Control · 2021. 1. 6. · Control. Announcements. Print and None (Demo) None Indicates that...

Control

Announcements

Print and None

(Demo)

None Indicates that Nothing is Returned

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The special value None represents nothing in Python

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A function that does not explicitly return a value will return None

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Careful: None is not displayed by the interpreter as the value of an expression

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4

>>> def does_not_return_square(x):

... x * x

...





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... x * x

... No return





4


... x * x

...

>>> does_not_return_square(4)

No return





4


... x * x

...


No return

None value is not displayed





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... x * x

...


>>> sixteen = does_not_return_square(4)

No return






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... x * x

...


>>> sixteen = does_not_return_square(4)The name sixteen is now bound to the value None

No return






4


... x * x

...


>>> sixteen = does_not_return_square(4)

>>> sixteen + 4

Traceback (most recent call last):

File "", line 1, in

TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

The name sixteen is now bound to the value None

No return


Pure Functions & Non-Pure Functions

Pure Functions just return values

Non-Pure Functions have side effects

5

abs




5

abs


-2Pure Functions just return values


5

abs


-22



5

abs


-22



Argument

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abs


-22



Argument

Return value

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abs


-22

pow



Argument

Return value

5

abs


-22

2, 100 pow



Argument

Return value

5

abs


-22

2, 100 pow



Argument

Return value

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2 Arguments

abs


-22

2, 1001267650600228229401496703205376

pow



Argument

Return value

5

2 Arguments

abs


-22

print

2, 1001267650600228229401496703205376

pow



Argument

Return value

5

2 Arguments

abs


-22

-2 print

2, 1001267650600228229401496703205376

pow



Argument

Return value

5

2 Arguments

abs


-22

-2None

print

2, 1001267650600228229401496703205376

pow



Argument

Return value

5

2 Arguments

abs


-22

-2None

print

Python displays the output “-2”

2, 1001267650600228229401496703205376

pow



Argument

Return value

5

2 Arguments

abs


-22

-2None

print


2, 1001267650600228229401496703205376

pow



Argument

Return value

Returns None!

5

2 Arguments

abs


-22

-2None

print


2, 1001267650600228229401496703205376

pow



Argument

Return value

A side effect isn't a value; it's anything that happens as a consequence of

calling a function

Returns None!

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2 Arguments

Nested Expressions with Print

6


print(print(1), print(2))

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func print(...)

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func print(...)print(1)

func print(...) 1

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func print(...)

print(...):1None

display “1”

print(1)

func print(...) 1

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func print(...)

print(...):1None

display “1”

print(1)

func print(...) 1

None

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func print(...)

print(...):1None

display “1”

print(1)

func print(...) 1

Noneprint(2)

2

6

func print(...)



func print(...)

print(...):1None

display “1”

print(...):2None

display “2”

print(1)

func print(...) 1

Noneprint(2)

2

6

func print(...)



func print(...)

print(...):1None

display “1”

print(...):2None

display “2”

print(1)

func print(...) 1

Noneprint(2)

2

None

6

func print(...)



func print(...)

print(...):1None

display “1”

print(...):2None

display “2”

print(...):None, NoneNone

display “None None”

print(1)

func print(...) 1

Noneprint(2)

2

None

6

func print(...)


Noneprint(print(1), print(2))

func print(...)

print(...):1None

display “1”

print(...):2None

display “2”



print(1)

func print(...) 1

Noneprint(2)

2

None

6

func print(...)


Noneprint(print(1), print(2))

func print(...)

print(...):1None

display “1”

print(...):2None

display “2”



print(1)

func print(...) 1

Noneprint(2)

2

None

6

Does not get displayed

func print(...)

Multiple Environments

Life Cycle of a User-Defined Function

Def statement:

Call expression:

Calling/Applying:

What happens?

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Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

What happens?

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

What happens?

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

Formal parameter What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

Formal parameter

Body

What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

Formal parameter

Body (return statement)

What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)

What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)

A new function is created!

What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)


Name bound to that function in the current frame

What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)



What happens?

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)



What happens?

operator: square function: func square(x)

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)



operand: 2+2 argument: 4

What happens?


Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)




Operator & operands evaluated

What happens?


Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)

Calling/Applying:

Def statement

Formal parameter

Body

Return expression

(return statement)





Function (value of operator) called on arguments (values of operands)

What happens?


Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)

Calling/Applying: square( x ):

Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

4

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

4

16

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

4

16Argument

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

4

16Argument

Return value

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

4

16

A new frame is created!

Argument

Return value

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

4

16


Parameters bound to argumentsArgument

Return value

Name

8


Def statement:

Call expression:

square( x ):

return mul(x, x)

>>> def

square(2+2)


Def statement

Formal parameter

Body

Return expression

(return statement)






What happens?


Signature

4

16


Parameters bound to arguments

Body is executed in that new environment

Argument

Return value

Name

8

Multiple Environments in One Diagram!

9http://pythontutor.com/composingprograms.html#code=from%20operator%20import%20mul%0Adef%20square%28x%29%3A%0A%20%20%20%20return%20mul%28x,%20x%29%0Asquare%28square%283%29%29&cumulative=true&curInstr=0&mode=display&origin=composingprograms.js&py=3&rawInputLstJSON=%5B%5D


square(square(3))



square(square(3))

func square(x)



square(square(3))

square(3)func square(x)



square(square(3))

square(3)func square(x)

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func square(x)

http://pythontutor.com/composingprograms.html#code=from%20operator%20import%20mul%0Adef%20square%28x%29%3A%0A%20%20%20%20return%20mul%28x,%20x%29%0Asquare%28square%283%29%29&cumulative=true&curInstr=0&mode=display&origin=composingprograms.js&py=3&rawInputLstJSON=%5B%5D


square(square(3))

square(3)

3

func square(x)

9

func square(x)



square(square(3))

square(3)

3

func square(x)

10

func square(x)



square(square(3))

square(3)9

3

func square(x)

10

func square(x)



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square(square(3))

square(3)9

3

func square(x)

func square(x)

81



An environment is a sequence of frames.

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square(square(3))

square(3)9

3

func square(x)

func square(x)

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• The global frame alone • A local, then the global frame

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square(square(3))

square(3)9

3

func square(x)

func square(x)

81




1


11

square(square(3))

square(3)9

3

func square(x)

func square(x)

81




1

2

1


11

square(square(3))

square(3)9

3

func square(x)

func square(x)

81




1

2

1

2

1


11

square(square(3))

square(3)9

3

func square(x)

func square(x)

81


Names Have No Meaning Without Environments



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1

2

1

2

1





12

Every expression is evaluated in the context of an environment.

1

2

1

2

1





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A name evaluates to the value bound to that name in the earliest frame of the current environment in which that name is found.

1

2

1

2

1





12



2

1


Names Have Different Meanings in Different Environments

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http://pythontutor.com/composingprograms.html#code=from%20operator%20import%20mul%0Adef%20square%28square%29%3A%0A%20%20%20%20return%20mul%28square,%20square%29%0Asquare%284%29&cumulative=true&curInstr=0&mode=display&origin=composingprograms.js&py=3&rawInputLstJSON=%5B%5D


13

A call expression and the body of the function being called are evaluated in different environments





13

1






13

1

2

1





Miscellaneous Python Features

Division Multiple Return Values

Source Files Doctests

Default Arguments

(Demo)

Conditional Statements

Statements

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A statement is executed by the interpreter to perform an action

: ... : ... ...

Compound statements:

Statements

16


: ... : ... ...


Statements

Statement

16


: ... : ... ...


Statements

StatementClause

16


: ... : ... ...


Statements

Statement

Suite

Clause

16


: ... : ... ...


Statements

Statement

Suite

ClauseThe first header determines a statement’s type

16


: ... : ... ...


Statements

Statement

Suite


The header of a clause “controls” the suite that follows

16


: ... : ... ...


Statements

Statement

Suite


The header of a clause “controls” the suite that follows

def statements are compound statements

16


Compound Statements


: ... : ... ...

Suite

17

Compound Statements


: ... : ... ...

Suite

A suite is a sequence of statements

17

Compound Statements


: ... : ... ...

