Awithdraw%2825%29%0Awithdraw%2825%29%0Awithdraw%2860%29%0Awithdraw%2815%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 19

Persistent Local State Using Environments

The parent frame contains the balance, the local state of the withdraw function Every call decreases the same balance by (a possibly different) amount

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pythontutor.com/composingprograms.html#code=def%20make_withdraw%28balance%29%3A%0A%20%20%20%20def%20withdraw%28amount%29%3A%0A%20%20%20%20%20%20%20%20nonlocal%20balance%0A%20%20%20%20%20%20%20%20if%20amount%20>%20balance%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20'Insufficient%20funds'%0A%20%20%20%20%20%20%20%20balance%20%3D%20balance%20- %20amount%0A%20%20%20%20%20%20%20%20return%20balance%0A%20%20%20%20return%20withdraw%0A%0Awithdraw%20%3D%20make_withdraw%28100%29%0Awithdraw%2825%29%0Awithdraw%2825%29%0Awithdraw%2860%29%0Awithdraw%2815%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 20

Persistent Local State Using Environments

The parent frame contains the balance, the local state of the withdraw function Every call decreases the same balance by (a possibly different) amount

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All calls to the same function have the same parent

pythontutor.com/composingprograms.html#code=def%20make_withdraw%28balance%29%3A%0A%20%20%20%20def%20withdraw%28amount%29%3A%0A%20%20%20%20%20%20%20%20nonlocal%20balance%0A%20%20%20%20%20%20%20%20if%20amount%20>%20balance%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20'Insufficient%20funds'%0A%20%20%20%20%20%20%20%20balance%20%3D%20balance%20- %20amount%0A%20%20%20%20%20%20%20%20return%20balance%0A%20%20%20%20return%20withdraw%0A%0Awithdraw%20%3D%20make_withdraw%28100%29%0Awithdraw%2825%29%0Awithdraw%2825%29%0Awithdraw%2860%29%0Awithdraw%2815%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 21

Reminder: Local Assignment

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pythontutor.com/composingprograms.html#code=def%20percent_difference%28x,%20y%29%3A%0A%20%20%20%20%20%20%20%20difference%20%3D%20abs%28x-y%29%0A%20%20%20%20%20%20%20%20return%20100%20*%20difference%20/%20x%0Adiff%20%3D%20percent_difference%2840,%2050%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=4

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SLIDE 22

Reminder: Local Assignment

Assignment binds name(s) to value(s) in the first frame of the current environment

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pythontutor.com/composingprograms.html#code=def%20percent_difference%28x,%20y%29%3A%0A%20%20%20%20%20%20%20%20difference%20%3D%20abs%28x-y%29%0A%20%20%20%20%20%20%20%20return%20100%20*%20difference%20/%20x%0Adiff%20%3D%20percent_difference%2840,%2050%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=4

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SLIDE 23

Reminder: Local Assignment

Assignment binds name(s) to value(s) in the first frame of the current environment

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pythontutor.com/composingprograms.html#code=def%20percent_difference%28x,%20y%29%3A%0A%20%20%20%20%20%20%20%20difference%20%3D%20abs%28x-y%29%0A%20%20%20%20%20%20%20%20return%20100%20*%20difference%20/%20x%0Adiff%20%3D%20percent_difference%2840,%2050%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=4

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SLIDE 24

Reminder: Local Assignment

Execution rule for assignment statements: Assignment binds name(s) to value(s) in the first frame of the current environment

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pythontutor.com/composingprograms.html#code=def%20percent_difference%28x,%20y%29%3A%0A%20%20%20%20%20%20%20%20difference%20%3D%20abs%28x-y%29%0A%20%20%20%20%20%20%20%20return%20100%20*%20difference%20/%20x%0Adiff%20%3D%20percent_difference%2840,%2050%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=4

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SLIDE 25

Reminder: Local Assignment

Execution rule for assignment statements:

  • 1. Evaluate all expressions right of =, from left to right
  • 2. Bind the names on the left to the resulting values in the current frame

Assignment binds name(s) to value(s) in the first frame of the current environment

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pythontutor.com/composingprograms.html#code=def%20percent_difference%28x,%20y%29%3A%0A%20%20%20%20%20%20%20%20difference%20%3D%20abs%28x-y%29%0A%20%20%20%20%20%20%20%20return%20100%20*%20difference%20/%20x%0Adiff%20%3D%20percent_difference%2840,%2050%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=4

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SLIDE 26

Non-Local Assignment & Persistent Local State

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SLIDE 27

Non-Local Assignment & Persistent Local State

def make_withdraw(balance):

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SLIDE 28

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance."""

