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SLIDE 1

Defining Functions

Lecture 4

SLIDE 2

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8/31/17 Defining Functions 2

SLIDE 3

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

§ Komodo Edit to make a module § Python to use the module

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Two different programs

SLIDE 4

We Write Programs to Do Things

Functions are the key doers

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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

SLIDE 5

We Write Programs to Do Things

Functions are the key doers

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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

Function Header

SLIDE 6

We Write Programs to Do Things

Functions are the key doers

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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

Function Header Function Body (indented)

SLIDE 7

We Write Programs to Do Things

Functions are the key doers

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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

Function Header Function Body (indented) declaration of parameter n argument to assign to n

SLIDE 8

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

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Function Header name parameters Docstring Specification Statements to execute when called

SLIDE 9

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

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Function Header name parameters Docstring Specification Statements to execute when called The vertical line indicates indentation

Use vertical lines when you write Python

n exams so we can see indentation

SLIDE 10

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

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SLIDE 11

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)

8/31/17 Defining Functions 11

? x

What is in the box?

SLIDE 12

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)

8/31/17 Defining Functions 12

? x

What is in the box?

A: 2 B: 3 C: 16 D: Nothing! E: I do not know

SLIDE 13

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)

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? x

What is in the box?

A: 2 B: 3 C: 16 D: Nothing! E: I do not know CORRECT

SLIDE 14

Number of statement in the

function body to execute next

Starts with 1

Draw parameters as variables (named boxes)

Understanding How Functions Work

Function Frame: Representation of function call
A conceptual model of Python

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function name local variables (later in lecture) parameters instruction counter

SLIDE 15

Text (Section 3.10) vs. Class

Textbook This Class

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def to_centigrade(x): return 5*(x-32)/9.0

Call: to_centigrade(50.0) Definition:

to_centigrade 1 50.0 x to_centigrade

x –> 50.0

SLIDE 16

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

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def to_centigrade(x): return 5*(x-32)/9.0 to_centigrade 1 x Initial call frame (before exec body) next line to execute 1 50.0

SLIDE 17

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

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

RETURN 10.0

SLIDE 18

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

8/31/17 Defining Functions 18

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

RETURN 10.0

SLIDE 19

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

8/31/17 Defining Functions 19

def to_centigrade(x): return 5*(x-32)/9.0 But don’t actually erase on an exam 1

SLIDE 20

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)

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1 a 2 b 1 2 3 swap 1 1 a 2 b

Global Variables Call Frame

SLIDE 21

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)

8/31/17 Defining Functions 21

1 a 2 b swap 2 1 a 2 b

Global Variables Call Frame

1 tmp 1 2 3

SLIDE 22

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)

8/31/17 Defining Functions 22

1 a 2 b swap 3 1 a 2 b

Global Variables Call Frame

1 tmp 2 1 2 3

x

SLIDE 23

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)

8/31/17 Defining Functions 23

1 a 2 b swap 1 a 2 b

Global Variables Call Frame

1 tmp 2 1 1 2 3

x x

SLIDE 24

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)

8/31/17 Defining Functions 24

1 a 2 b

Global Variables Call Frame

1 2 3

SLIDE 25

Global Space (for globals.py)

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

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get_a 1 4 a

# globals.py """Show how globals work""" a = 4 # global space def get_a(): return a # returns global

SLIDE 26

Global Space (for globals.py)

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

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change_a 3.5 a 4 a

# globals.py """Show how globals work""" a = 4 # global space def change_a(): a = 3.5 # local variable return a

SLIDE 27

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)

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1 2 3

What does the frame look like at the start?

SLIDE 28

Which One is Closest to Your Answer?

A: B:

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C: D:

foo 1 3 a 4 b 3 x foo 1 3 a 4 b x y foo 3 a 4 b foo 1 3 a 4 b

SLIDE 29

Which One is Closest to Your Answer?

A: B:

8/31/17 Defining Functions 29

C: D:

foo 1 3 a 4 b 3 x foo 1 3 a 4 b x y foo 3 a 4 b foo 1 3 a 4 b

E:

¯\_(ツ)_/¯

SLIDE 30

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)

B:

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1 2 3 foo 1 3 a 4 b

SLIDE 31

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)

B:

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1 2 3 foo 1 3 a 4 b

What is the next step?

SLIDE 32

Which One is Closest to Your Answer?

A: B:

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C: D:

foo 2 3 a 4 b 3 x foo 2 3 a 4 b 3 x y foo 2 3 a 4 b foo 1 3 a 4 b 3 x

SLIDE 33

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)

C:

8/31/17 Defining Functions 33

1 2 3 foo 2 3 a 4 b 3 x

SLIDE 34

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)

C:

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1 2 3 foo 2 3 a 4 b 3 x

What is the next step?

SLIDE 35

Which One is Closest to Your Answer?

A: B:

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C: D:

foo 3 3 a 4 b 3 x 4 y foo 3 a 4 b 3 x 4 y

RETURN

3 foo 3 a 4 b 3 x 4 y

RETURN

16

SLIDE 36

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)

A:

8/31/17 Defining Functions 36

1 2 3 foo 3 3 a 4 b 3 x 4 y

SLIDE 37

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)

A:

8/31/17 Defining Functions 37

1 2 3 foo 3 3 a 4 b 3 x 4 y

What is the next step?

SLIDE 38

Which One is Closest to Your Answer?

A: B:

8/31/17 Defining Functions 38

C: D:

foo 3 foo 3 a 4 b 3 x 4 y

RETURN

16 3 foo 3 a 4 b 3 x 4 y

RETURN

16

RETURN

16

SLIDE 39

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)

C:

8/31/17 Defining Functions 39

1 2 3 foo 3 a 4 b 3 x 4 y

RETURN

16

SLIDE 40

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)

C:

8/31/17 Defining Functions 40

1 2 3 foo 3 a 4 b 3 x 4 y

RETURN

16

What is the next step?

SLIDE 41

Which One is Closest to Your Answer?

A: B:

8/31/17 Defining Functions 41

C: D:

16 x foo foo 16 x

RETURN

16

SLIDE 42

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)

D:

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1 2 3 16 x

SLIDE 43

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)

D:

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1 2 3 16 x Variable in global space

SLIDE 44

Visualizing Frames: The Python Tutor

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SLIDE 45

Visualizing Frames: The Python Tutor

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Global Space Call Frame

SLIDE 46

Visualizing Frames: The Python Tutor

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Global Space Call Frame

Variables from second lecture go in here

SLIDE 47

Visualizing Frames: The Python Tutor

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Missing line numbers!

SLIDE 48

Visualizing Frames: The Python Tutor

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

SLIDE 49

Next Time: Concrete Examples

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