Lecture 9: Memory in Python CS 1110 Introduction to Computing - - PowerPoint PPT Presentation

Lecture 9: Memory in Python

CS 1110 Introduction to Computing Using Python

[E. Andersen, A. Bracy, D. Gries, L. Lee, S. Marschner, C. Van Loan, W. White]

http://www.cs.cornell.edu/courses/cs1110/2018sp

Feb 27: CS 1110: Announcements

• Last call for a one-on-one!

§ CMS: OPTIONAL: one-on-ones

• Prelim 1 is March 13. You have until March

1st, 11:59pm to register a conflict or a need for accommodation. There is no single makeup

• session. See website: “Assessment à Exams”

§ CMS: Prelim 1 conflicts

• A1 Revisions: open from Mar 1st thru Mar 7,

11:59pm.

Storage in Python – Round 1

• Global Space

§ What you “start with” § Stores global variables § Lasts until you quit Python

id2

p Global Space

Folders Live on the Heap

p = shapes.Point3(1,2,3) id5

p

id5 Point3

Heap Space Global Space

1 x 2 y 3 z

p lives in the Global Space. Its folder lives on the Heap.

Storage in Python – Round 2

• Global Space

§ What you “start with” § Stores global variables § Lasts until you quit Python

• Heap Space

§ Where “folders” are stored § Have to access indirectly

id2

p Global Space id2 Heap Space

Calling a Function Creates a Call Frame

p = shapes.Point3(1,2,3) incr_x(p) id5

p

id5 Point3

Heap Space Global Space

1 x 2 y 3 z

Call Frame incr_x

1

id5 the_point

What goes in a Call Frame?

p = shapes.Point3(1,2,3) def incr_x(the_point): the_point.x = the_point.x+1 incr_x(p) id5

p

Global Space Call Frame incr_x

1

id5 the_point

(1) Boxes for parameters at the start of the function (2) Boxes for variables local to the function as they are created

Storage in Python – Round 3

• Global Space

§ What you “start with” § Stores global variables § Lasts until you quit Python

• Heap Space

§ Where “folders” are stored § Have to access indirectly

• Call Frames

§ Parameters § Other variables local to function § Lasts until function returns

id2

p Global Space id2 Heap Space f1 f2 Call Frames

Frames and Helper Functions

• Functions can call each other!
• Each call creates a new call frame
• Writing the same several lines of code in 2

places? Or code that accomplishes some conceptual sub-task? Write a helper function! Makes your code easier to:

§ Read § Write § Edit § Debug

9

From before: last_name_first

def last_name_first(n): """Returns: copy of <n> but in the form <last-name>, <first-name> Precondition: <n> is in the form <first-name> <last-name> with one or more blanks between the two names. """ space_index = n.find(' ') first = n[:space_index] last = n[space_index+1:].strip() return last+', '+first

• last_name_first('Haruki Murakami') gives 'Murakami, Haruki'
• last_name_first('Haruki Murakami') gives '

Murakami, Haruki'

1 2 3 4

10

Frames and Helper Functions

def first_name(s): """Prec: see last_name_first""" end = s.find(' ') return s[0:end] def last_name(s): """Prec: see last_name_first""" end = s.rfind(' ') return s[end+1:]

11

4 5 6 7 rfind gets the last instance of substring def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + ',' + first 1 2 3

Drawing Frames for Helper Functions (1)

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + ',' + first def first_name(s): """Prec: see last_name_first""" end = s.find(' ') return s[0:end]

12

1 2 3 4 5

last_name_first 1 'Haruki Murakami'

s

last_name_first('Haruki Murakami')

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + ',' + first def first_name(s): """Prec: see last_name_first""" end = s.find(' ') return s[0:end]

13

1 2 3

last_name_first 1

s

first_name 'Haruki Murakami'

s

4

4 5

'Haruki Murakami'

Not done. Do not erase!

Drawing Frames for Helper Functions (2)

last_name_first('Haruki Murakami')

Drawing Frames for Helper Functions (3)

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + ',' + first def first_name(s): """Prec: see last_name_first""" end = s.find(' ') return s[0:end]

14

1 2 3

last_name_first 1

s

first_name

s end

6 5

4 5

'Haruki Murakami' 'Haruki Murakami'

last_name_first('Haruki Murakami')

Drawing Frames for Helper Functions (4)

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + ',' + first def first_name(s): """Prec: see last_name_first""" end = s.find(' ') return s[0:end]

15

last_name_first 1

1 2 3

s

first_name

s end

6 RETURN 'Haruki'

4 5

'Haruki Murakami' 'Haruki Murakami'

last_name_first('Haruki Murakami')

B:

Question: What Happens Next?

