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Lecture 7
Memory in Python
Announcements For This Lecture
Readings
Assignment 1
•  Reread Chapter 3
•  No reading for Thursday
Lab
•  Moved to Fri, Sep. 19
§  Worried if I push to Sun.,
people will start too late
§  Resubmit until Sep. 28
•  Work on Assignment 1
•  Posted a Survey in CMS
§  Credit when submit A1
§  Questions on A1
§  Nothing else to do
§  Fill it out when done
9/16/14
Python Memory
2
Modeling Storage in Python
•  Global Space
§ 
§ 
§ 
§ 
Global Space
What you “start with”
Stores global variables
Also modules & functions!
Lasts until you quit Python
p
Call Frame
incr_x
•  Call Frame
q
§  Variables in function call
§  Deleted when call done
id2
Heap Space
id2
•  Heap Space
§  Where “folders” are stored
§  Have to access indirectly
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id2
Python Memory
Point
x
1.0
y
2.0
x
3.0
3
Modeling Storage in Python
•  Global Space
§ 
§ 
§ 
§ 
Global Space
What you “start with”
Stores global variables
Also modules & functions!
Lasts until you quit Python
p
Call Frame
incr_x
•  Call Frame
q
§  Variables in function call
§  Deleted when call done
id2
•  Heap Space
§  Where “folders” are stored
§  Have to access indirectly
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id2
Python Memory
x
id2
Heap rSpace
te
a
l
r
e
v
o
Will c course
Point
his 2.0 x 3.0
1.0in ty
4
Memory and the Python Tutor
Heap
Space
Global
Space
Call Frame
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Python Memory
5
Functions and Global Space
•  A function definition…
def to_centigrade(x):"
§  Creates a global variable
(same name as function)
return 5*(x-32)/9.0
§  Creates a folder for body
Global Space Body
to_centigrade id6
§  Puts folder id in variable
•  Variable vs. Call
Heap Space
>>> to_centigrade
id6
<fun to_centigrade at 0x100498de8>
function
>>> to_centigrade (32)
Body
0.0
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Python Memory
6
Modules and Global Space
•  Importing a module:
§  Creates a global variable
(same name as module)
import math
Global Space math id5
Heap Space
§  Puts contents in a folder
id5
•  Module variables
module
•  Module functions
pi
3.141592
§  Puts folder id in variable
e
2.718281
•  from keyword dumps
contents to global space
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Python Memory
functions
7
Modules vs Objects
Module
math
Object
id2
p
id2
id3
module
pi 3.141592
e 2.718281
functions
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id3
x
y
z
Python Memory
5.0
Point
2.0
3.0
8
Modules vs Objects
Module
math
Object
id2
p
id2
id3
module
pi 3.141592
e 2.718281
functions
math.pi
math.cos(1)
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id3
Python Memory
x
y
z
5.0
Point
2.0
3.0
p.x
p.clamp(-1,1)
9
Modules vs Objects
Module
math
id2
Object
id2
p
id3
s
n
a
e
m
)
.
(
d
o
i
r
e
module
The p
5.0
”
r
x
e
d
l
o
f
e
th
3.141592 “go inside of Point
id3
pi
e 2.718281
functions
math.pi
math.cos(1)
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Python Memory
y
z
2.0
3.0
p.x
p.clamp(-1,1)
10
Recall: Everything is an Object!
•  Including basic values
x
id5
§  int, float, bool, str
•  Example:
id5
>>> x = 2.5
>>> id(x)
float
2.5
•  But basics are immutable
§  Contents cannot change
§  Distinction between value
and identity is immaterial
§  So we can ignore the folder
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Python Memory
x
2.5
11
When Do We Need to Draw a Folder?
Yes •  Variable holds a
§  function
§  module
§  object
§  (more????)
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No
•  Variable holds a §  base type
§  bool, int, float, str
x
id2
id2
Python Memory
float
2.5
12
Recall: Call Frames
1.  Draw a frame for the call
2.  Assign the argument value
to the parameter (in frame)
3.  Execute the function body
Call: to_centigrade(50.0) to_centigrade
x 50.0
§  Look for variables in the frame
§  If not there, look for global
variables with that name
4.  Erase the frame for the call
What is happening here?
def to_centigrade(x):
1
return 5*(x-32)/9.0
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1
Only at the End!
