Python Course Lecture Defining Functions

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• Reminder: Assignments can be done in pairs
§ Can work with anyone in class (in any section)
§ Can change partners every assignment
• Having a mixer to help find a partner
§ 4-6pm Tuesday, September 6th
§ Upson Lounge (adjacent to Duffield)
• Please register if you are coming
§ Need to know how much food to have
§ Link will be posted on Ed Discussions
9/1/22 Defining Functions 2

Announcement: Partner Finding Social
• Reminder: Assignments can be done in pairs
§ Can work with anyone in class (in any section)
§ Can change partners every assignment
• Having a mixer to help find a partner
§ 4-6pm Tuesday, September 6th
§ Upson Lounge (adjacent to Duffield)
• Please register if you are coming
§ Need to know how much food to have
§ Link will be posted on Ed Discussions

Academic Integrity Quiz
• Remember: quiz about the course AI policy
§ Have posted grades for completed quizes
§ Right now, missing ~40 enrolled students
§ If did not receive 9/10 (~124 students), take it again
• If you are not aware of the quiz
§ Go to http://www.cs.cornell.edu/courses/cs1110/
§ Click Academic Integrity under Assessments
§ Read and take quiz in CMS

Recall: Modules
• Modules provide extra functions, variables
§ Example: math provides math.cos(), math.pi
§ Access them with the import command
• Python provides a lot of them for us
• This Lecture: How to make modules
§ VS Code to make a module
§ Python to use the module
Two different
programs

We Write Programs to Do Things
• Functions are the key doers
Function Call Function Definition
• Command to do the function
>>> plus(23)
24
>>>
• Defines what function does
def plus(n):
return n+1
• Parameter: variable that is listed within
the parentheses of a method header.
• Argument: a value to assign to the method
parameter when it is called

>>> plus(23)
24
>>>
def plus(n):
return n+1
Function
Header

>>> plus(23)
24
>>>
def plus(n):
return n+1
Function
Header Function
Body
(indented)

>>> plus(23)
24
>>>
def plus(n):
return n+1
Function
Header Function
Body
(indented)
declaration of
parameter n
argument to
assign to n

Anatomy of a Function Definition
def plus(n):
"""Returns the number n+1
Parameter n: number to add to
Precondition: n is a number"""
x = n+1
return x
Function Header
name parameters
Docstring
Specification
Statements to
execute when called

Anatomy of a Function Definition
def plus(n):
"""Returns the number n+1
Parameter n: number to add to
Precondition: n is a number"""
x = n+1
return x
Function Header
name parameters
Docstring
Specification
Statements to
execute when called
The vertical line
indicates indentation
Use vertical lines when you write Python
on exams so we can see indentation

The return Statement
• Format: return <expression>
§ Used to evaluate function call (as an expression)
§ Also stops executing the function!
§ Any statements after a return are ignored
• Example: temperature converter function
def to_centigrade(x):
"""Returns: x converted to centigrade"""
return 5*(x-32)/9.0

A More Complex Example
Function Definition
def foo(a,b):
"""Return something
Param a: number
Param b: number"""
x = a
y = b
return x*y+y
Function Call
>>> x = 2
>>> foo(3,4)
?
x
What is in the box?

Function Definition
def foo(a,b):
"""Return something
Param a: number
Param b: number"""
x = a
y = b
return x*y+y
Function Call
>>> x = 2
>>> foo(3,4)
?
x
What is in the box?
A: 2
B: 3
C: 16
D: Nothing!
E: I do not know

Function Definition
def foo(a,b):
"""Return something
Param a: number
Param b: number"""
x = a
y = b
return x*y+y
Function Call
>>> x = 2
>>> foo(3,4)
?
x
What is in the box?
A: 2
B: 3
C: 16
D: Nothing!
E: I do not know
CORRECT

• Number of statement in the
function body to execute next
• Starts with line no. of body
Draw parameters
as variables
(named boxes)
Understanding How Functions Work
• Function Frame: Representation of function call
• A conceptual model of Python
function name
local variables (later in lecture)
parameters
instruction counter