Suite


To “execute” a suite means to execute its sequence of statements, in order

17

Compound Statements


: ... : ... ...

Execution Rule for a sequence of statements:

• Execute the first statement

• Unless directed otherwise, execute the rest

Suite


To “execute” a suite means to execute its sequence of statements, in order

17


18


18

def absolute_value(x): """Return the absolute value of x.""" if x < 0: return -x elif x == 0: return 0 else: return x


1 statement, 3 clauses, 3 headers, 3 suites

18




18


Execution Rule for Conditional Statements:



Each clause is considered in order.

1. Evaluate the header's expression.

2. If it is a true value, execute the suite & skip the remaining clauses.

18








18

Syntax Tips:








18

Syntax Tips:

1. Always starts with "if" clause.

2. Zero or more "elif" clauses.

3. Zero or one "else" clause, always at the end.



Boolean Contexts

19


George Boole


Boolean Contexts

George Boole

20


Boolean Contexts

George Boole

20

Two boolean contextsTwo boolean contexts


Boolean Contexts

False values in Python: False, 0, '', None

George Boole

20



Boolean Contexts

False values in Python: False, 0, '', None (more to come)

George Boole

20



Boolean Contexts


True values in Python: Anything else (True)

(more to come)

George Boole

20



Boolean Contexts



(more to come)

George Boole

Read Section 1.5.4!

20


Reading: http://composingprograms.com/pages/15-control.html#conditional-statements


Boolean Contexts



(more to come)

George Boole

Read Section 1.5.4!

20


Reading: http://composingprograms.com/pages/15-control.html#conditional-statements

(Demo)

Iteration

While Statements

22

(Demo)

While Statements

1. Evaluate the header’s expression.

2. If it is a true value, execute the (whole) suite, then return to step 1.

22

(Demo)

Execution Rule for While Statements:

George Boole

While Statements



22

(Demo)


George Boole

While Statements



1

22

(Demo)


George Boole

While Statements



11

22

(Demo)


George Boole

While Statements



1 21

22

(Demo)


George Boole

While Statements



1 21 3

22

(Demo)


George Boole

While Statements



1 2 31 3

22

(Demo)


George Boole

While Statements



1 2 31 3 6

22

(Demo)


George Boole

While Statements



1 2 31 3 6

22

(Demo)


(Demo)

Example: Prime Factorization

Prime Factorization

24

Prime Factorization

Each positive integer n has a set of prime factors: primes whose product is n

24

Prime Factorization


... 8 = 2 * 2 * 2 9 = 3 * 3 10 = 2 * 5 11 = 11 12 = 2 * 2 * 3 ...

24

Prime Factorization


... 8 = 2 * 2 * 2 9 = 3 * 3 10 = 2 * 5 11 = 11 12 = 2 * 2 * 3 ...

One approach: Find the smallest prime factor of n, then divide by it

24

Prime Factorization


... 8 = 2 * 2 * 2 9 = 3 * 3 10 = 2 * 5 11 = 11 12 = 2 * 2 * 3 ...


24

858

Prime Factorization


... 8 = 2 * 2 * 2 9 = 3 * 3 10 = 2 * 5 11 = 11 12 = 2 * 2 * 3 ...


24

858 = 2 * 429

Prime Factorization


... 8 = 2 * 2 * 2 9 = 3 * 3 10 = 2 * 5 11 = 11 12 = 2 * 2 * 3 ...


24

858 = 2 * 429 = 2 * 3 * 143

Prime Factorization


... 8 = 2 * 2 * 2 9 = 3 * 3 10 = 2 * 5 11 = 11 12 = 2 * 2 * 3 ...


24

858 = 2 * 429 = 2 * 3 * 143 = 2 * 3 * 11 * 13

Prime Factorization


... 8 = 2 * 2 * 2 9 = 3 * 3 10 = 2 * 5 11 = 11 12 = 2 * 2 * 3 ...


24

858 = 2 * 429 = 2 * 3 * 143 = 2 * 3 * 11 * 13

(Demo)

Date post:	29-Jan-2021
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Control · 2021. 1. 6. · Control. Announcements. Print and None (Demo) None Indicates that...

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