Conditionals

if statement
if-else statement
if-elif-else statement
if-else expression
Incorrect usage

if statement

def f(x):
    print("A", end="")
    if (x == 0):
        print("B", end="")
        print("C", end="")
    print("D")

f(0)
f(1)

A more interesting example:

# These examples define abs(n), which is a nice example here, but it is
# also a builtin function, so you do not need to define it to use it.

def abs1(n):
    if (n < 0):
        n = -n
    return n

# again, with same-line indenting

def abs2(n):
    if (n < 0): n = -n # only indent this way for very short lines (if at all)
    return n

# again, with multiple return statements

def abs3(n):
    if (n < 0):
        return -n
    return n

# aside: you can do this with boolean arithmetic, but don't!

def abs4(n):
    return (n < 0)*(-n) + (n>=0)*(n) # this is awful!

# now show that they all work properly:

print("abs1(5) =", abs1(5), "and abs1(-5) =", abs1(-5))
print("abs2(5) =", abs2(5), "and abs2(-5) =", abs2(-5))
print("abs3(5) =", abs3(5), "and abs3(-5) =", abs3(-5))
print("abs4(5) =", abs4(5), "and abs4(-5) =", abs4(-5))

if-else statement

def f(x):
    print("A", end="")
    if (x == 0):
        print("B", end="")
        print("C", end="")
    else:
        print("D", end="")
        if (x == 1):
            print("E", end="")
        else:
            print("F", end="")
    print("G")

f(0)
f(1)
f(2)

Revisiting abs(n):

def abs5(n):
    if (n >= 0):
        return n
    else:
        return -n

# or, if you prefer...

def abs6(n):
    if (n >= 0):
        sign = +1
    else:
        sign = -1
    return sign * n

print("abs5(5) =", abs5(5), "and abs5(-5) =", abs5(-5))
print("abs6(5) =", abs6(5), "and abs6(-5) =", abs6(-5))

if-elif-else statement

def f(x):
    print("A", end="")
    if (x == 0):
        print("B", end="")
        print("C", end="")
    elif (x == 1):
        print("D", end="")
    else:
        print("E", end="")
        if (x == 2):
            print("F", end="")
        else:
            print("G", end="")
    print("H")

f(0)
f(1)
f(2)
f(3)

A more interesting example:

def numberOfRoots(a, b, c):
    # Returns number of roots (zeros) of y = a*x**2 + b*x + c
    d = b**2 - 4*a*c
    if (d > 0):
        return 2
    elif (d == 0):
        return 1
    else:
        return 0

print("y = 4*x**2 + 5*x + 1 has", numberOfRoots(4,5,1), "root(s).")
print("y = 4*x**2 + 4*x + 1 has", numberOfRoots(4,4,1), "root(s).")
print("y = 4*x**2 + 3*x + 1 has", numberOfRoots(4,3,1), "root(s).")

Another example:

def getGrade(score):
    if (score >= 90):
        grade = "A"
    elif (score >= 80):
        grade = "B"
    elif (score >= 70):
        grade = "C"
    elif (score >= 60):
        grade = "D"
    else:
        grade = "F"
    return grade

print("103 -->", getGrade(103))
print(" 88 -->", getGrade(88))
print(" 70 -->", getGrade(70))
print(" 61 -->", getGrade(61))
print(" 22 -->", getGrade(22))

if-else expression

# if-else expression (not an if-else statement!)

def abs7(n):
    return n if (n >= 0) else -n

print("abs7(5) =", abs7(5), "and abs7(-5) =", abs7(-5))

Incorrect usage
Note: this section concerns style and not correctness. All the examples here 'work', but some work more artfully than others. With that...

Negated Condition (with "else" clause)

# Wrong:
b = True
if (not b):
    print('no')
else:
    print('yes')

# Right:
b = True
if (b):
    print('yes')
else:
    print('no')

Empty "if" Clause

# Wrong:
b = False
if (b):
    pass
else:
    print('no')

# Right:
b = False
if (not b):
    print('no')

Using "if" instead of "and"

# Wrong (though not so wrong...):
b1 = True
b2 = True
if (b1):
    if (b2):
        print('both!')

# Right (well, maybe just "preferred"):
b1 = True
b2 = True
if (b1 and b2):
        print('both!')

Using another "if" instead of "else"

# Wrong:
b = True
if (b):
    print('yes')
if (not b):
    print('no')

# Right:
b = True
if (b):
    print('yes')
else:
    print('no')

Another example:

# Wrong:
x = 10
if (x < 5):
    print('small')
if ((x >= 5) and (x < 10)):
    print('medium')
if ((x >= 10) and (x < 15)):
    print('large')
if (x >= 15):
    print('extra large')

# Right:
x = 10
if (x < 5):
    print('small')
elif (x < 10):
    print('medium')
elif (x < 15):
    print('large')
else:
    print('extra large')

Yet Another Example

# Wrong:
c = 'a'
if ((c >= 'A') and (c <= 'Z')):
    print('Uppercase!')
if ((c >= 'a') and (c <= 'z')):
    print('lowercase!')
if ((c < 'A') or
    ((c > 'Z') and (c < 'a')) or
    (c > 'z')):
    print ('not a letter!')

# Right:
c = 'a'
if ((c >= 'A') and (c <= 'Z')):
    print('Uppercase!')
elif ((c >= 'a') and (c <= 'z')):
    print('lowercase!')
else:
    print('not a letter!')

Using "arithmetic logic" instead of "boolean logic"

# Wrong:
x = 42
y = ((x > 0) and 99)

# Right:
x = 42
if (x > 0):
    y = 99