Example: to_centigrade(50.0)
1. Draw a frame for the call
2. Assign the argument value
to the parameter (in frame)
3. Execute the function body
§ Look for variables in the frame
§ If not there, look for global
variables with that name
4. Erase the frame for the call
return 5*(x-32)/9.0
to_centigrade 2
x
Initial call frame
(before exec body)
next line to execute
50.0
2
1

return 5*(x-32)/9.0
to_centigrade
50.0
x
Executing the
return statement
Return statement creates a
special variable for result
RETURN 10.0
2
1

return 5*(x-32)/9.0
to_centigrade
50.0
x
Executing the
return statement
The return terminates;
no next line to execute
RETURN 10.0
2
1

return 5*(x-32)/9.0
ERASE
W
H
O
LE
FRAM
E
But don’t actually
erase on an exam
2
1

Call Frames vs. Global Variables
The specification is a lie:
def swap(a,b):
"""Swap global a & b"""
tmp = a
a = b
b = tmp
>>> a = 1
>>> b = 2
>>> swap(a,b)
1
a 2
b
swap 3
1
a 2
b
Global Variables
Call Frame
5
4
3
2
1

def swap(a,b):
tmp = a
a = b
b = tmp
>>> a = 1
>>> b = 2
>>> swap(a,b)
1
a 2
b
swap 4
1
a 2
b
Global Variables
Call Frame
1
tmp
5
4
3
2
1

def swap(a,b):
tmp = a
a = b
b = tmp
>>> a = 1
>>> b = 2
>>> swap(a,b)
1
a 2
b
swap 5
1
a 2
b
Global Variables
Call Frame
1
tmp
2
x
5
4
3
2
1

def swap(a,b):
tmp = a
a = b
b = tmp
>>> a = 1
>>> b = 2
>>> swap(a,b)
1
a 2
b
swap
1
a 2
b
Global Variables
Call Frame
1
tmp
2 1
x x
5
4
3
2
1

def swap(a,b):
tmp = a
a = b
b = tmp
>>> a = 1
>>> b = 2
>>> swap(a,b)
1
a 2
b
Global Variables
Call Frame
ERASE THE FRAME
5
4
3
2
1

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
Param y: a number"""
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
5
6
7
What does the
frame look like
at the start?
4
3
2
1

Which One is Closest to Your Answer?
A: B:
C: D:
foo 5
3
a
foo 5
3
a 4
b
x y
foo 4
3
a 4
b
foo 5
3
a 4
b

A: B:
C: D:
foo 5
3
a
foo 5
3
a 4
b
x y
foo 4
3
a 4
b
foo 5
3
a 4
b
E:
¯_(ツ)_/¯

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
B:
5
6
7
4
3
2
1
foo 5
3
a 4
b

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
B:
5
6
7
4
3
2
1
foo 5
3
a 4
b
What is the next step?

A: B:
C: D:
foo 6
3
a 4
b
3
x
foo 6
3
a 4
b
3
x y
foo 6
3
a 4
b
foo 5
3
a 4
b
3
x

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
C:
foo 6
3
a 4
b
3
x
5
6
7
4
3
2
1

A: B:
C: D:
foo 7
3
a 4
b
3
x 4
y
foo
3
a 4
b
3
x 4
y
RETURN
7
foo
3
a 4
b
3
x 4
y
RETURN 16
ERASE THE FRAME

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
A:
foo 7
3
a 4
b
3
x 4
y
5
6
7
4
3
2
1

A: B:
C: D:
foo 7 foo
3
a 4
b
3
x 4
y
RETURN 16
7
foo
3
a 4
b
3
x 4
y
RETURN 16
ERASE THE FRAME
RETURN 16

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
C:
foo
3
a 4
b
3
x 4
y
RETURN 16
5
6
7
4
3
2
1

A: B:
C: D:
16
x
foo
foo
16
x
RETURN 16
ERASE THE FRAME
ERASE THE FRAME

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
D:
16
x
ERASE THE FRAME
5
6
7
4
3
2
1

Exercise Time
Function Definition
def foo(a,b):
"""Return something
Param x: a number
x = a
y = b
return x*y+y
Function Call
>>> x = foo(3,4)
D:
16
x
ERASE THE FRAME
Variable in
global space
5
6
7
4
3
2
1

Visualizing Frames: The Python Tutor

Global
Space
Call Frame

Global
Space
Call Frame
Variables from
second lecture
go in here

Missing line
numbers!

Missing line
numbers!
Line number
marked here
(sort-of)

Next Time: Text Processing

Python Course Lecture Defining Functions

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