1. Winter 2019 CISC101
4/8/2019
CISC101 Reminders
Quiz 1 this week, starting today. More info in last Tuesday’s lecture.
Assignment 1 due this Friday.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

2. Winter 2019 CISC101
4/8/2019
Today
Mistake in last Friday’s slides – slide #11 – “Expanded Precedence Rules”. Unary operators were not in the right places. This will not have any effect on your answers for Quiz 1.
Finish Building Conditionals.
Commenting Code and Coding Style (also applicable to your Assignment 1 code).
Variable Scope
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

3. So, addition and < is taking place before the not operation.
>>> not < 8 True >>> not (4 + 5 < 8) >>> not (4 + 5) False >>> not 4 >>> not 0
So, addition and < is taking place before the not operation.
And, not will operate on more than just a True or False!
In fact, and and or will work with integers, but the results are not useful.
Winter 2019
CISC101 - Prof. McLeod

4. So, negation takes place after ** but before +.
>>> aVal = 5 >>> -aVal >>> -aVal ** 2 -25
So, negation takes place after ** but before +.
Use ( ) to control precedence if in doubt!
Winter 2019
CISC101 - Prof. McLeod

5. Expanded Precedence Rules - Fixed
Math operators in usual order: **
- (unary)
* / // %
+ -
Binary boolean comparison operators:
>= < <= == !=
not
Logical operator: and
Logical operator: or
Assignment operator: =
Winter 2019
CISC101 - Prof. McLeod

6. Back To: Nested if Statements - Demo
What to wear? Depends on time to destination and outdoor temperature:
If time < 5 minutes, wear shorts & tshirt
if time between 5 and 20 minutes:
if temp >= 5 wear shorts & tshirt
if temp between -10 and 5 wear pants & sweatshirt
if temp less than -10 wear pants & jacket
if time > 20 minutes:
if temp >= 10 wear shorts & tshirt
if temp between 0 and 10 wear shorts & sweatshirt
if temp between – 10 and 0 wear pants & jacket
if temp < -10 wear pants & parka
Winter 2019
CISC101 - Prof. McLeod

7. Nested if Statements – Demo, Cont.
See:
ClothesVersion0.py
ClothesVersion1.py
ClothesVersion2.py
ClothesVersion3.py
All work correctly and all do the same thing.
Which one is “best” and why?
Winter 2019
CISC101 - Prof. McLeod

8. Building Conditionals
Nesting conditionals leads to the best structure for this example.
If you have to nest conditionals, try writing the test with the fewest conditions for the outermost if statement.
When testing, test every possible combination of inputs to make sure your logic is good.
Writing nested conditionals can be tricky!
Winter 2019
CISC101 - Prof. McLeod

9. Commenting Code
An important part of how you will make your code more readable for yourself and anyone else (your grader!) who has to read your code.
But, where to put comments, and how much commenting is enough?
Winter 2019
CISC101 - Prof. McLeod

10. Programming Style & Documentation
CISC101
Programming Style & Documentation
Purpose is to make your code readable (and “debuggable”) by you or another programmer who is familiar with the language you are using:
“Code is read more often than it is written.”
(Guido van Rossum)
Internal style elements are documentation (comments), spacing, and descriptive variable and function names.
Select the conventions you want to use and be consistent.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

11. Comments
Add a block comment to the top of the module (your program) and at the beginning of each function. Describe overall purpose of your program or function, the main algorithm used, author, date created, and any assumptions made and/or bugs found.
Function comments should state what parameters are expected by the function and what the function returns, if anything. Function comments should also state any assumptions made about the state of the arguments.
Winter 2019
CISC101 - Prof. McLeod

12. Documentation Strings
CISC101
Documentation Strings
If you put a string literal right after the def … statement it is a documentation string.
It will be regurgitated by the help system and can be accessed using special variables.
You can use triple quotes to place a large document here!
Used mostly when creating object definitions (classes).
Still - not a bad habit to get into…
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

13. Documentation Strings, Cont.
If you describe your function in a doc string you don’t need as much in a block comment above the def fcn() : line.
What would you not include in a doc string?
Author of function.
Date/revision number.
Code history.
Problem areas.
Incomplete section(s).
License/copyright, etc.
Winter 2019
CISC101 - Prof. McLeod

14. Comments, Cont.
Add in-line comments for variables when first initialized, when the name of variable is not self-explanatory.
Add comments at the beginnings of logical blocks of code. Indent comment same as start of block.
You don’t need to explain code that is obvious – just focus on code that is tricky to understand (maybe it needs to be re-written?).
# TODO comments can be used to mark where more work is needed.
Winter 2019
CISC101 - Prof. McLeod