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SLIDE 29

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount):

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SLIDE 30

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance

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SLIDE 31

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance:

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SLIDE 32

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance: return 'Insufficient funds'

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SLIDE 33

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance: return 'Insufficient funds' balance = balance - amount

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SLIDE 34

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance: return 'Insufficient funds' balance = balance - amount return balance

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SLIDE 35

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance: return 'Insufficient funds' balance = balance - amount return balance return withdraw

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SLIDE 36

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance: return 'Insufficient funds' balance = balance - amount return balance return withdraw Declare the name "balance" nonlocal at the top of the body of the function in which it is re-assigned

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SLIDE 37

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance: return 'Insufficient funds' balance = balance - amount return balance return withdraw Declare the name "balance" nonlocal at the top of the body of the function in which it is re-assigned Re-bind balance in the first non-local frame in which it was bound previously

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SLIDE 38

Non-Local Assignment & Persistent Local State

def make_withdraw(balance): """Return a withdraw function with a starting balance.""" def withdraw(amount): nonlocal balance if amount > balance: return 'Insufficient funds' balance = balance - amount return balance return withdraw Declare the name "balance" nonlocal at the top of the body of the function in which it is re-assigned Re-bind balance in the first non-local frame in which it was bound previously

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(Demo)

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SLIDE 39

Non-Local Assignment

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SLIDE 40

The Effect of Nonlocal Statements

nonlocal <name>

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SLIDE 41

The Effect of Nonlocal Statements

Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>

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SLIDE 42

The Effect of Nonlocal Statements

Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name> Python Docs: an "enclosing scope"

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SLIDE 43

The Effect of Nonlocal Statements

Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>, <name>, ... Python Docs: an "enclosing scope"

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SLIDE 44

The Effect of Nonlocal Statements

From the Python 3 language reference: Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>, <name>, ... Python Docs: an "enclosing scope"

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SLIDE 45

The Effect of Nonlocal Statements

From the Python 3 language reference: Names listed in a nonlocal statement must refer to pre-existing bindings in an enclosing scope. Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>, <name>, ... Python Docs: an "enclosing scope"

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SLIDE 46

The Effect of Nonlocal Statements

From the Python 3 language reference: Names listed in a nonlocal statement must refer to pre-existing bindings in an enclosing scope. Names listed in a nonlocal statement must not collide with pre-existing bindings in the local scope. Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>, <name>, ... Python Docs: an "enclosing scope"

9

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SLIDE 47

The Effect of Nonlocal Statements

From the Python 3 language reference: Names listed in a nonlocal statement must refer to pre-existing bindings in an enclosing scope. Names listed in a nonlocal statement must not collide with pre-existing bindings in the local scope. Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>, <name>, ... Python Docs: an "enclosing scope"

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Current frame

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SLIDE 48

The Effect of Nonlocal Statements

From the Python 3 language reference: Names listed in a nonlocal statement must refer to pre-existing bindings in an enclosing scope. Names listed in a nonlocal statement must not collide with pre-existing bindings in the local scope.

https://docs.python.org/3/reference/simple_stmts.html#the-nonlocal-statement

Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>, <name>, ... Python Docs: an "enclosing scope"

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Current frame

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SLIDE 49

The Effect of Nonlocal Statements

http://www.python.org/dev/peps/pep-3104/

From the Python 3 language reference: Names listed in a nonlocal statement must refer to pre-existing bindings in an enclosing scope. Names listed in a nonlocal statement must not collide with pre-existing bindings in the local scope.

https://docs.python.org/3/reference/simple_stmts.html#the-nonlocal-statement

Effect: Future assignments to that name change its pre-existing binding in the first non-local frame of the current environment in which that name is bound. nonlocal <name>, <name>, ... Python Docs: an "enclosing scope"