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + ',' + first def first_name(s): """Prec: see last_name_first""" end = s.find(' ') return s[0:end]

last_name_first 1

1 2 3

s

first_name

s end

6 RETURN 'Haruki'

4 5

'Haruki Murakami' 'Haruki Murakami' last_name_first 2

Stuff

A:

ERASE FRAME #2

C: ERASE FRAME #1

ERASE FRAME #2 first_name

Stuff

last_name_first 2

Stuff

last_name_first('Haruki Murakami')

B:

Answer: What Happens Next?

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + ',' + first def first_name(s): """Prec: see last_name_first""" end = s.find(' ') return s[0:end]

last_name_first 1

1 2 3

s

first_name

s end

6 RETURN 'Haruki'

4 5

'Haruki Murakami' 'Haruki Murakami' last_name_first 2

Stuff

A:

ERASE FRAME #2

C: ERASE FRAME #1

ERASE FRAME #2 first_name

Stuff

last_name_first 2

Stuff

• last_name_first('Haruki Murakami')

Drawing Frames for Helper Functions (5)

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + '.' + first def last_name(s): """Prec: see last_name_first""" end = s.rfind(' ') return s[end+1:] 1 2 3

last_name_first 2

s first

6 7

'Haruki Murakami' 'Haruki'

last_name_first('Haruki Murakami')

Drawing Frames for Helper Functions (6)

def last_name_first(s): """Precondition: s in the form <first-name> <last-name>""" first = first_name(s) last = last_name(s) return last + '.' + first def last_name(s): """Prec: see last_name_first""" end = s.rfind(' ') return s[end+1:]

19

1 2 3

last_name_first 2

s

last_name

s first

6

6 7

'Haruki Murakami' 'Haruki' 'Haruki Murakami'

last_name_first('Haruki Murakami')

The Call Stack

• Functions frames are “stacked”

§ Cannot remove one above w/o removing one below

• Python must keep the entire

stack in memory

§ Error if it cannot hold stack (“stack overflow”)

20

function1

calls calls calls calls

function2 function3 function4 function5

Call Stack Example (1)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Call Frame Stack

Call Stack Example (2)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() basic_line 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14 11

Call Frame Stack

Call Stack Example (3)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() basic_line happy_birthday 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14 11

Call Frame Stack

Call Stack Example (4)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() Happy Birthday basic_line happy_birthday

RETURN

None 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14 11

Call Frame Stack

Call Stack Example (5)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() Happy Birthday basic_line 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14 12

Call Frame Stack

Call Stack Example (6)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() Happy Birthday basic_line to_you 6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14 12

Call Frame Stack

Call Stack Example (7)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() Happy Birthday to you basic_line to_you

RETURN

None 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14 12

Call Frame Stack

Call Stack Example (8)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() Happy Birthday to you basic_line

RETURN

None 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 14

Call Frame Stack

Call Stack Example (9)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() Happy Birthday to you 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 15

Call Frame Stack

Call Stack Example (9)

song def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear James”) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() Happy Birthday to you basic_line 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 15

Call Frame Stack

31

Errors and the Call Stack

def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear ”+name) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Happy Birthday to you Happy Birthday to you Happy Birthday Traceback (most recent call last): File "birthday_error.py", line 18, in <module> song() File "birthday_error.py", line 16, in song line_with_name() File "birthday_error.py", line 9, in line_with_name dear() File "birthday_error.py", line 4, in dear print("Dear "+name) NameError: name 'name' is not defined

Make sure you can see line numbers in Komodo. Preferences è Editor

32

Errors and the Call Stack, in Color!

def happy_birthday(): print(“Happy Birthday”) def dear(): print(“Dear ”+name) def to_you(): print(“to you”) def line_with_name(): happy_birthday() dear() def basic_line(): happy_birthday() to_you() def song(): basic_line() basic_line() line_with_name() basic_line() song() 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Happy Birthday to you Happy Birthday to you Happy Birthday Traceback (most recent call last): File "birthday_error.py", line 18, in <module> song() File "birthday_error.py", line 16, in song line_with_name() File "birthday_error.py", line 9, in line_with_name dear() File "birthday_error.py", line 4, in dear print("Dear "+name) NameError: name 'name' is not defined

Where error was found Script code. Global space

33

Functions and Global Space

A function definition

• Creates a global variable (same name as function)
• Creates a folder for body
• Puts folder id in variable