15. Aside - Commenting During Development
In addition to the # TODO comment:
You can temporarily remove a chunk of code that you may not need, but don’t want to completely delete by:
Selecting the lines of code, and
Choosing “Format” from the menu bar, followed by
“Comment Out Region”.
(Or use <Alt>3)
You can also “Un-Comment Out” the region. (Use <Alt>4)
However, don’t leave these chunks in your final program version…
Winter 2019
CISC101 - Prof. McLeod

16. Commenting, Example See ClothesCommented.py
You do not have to have doc strings in your assignment 1 code.
Winter 2019
CISC101 - Prof. McLeod

17. CISC101
Another Demo Program
See another example program called WindowWeight.py.
It calculates the weight of a piece of window glass given its dimensions.
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

18. Features Use of global constants. Good commenting style.
Use of a doc string.
Error checking.
Winter 2019
CISC101 - Prof. McLeod

19. CISC101
Variable Scope
A variable created inside a function is known only inside that function.
A variable created at the same level as the function headers is known everywhere in the program. These variables are called “global variables”.
What do I mean by “known”?
Winter 2019
CISC101 - Prof. McLeod
Prof. Alan McLeod

20. Global Variables Simple rules:
Don’t declare global variables unless the majority of your functions will use this variable and you think your code will be easier to work with and read as a result.
You can declare constants as global variables. By convention, the constant’s variable name should be in all upper case.
Winter 2019
CISC101 - Prof. McLeod

21. Aside - “Magic Numbers”
These are numeric literals that just appear in your program.
Sometimes they make sense, like assigning:
sum = 0
But, sometimes they don’t:
distance = measurement / 2.54
What is this number 2.54? Where did it come from and what does it mean?
Better:
distance = measurement / CM_PER_INCH
Winter 2019
CISC101 - Prof. McLeod

22. Style, Cont. - Spacing
Use a tab that generates 4 spaces for indentation.
Don’t mix tabs and spaces (not a problem if you are only using IDLE. When you hit the <tab> key you automatically get 4 spaces.)
Long lines:
Keep lines < 80 characters in length.
Use the Python continuation character: \
Break a line after a binary operator, not before.
Indent a continued line so that it lines up as shown by the examples on the next slide:
Winter 2019
CISC101 - Prof. McLeod

23. Spacing, Cont. Continuation examples:
longAssignment = aLongName + anotherLongName - \
anotherVariable * 2.0
returnedVal = functionCall(param1, anotherParam, \
param2, param3)
Don’t put multiple lines of code on a single line. For example:
if bingo < 3 : bork = try + again
else : we = are + all + winners
Winter 2019
CISC101 - Prof. McLeod

24. Spacing, Cont. Blank Lines:
Use one blank line above a def statement - no blank lines below.
A blank line inside a function can be used to delineate a block of code, but don’t put too many blank lines inside a function. (Don’t double space your code!!)
Winter 2019
CISC101 - Prof. McLeod

25. Spacing, Cont. Spaces: space on both sides of a :
(Google style says no space on the left of a : )
space after a comma, but not before.
spaces on both sides of a binary operator.
spaces on both sides of keywords like in, not in, is, and, or, not.
No space after a unary operator.
Don’t use spaces around a = when used in a function’s parameter list (default and keyword arguments).
Winter 2019
CISC101 - Prof. McLeod

26. Spacing, Cont. Spaces and brackets:
No space before or after ( and ) unless an operator comes before or after the brackets.
Same rules for [ ] and { }.
Winter 2019
CISC101 - Prof. McLeod

27. Using Round Brackets
Don’t use round brackets when they are not necessary. For example, don’t do:
if (x > 2): while (x < 3): if not(x): if ((x < 3) and (not y)): return (foo) Understanding precedence will help!
Winter 2019
CISC101 - Prof. McLeod

28. Misc. No code after a : when it is used with a def, if or while loop.
Define main at the top of your program or at the end - not in the middle!
One import statement per line.
Put import statements before globals but after block comments and module-level doc strings.
(There is a big set of additional rules for doc strings themselves that I won’t get into here…)
Winter 2019
CISC101 - Prof. McLeod

29. Do # This program is used to demonstrate better style.
# Version 1, by Alan McLeod, 27 Oct. 2011
def product(num1, num2) :
'''This is a useless little function that does not do much'''
print('Hello')
return num1 * num2
def main() :
'''main invokes product and then waves goodbye!'''
print(product(3, 4))
print('Goodbye!')
main()
Winter 2019
CISC101 - Prof. McLeod

30. Don’t! This works! How many things are wrong with this code?
def m(ll1,l1):print('Hello');return(ll1*l1);
def main():print(m(3,4));print("Goodbye!")
main ()
This works!
How many things are wrong with this code?
Winter 2019
CISC101 - Prof. McLeod