CMIS 102
logical operators

Logical operators are very important.  
'and' is the logical AND operator. 
'or' is logical OR.  
'not' is logical NOT.  
The result of and-ing two values is True only if both values are True,
 it's False otherwise (i.e. if either or both are False).  
The result of or-ing two values is True if either or both of the values are True,
 it's False only if both values are False.    
These are straight out of classical logic, which is based on human thought and language. 
In real-life and human language:

if sunny OR weekend   then go outside    #either sunny or weekend (or both)
if sunny AND weekend  then go outside    #a sunny weekend
if NOT rainy   then go outside


In logic and programming language:

 two truth values OR'ed together are True if either or both are True.
  Ex.      booleanExpr1 or booleanExpr2
   Is True if booleanExpr1 is True or if booleanExpr2 is True (or if both are True).  
   Is False only if both are False.
   "Easy" to be True.

 two truth values AND'ed together are True only if both are True.
  Ex.      booleanExpr1 and booleanExpr2
   Is True if booleanExpr1 is True and booleanExpr2 is True. 
   Is False if either or both are False.
   "Hard" to be True.

 a truth value NOT'ed reverses its truth value.
  Ex.  NOT booleanExpr
   Is True if booleanExpr is False
   Is False if booleanExpr is True

Truth tables: 
x  not x
-  -----
F    T
T    F

x y  x or y
- -  ------
F F     F
F T     T
T F     T
T T     T

x y  x and y
- -  -------
F F     F
F T     F
T F     F
T T     T

Here's some natural examples used as conditions for if statements: 
notice the spacing style

midExam = input("Input the midterm exam grade: ")
if midExam=='A' or midExam=='B':
    print("congratulations!")
elif midExam == 'C':
    print("could use some improvement")
elif midExam == 'F':
    print("you're failing")


score= eval(input("Enter the score: "))
if score<0 or score>100:
    print("Error.  score must be between 0 and 100")

if score>=90 and score<=100:    #between 90 and 100, inclusive
    print("A")


ans = input("Input your answer: ")
if ans=='y' or ans=='Y':    #allows user to enter either case y/Y
    print("thank you for the y or Y")

if ans!='y' and ans!='Y':     #anything but a y/Y
    print("whatever it is, it isn't a y or Y")

month = eval(input("Enter month number: "))
if month==1 or month==3 or month==5 or month==7 or month==8 or month==10 or month==12:
    print("31 day month")
elif month==4 or month==6 or month==9 or month==11:
    print("30 day month")
else:
    print("February: 28 or 29 days")

Operator precedence is important. In an expression like
 a<b and b<c
the comparison operators are done first because they have higher
precedence than the logical operator.
In an expression like
 a+2<b and b*2<c/3
the arithmetic operators are done first, followed by the comparison,
and then the And.
And in
 a*c>b+2 or a<c and b==a%2
the evaluation is as if this full parenthesization existed:
 (((a*c) > (b+2)) or ((a<c) and (b==(a%2))))
Note that the Or is done last because it has the lowest precedence.
rainy or windy and cold    is evaluated: rainy or (windy and cold)

You can use parentheses just like in math to force a certain order of
evaluation, or just to make the expression's intention clearer.
 (a*c > b+2) or (a<c and b==a%2)    makes structure clearer

 (a*c > b+2 or a<c) and b==a%2    changes the structure.  Now the And is
the last operator that will be evaluated.


Errors in logical expressions: 
mid = 'b'
final = 'c'
if mid or final == 'A':   #Not a syntax error.  but WRONG!
    print("always is True")

i = 2
if i == 3 or 4:         # Not a syntax error.  but WRONG!  it's Always True.
    print("always is True")
 #Expressions that are Always True or Always False are in error.

if i==3 or ==4:      #Syntax error.  

Math-like syntax to test if variable is in an interval:
if 12 < y < 20:
    print("y is between 12 and 20, exclusive")

Programming takes practice in order to learn it.