Lecture 9: Memory in Python

Transcription

1 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]

2 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 1 st thru Mar 7, 11:59pm.

3 Storage in Python Round 1 Global Space What you start with Stores global variables Lasts until you quit Python Global Space p id2

4 Folders Live on the Heap p = shapes.point3(1,2,3) p lives in the Global Space. Its folder lives on the Heap. Global Space p id5 Heap Space id5 Point3 x 1 y 2 z 3

5 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 Global Space p id2 Heap Space id2

6 Calling a Function Creates a Call Frame p = shapes.point3(1,2,3) incr_x(p) Global Space Heap Space p id5 id5 Point3 Call Frame x 1 y 2 z 3 incr_x 1 the_point id5

7 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) (1) Boxes for parameters at the start of the function (2) Boxes for variables local to the function as they are created Global Space p id5 Call Frame incr_x 1 the_point id5

8 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 Global Space Call Frames p f1 f2 id2 Heap Space id2

9 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

10 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' 10

11 Frames and Helper Functions def first_name(s): """Prec: see last_name_first""" 4 end = s.find(' ') 5 return s[0:end] 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""" 6 7 end = s.rfind(' ') return s[end+1:] rfind gets the last instance of substring 11

12 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 last_name_first 1 s 'Haruki Murakami' def first_name(s): """Prec: see last_name_first""" 4 end = s.find(' ') 5 return s[0:end] last_name_first('haruki Murakami') 12

13 Drawing Frames for Helper Functions (2) 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] Not done. Do not erase! last_name_first 1 s 'Haruki Murakami' first_name 4 s 'Haruki Murakami' last_name_first('haruki Murakami') 13

14 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] last_name_first 1 s 'Haruki Murakami' first_name 5 s 'Haruki Murakami' end 6 last_name_first('haruki Murakami') 14

15 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] last_name_first 1 s 'Haruki Murakami' first_name s 'Haruki Murakami' end 6 RETURN 'Haruki' last_name_first('haruki Murakami') 15

16 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] C: last_name_first('haruki Murakami') last_name_first 1 s first_name s 'Haruki Murakami' 'Haruki Murakami' end 6 RETURN 'Haruki' ERASE FRAME #1 ERASE FRAME #2 A: last_name_first 2 B: Stuff ERASE FRAME #2 last_name_first 2 Stuff first_name Stuff

17 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] C: last_name_first('haruki Murakami') last_name_first 1 s first_name s 'Haruki Murakami' 'Haruki Murakami' end 6 RETURN 'Haruki' ERASE FRAME #1 ERASE FRAME #2 A: last_name_first 2 B: Stuff ERASE FRAME #2 last_name_first 2 Stuff first_name Stuff

18 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 last_name_first 2 s first 'Haruki Murakami' 'Haruki' def last_name(s): """Prec: see last_name_first""" 6 end = s.rfind(' ') 7 return s[end+1:] last_name_first('haruki Murakami')

19 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:] last_name_first 2 s 'Haruki Murakami' first 'Haruki' last_name 6 s 'Haruki Murakami' last_name_first('haruki Murakami') 19

20 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 ) function1 function2 function3 function4 function5 calls calls calls calls 20

21 Call Stack Example (1) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 14

22 Call Stack Example (2) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 14 basic_line 11

23 Call Stack Example (3) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 14 basic_line 11 happy_birthday 2

24 def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Stack Example (4) Call Frame Stack song basic_line happy_birthday RETURN None Happy Birthday

25 Call Stack Example (5) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 14 basic_line 12 Happy Birthday

26 Call Stack Example (6) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 14 basic_line 12 to_you 6 Happy Birthday

27 Call Stack Example (7) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 14 basic_line 12 to_you RETURN None Happy Birthday to you

28 Call Stack Example (8) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 14 basic_line RETURN None Happy Birthday to you

29 Call Stack Example (9) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 15 Happy Birthday to you

30 Call Stack Example (9) def : print( Happy Birthday ) def dear(): print( Dear James ) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Call Frame Stack song 15 basic_line 11 Happy Birthday to you

31 Errors and the Call Stack def : print( Happy Birthday ) def dear(): print( Dear +name) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() 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. 31 Preferences è Editor

32 Errors and the Call Stack, in Color! def : print( Happy Birthday ) def dear(): print( Dear +name) def to_you(): print( to you ) def line_with_name(): dear() def : to_you() def song(): line_with_name() song() Script code. Global space 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 32

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 Body Global Space Heap Space INCHES_PER_FT 12 get_feet id6 See for yourself: id6 Body function 33

34 Function Definition vs. Call Frame Global Space Heap Space (Function definition goes here) Call Frame (memory for function call goes here) It s alive! 34

35 Modules and Global Space import Creates a global variable (same name as module) Puts variables, functions in a folder Puts folder id in variable import math Global Space math id5 Heap Space id5 module pi e functions 35

36 Modules, Objects, Call Frames (1) import shapes def incr_x(pt): pt.x = pt.x+1 Global Space Heap Space 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).

37 Modules, Objects, Call Frames (2) import shapes def incr_x(pt): pt.x = pt.x+1 Global Space Heap Space 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).

38 Modules, Objects, Call Frames (3) import shapes def incr_x(pt): pt.x = pt.x+1 Global Space Heap Space 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.

39 Modules, Objects, Call Frames (4) import shapes def incr_x(pt): pt.x = pt.x+1 Global Space Heap Space p = shapes.point3(1,2,3) incr_x(p) Call Frame 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.

40 Modules, Objects, Call Frames (5) import shapes def incr_x(pt): pt.x = pt.x+1 Global Space Heap Space p = shapes.point3(1,2,3) incr_x(p) Call Frame Narration: Python is about to execute the first instruction in the function incr_x

41 Modules, Objects, Call Frames (6) import shapes def incr_x(pt): pt.x = pt.x+1 Global Space Heap Space p = shapes.point3(1,2,3) incr_x(p) Call Frame 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.

42 Modules, Objects, Call Frames (7) import shapes def incr_x(pt): pt.x = pt.x+1 Global Space Heap Space 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.

43 Storage in Python (Final Version!) 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 Global Space Call Frame Stack p f1 f2 id2 Heap Space id2

44 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) 45

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 Question: What if these parameters were called first & last? new_first = make_name(first, last) new_last = make_name(middle, town) print("your Star Wars name is: "+new_first+" "+new_last) 46

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 Question: What if this variable were called first? new_first = make_name(first, last) new_last = make_name(middle, town) print("your Star Wars name is: "+new_first+" "+new_last) 47