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Current frame

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SLIDE 50

The Many Meanings of Assignment Statements

x = 2

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SLIDE 51

The Many Meanings of Assignment Statements

x = 2 Status Effect

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SLIDE 52

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

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SLIDE 53

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

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SLIDE 54

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

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SLIDE 55

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

Re-bind name "x" to object 2 in the first frame

  • f the current environment

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SLIDE 56

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

Re-bind name "x" to object 2 in the first frame

  • f the current environment
  • nonlocal x
  • "x" is bound in a non-local

frame

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SLIDE 57

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

Re-bind name "x" to object 2 in the first frame

  • f the current environment
  • nonlocal x
  • "x" is bound in a non-local

frame Re-bind "x" to 2 in the first non-local frame of the current environment in which "x" is bound

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SLIDE 58

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

Re-bind name "x" to object 2 in the first frame

  • f the current environment
  • nonlocal x
  • "x" is not bound in a non-

local frame

  • nonlocal x
  • "x" is bound in a non-local

frame Re-bind "x" to 2 in the first non-local frame of the current environment in which "x" is bound

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SLIDE 59

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

Re-bind name "x" to object 2 in the first frame

  • f the current environment
  • nonlocal x
  • "x" is not bound in a non-

local frame SyntaxError: no binding for nonlocal 'x' found

  • nonlocal x
  • "x" is bound in a non-local

frame Re-bind "x" to 2 in the first non-local frame of the current environment in which "x" is bound

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SLIDE 60

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

Re-bind name "x" to object 2 in the first frame

  • f the current environment
  • nonlocal x
  • "x" is bound in a

non-local frame

  • "x" also bound locally
  • nonlocal x
  • "x" is not bound in a non-

local frame SyntaxError: no binding for nonlocal 'x' found

  • nonlocal x
  • "x" is bound in a non-local

frame Re-bind "x" to 2 in the first non-local frame of the current environment in which "x" is bound

10

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SLIDE 61

The Many Meanings of Assignment Statements

x = 2 Status Effect

  • No nonlocal statement
  • "x" is not bound locally

Create a new binding from name "x" to object 2 in the first frame of the current environment

  • No nonlocal statement
  • "x" is bound locally

Re-bind name "x" to object 2 in the first frame

  • f the current environment
  • nonlocal x
  • "x" is bound in a

non-local frame

  • "x" also bound locally

SyntaxError: name 'x' is parameter and nonlocal

  • nonlocal x
  • "x" is not bound in a non-

local frame SyntaxError: no binding for nonlocal 'x' found

  • nonlocal x
  • "x" is bound in a non-local

frame Re-bind "x" to 2 in the first non-local frame of the current environment in which "x" is bound

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SLIDE 62

Python Particulars

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SLIDE 63

Python Particulars

Python pre-computes which frame contains each name before executing the body of a function.

11

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SLIDE 64

Python Particulars

Python pre-computes which frame contains each name before executing the body of a function. Within the body of a function, all instances of a name must refer to the same frame.

11

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SLIDE 65

Python Particulars

Python pre-computes which frame contains each name before executing the body of a function. Within the body of a function, all instances of a name must refer to the same frame.

11

pythontutor.com/ composingprograms.html#code=def%20make_withdraw%28balance%29%3A%0A%20%20%20%20def%20withdraw%28amount%29%3A%0A%20%20%20%20%20%20%20%20if%20amount%20>%20balance%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20'Insufficient%20funds'%0A%20%20%20%20%20%20%20%20balance%20%3D%20balance%20- %20amount%0A%20%20%20%20%20%20%20%20return%20balance%0A%20%20%20%20return%20withdraw%0A%0Awithdraw%20%3D%20make_withdraw%2820%29%0Awithdraw%285%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 66

Python Particulars

Python pre-computes which frame contains each name before executing the body of a function. Within the body of a function, all instances of a name must refer to the same frame. Local assignment

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SLIDE 67

Python Particulars

Python pre-computes which frame contains each name before executing the body of a function. Within the body of a function, all instances of a name must refer to the same frame. Local assignment

11

pythontutor.com/ composingprograms.html#code=def%20make_withdraw%28balance%29%3A%0A%20%20%20%20def%20withdraw%28amount%29%3A%0A%20%20%20%20%20%20%20%20if%20amount%20>%20balance%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20'Insufficient%20funds'%0A%20%20%20%20%20%20%20%20balance%20%3D%20balance%20- %20amount%0A%20%20%20%20%20%20%20%20return%20balance%0A%20%20%20%20return%20withdraw%0A%0Awithdraw%20%3D%20make_withdraw%2820%29%0Awithdraw%285%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 68

Mutable Values & Persistent Local State

Mutable values can be changed without a nonlocal statement.