INCHES_PER_FT = 12 def get_feet(ht_in_inches): return ht_in_inches // INCHES_PER_FT Global Space 12 INCHES_PER_FT

See for yourself: https://tinyurl.com/get-feet

id6 get_feet id6

Body

function

Heap Space

Body

Function Definition vs. Call Frame

34

Global Space Call Frame (memory for function call goes here) It’s alive! Heap Space (Function definition goes here)

Modules and Global Space

import math

35

id5 math id5

module

import

• Creates a global variable (same name as module)
• Puts variables, functions in a folder
• Puts folder id in variable

pi 3.141592 e 2.718281 functions

Heap Space Global Space

Modules, Objects, Call Frames (1)

import shapes def incr_x(pt): pt.x = pt.x+1 p = shapes.Point3(1,2,3) incr_x(p)

Narration: Python knows there is a shapes module And it knows where to find it (à on the Heap).

Heap Space Global Space

Modules, Objects, Call Frames (2)

import shapes def incr_x(pt): pt.x = pt.x+1 p = shapes.Point3(1,2,3) incr_x(p)

Narration: Python knows there is a function called incr_x And it knows where to find the definition (à on the Heap).

Heap Space Global Space

Modules, Objects, Call Frames (3)

import shapes def incr_x(pt): pt.x = pt.x+1 p = shapes.Point3(1,2,3) incr_x(p)

Narration: Python just created a point p. p lives in the Global Space. Its folder lives on the Heap.

Heap Space Global Space

Modules, Objects, Call Frames (4)

import shapes def incr_x(pt): pt.x = pt.x+1 p = shapes.Point3(1,2,3) incr_x(p)

Narration: Python just created a call frame for the function incr_x Parameter pt lives in the call frame. Its value is whatever the argument to incr_x was.

Call Frame Heap Space Global Space

Modules, Objects, Call Frames (5)

import shapes def incr_x(pt): pt.x = pt.x+1 p = shapes.Point3(1,2,3) incr_x(p)

Narration: Python is about to execute the first instruction in the function incr_x

Call Frame Heap Space Global Space

Modules, Objects, Call Frames (6)

import shapes def incr_x(pt): pt.x = pt.x+1 p = shapes.Point3(1,2,3) incr_x(p)

Narration: Python just executed the lone instruction in the function incr_x. Since there is no return statement, the return value of NONE is created in the call frame.

Call Frame Heap Space Global Space

Modules, Objects, Call Frames (7)

import shapes def incr_x(pt): pt.x = pt.x+1 p = shapes.Point3(1,2,3) incr_x(p)

Narration: Python just returned from the function incr_x. Its call frame is destroyed because the function is done.

Heap Space Global Space

Storage in Python (Final Version!)

id2

p id2 f1 f2

• Global Space

§ What you “start with” § Stores global variables, modules & functions § Lasts until you quit Python

• Heap Space

§ Where “folders” are stored § Have to access indirectly

• Call Frame Stack

§ Parameters § Other variables local to function § Lasts until function returns

Heap Space Global Space Call Frame Stack

45

Global & Local Variables

print("Welcome to the Star Wars Name Generator!") print("Inputs must be 3 letters or longer. What is your...") first = input("...First name? ") middle = input("...Middle name? ") last = input("...Last name? ") town = input("...Hometown? ") def make_name(name1, name2): name = name1[0:3]+name2[0:3].lower() return name new_first = make_name(first, last) new_last = make_name(middle, town) print("Your Star Wars name is: "+new_first+" "+new_last)

46

Global & Local Variables

print("Welcome to the Star Wars Name Generator!") print("Inputs must be 3 letters or longer. What is your...") first = input("...First name? ") middle = input("...Middle name? ") last = input("...Last name? ") town = input("...Hometown? ") def make_name(name1, name2): name = name1[0:3]+name2[0:3].lower() return name new_first = make_name(first, last) new_last = make_name(middle, town) print("Your Star Wars name is: "+new_first+" "+new_last)

Question: What if these parameters were called first & last?

47

Global & Local Variables

print("Welcome to the Star Wars Name Generator!") print("Inputs must be 3 letters or longer. What is your...") first = input("...First name? ") middle = input("...Middle name? ") last = input("...Last name? ") town = input("...Hometown? ") def make_name(name1, name2): name = name1[0:3]+name2[0:3].lower() return name new_first = make_name(first, last) new_last = make_name(middle, town) print("Your Star Wars name is: "+new_first+" "+new_last)

Question: What if this variable were called first?