Python Memory
13
Recall: Call Frames
1.  Draw a frame for the call
2.  Assign the argument value
to the parameter (in frame)
3.  Execute the function body
Call: to_centigrade(50.0) to_centigrade
§  Look for variables in the frame
§  If not there, look for global
variables with that name
x 50.0
RETURN 10.0
4.  Erase the frame for the call
def to_centigrade(x):
1
return 5*(x-32)/9.0
9/16/14
Python Memory
14
Recall: Call Frames
1.  Draw a frame for the call
2.  Assign the argument value
to the parameter (in frame)
3.  Execute the function body
Call: to_centigrade(50.0) §  Look for variables in the frame
§  If not there, look for global
variables with that name
4.  Erase the frame for the call
def to_centigrade(x):
1
return 5*(x-32)/9.0
9/16/14
But don’t actually
erase on an exam
Python Memory
15
Aside: What Happens Each Frame Step?
•  The instruction counter always changes
•  The contents only change if
§  You add a new variable
§  You change an existing variable
§  You delete a variable
•  If a variable refers to a mutable object
§  The contents of the folder might change
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Python Memory
16
Call Frames vs. Global Variables
•  This does not work:
1
2
3
def
swap(a,b):
"""Swap vars a & b"""
tmp = a
a = b
b = tmp
Global Variables
a
1
b
2
Call Frame
swap
1
>>> a = 1
>>> b = 2
>>> swap(a,b) 9/16/14
a
Python Memory
1
b
2
17
Call Frames vs. Global Variables
•  The specification is false:
1
2
3
def
swap(a,b):
"""Swap vars a & b"""
tmp = a
a = b
b = tmp
Global Variables
a
b
1
2
Call Frame
swap
2
>>> a = 1
>>> b = 2
>>> swap(a,b) 9/16/14
a
b
1
tmp
Python Memory
2
1
18
Call Frames vs. Global Variables
•  The specification is false:
1
2
3
def
swap(a,b):
"""Swap vars a & b"""
tmp = a
a = b
b = tmp
Global Variables
a
b
1
2
Call Frame
swap
3
>>> a = 1
>>> b = 2
>>> swap(a,b) 9/16/14
a
1
2
✗
tmp
Python Memory
b
2
1
19
Call Frames vs. Global Variables
•  The specification is false:
1
2
3
def
swap(a,b):
"""Swap vars a & b"""
tmp = a
a = b
b = tmp
Global Variables
a
b
1
2
Call Frame
swap
>>> a = 1
>>> b = 2
>>> swap(a,b) 9/16/14
a
1
2
✗
tmp
Python Memory
b
2
1
✗
1
20
Call Frames vs. Global Variables
•  The specification is false:
1
2
3
def
swap(a,b):
"""Swap vars a & b"""
tmp = a
a = b
b = tmp
Global Variables
a
1
b
2
Call Frame
>>> a = 1
>>> b = 2
>>> swap(a,b) 9/16/14
Python Memory
21
Function Access to Global Space
•  All function definitions are in some module
•  Call can access global
space for that module
§  math.cos: global for math
§  temperature.to_centigrade
uses global for temperature
•  But cannot change values
§  Assignment to a global
makes a new local variable!
§  Why we limit to constants
9/16/14
Python Memory
Global Space (for globals.py)
a 4
show_a
1
# globals.py
"""Show how globals work"""
a = 4 # global space
def show_a():
print a # shows global
22
Function Access to Global Space
•  All function definitions are in some module
•  Call can access global
space for that module
§  math.cos: global for math
§  temperature.to_centigrade
uses global for temperature
•  But cannot change values
§  Assignment to a global
makes a new local variable!
§  Why we limit to constants
9/16/14
Python Memory
Global Space (for globals.py)
a 4
change_a
a
3.5
# globals.py
"""Show how globals work"""
a = 4 # global space
def change_a():
a = 3.5 # local variable
23
Call Frames and Objects
•  Mutable objects can be
altered in a function call
§  Object vars hold names!
§  Folder accessed by both
global var & parameter
•  Example:
def incr_x(q):
1
q.x = q.x + 1
>>> p = Point(0,0,0)
>>> incr_x(p)
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Global Space p
id5
Heap Space id5
x 0.0
…
Call Frame
incr_x
q
Python Memory
Point
1
id5
24
Call Frames and Objects
•  Mutable objects can be
altered in a function call
§  Object vars hold names!
§  Folder accessed by both
global var & parameter
•  Example:
def incr_x(q):
1
q.x = q.x + 1
>>> p = Point(0,0,0)
>>> incr_x(p)
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Global Space p
id5
Heap Space id5
Point
x 0.0
✗
1.0
…
Call Frame
incr_x
q
Python Memory
id5
25
Call Frames and Objects
•  Mutable objects can be
altered in a function call
§  Object vars hold names!
§  Folder accessed by both
global var & parameter
•  Example:
def incr_x(q):
1
q.x = q.x + 1
>>> p = Point(0,0,0)
>>> incr_x(p)
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Global Space p
id5
Heap Space id5
Point
x 0.0
✗
1.0
…
Call Frame
Python Memory
26