12

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SLIDE 69

Mutable Values & Persistent Local State

Mutable values can be changed without a nonlocal statement.

12

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SLIDE 70

Mutable Values & Persistent Local State

Mutable values can be changed without a nonlocal statement.

12

Name bound

  • utside of

withdraw def

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SLIDE 71

Mutable Values & Persistent Local State

Mutable values can be changed without a nonlocal statement.

12

Name bound

  • utside of

withdraw def Element assignment changes a list

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SLIDE 72

Mutable Values & Persistent Local State

Mutable values can be changed without a nonlocal statement.

12

Name bound

  • utside of

withdraw def Element assignment changes a list

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SLIDE 73

Mutable Values & Persistent Local State

Mutable values can be changed without a nonlocal statement. Name-value binding cannot change because there is no nonlocal statement

12

Name bound

  • utside of

withdraw def Element assignment changes a list

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SLIDE 74

Mutable Values & Persistent Local State

Mutable values can be changed without a nonlocal statement. Name-value binding cannot change because there is no nonlocal statement Mutable value can change

12

Name bound

  • utside of

withdraw def Element assignment changes a list

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SLIDE 75

Multiple Mutable Functions

(Demo)

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SLIDE 76

Referential Transparency, Lost

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 77

Referential Transparency, Lost

  • Expressions are referentially transparent if substituting an expression with its value

does not change the meaning of a program.

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 78

Referential Transparency, Lost

  • Expressions are referentially transparent if substituting an expression with its value

does not change the meaning of a program. mul(add(2, mul(4, 6)), add(3, 5))

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 79

Referential Transparency, Lost

  • Expressions are referentially transparent if substituting an expression with its value

does not change the meaning of a program. mul(add(2, mul(4, 6)), add(3, 5)) mul(add(2, 24 ), add(3, 5))

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 80

Referential Transparency, Lost

  • Expressions are referentially transparent if substituting an expression with its value

does not change the meaning of a program. mul(add(2, mul(4, 6)), add(3, 5)) mul(add(2, 24 ), add(3, 5)) mul( 26 , add(3, 5))

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 81

Referential Transparency, Lost

  • Expressions are referentially transparent if substituting an expression with its value

does not change the meaning of a program.

  • Mutation operations violate the condition of referential transparency because they do

more than just return a value; they change the environment. mul(add(2, mul(4, 6)), add(3, 5)) mul(add(2, 24 ), add(3, 5)) mul( 26 , add(3, 5))

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 82

Referential Transparency, Lost

  • Expressions are referentially transparent if substituting an expression with its value

does not change the meaning of a program.

  • Mutation operations violate the condition of referential transparency because they do

more than just return a value; they change the environment. mul(add(2, mul(4, 6)), add(3, 5)) mul(add(2, 24 ), add(3, 5)) mul( 26 , add(3, 5))

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 83

Referential Transparency, Lost

  • Expressions are referentially transparent if substituting an expression with its value

does not change the meaning of a program.

  • Mutation operations violate the condition of referential transparency because they do

more than just return a value; they change the environment. mul(add(2, mul(4, 6)), add(3, 5)) mul(add(2, 24 ), add(3, 5)) mul( 26 , add(3, 5))

14

pythontutor.com/ composingprograms.html#code=def%20f%28x%29%3A%0A%20%20%20%20x%20%3D%204%0A%20%20%20%20def%20g%28y%29%3A%0A%20%20%20%20%20%20%20%20def%20h%28z%29%3A%0A%20%20%20%20%20%20%20%20%20%20%20%20nonlocal%20x%0A%20%20%20%20%20%20%20%20%20%20%20%20x%20%3D%20x%20%2B%201%0A%20%20%20%20%20%20%20%20%20%20%20%20return%20x%20%2B%20y%20%2B%20z%0A%20%20%20%20%20%20%20%20return%20h%0A%20%20%20%20return%20g%0Aa%20 %3D%20f%281%29%0Ab%20%3D%20a%282%29%0Atotal%20%3D%20b%283%29%20%2B%20b%284%29&mode=display&origin=composingprograms.js&cumulative=true&py=3&rawInputLstJSON=[]&curInstr=0

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SLIDE 84

Review Problem