3.1 結構化程式設計；
3.2 條件判斷語句；
     3.2.1 if 條件語句；
if (表示式):
    語句1
else:
    語句2
x = input("x:")
x = int(x)
x = x + 1
print (x)
輸入：x:9
     10

執行 if 內的程式
a = input("a:")
a = int(a)
b = input("b:")
b = int(b)
if (a > b):
    print (a, ">", b)
輸出：a:8
     b:3
     8 > 3
     a:2
     b:7

if else 語句
a = input("a:")
a = int(a)
b = input("b:")
b = int(b)
if (a > b):
    print (a, ">", b)
else:
    print (a, "<", b)

        3.2.2 if...elif...else 判斷語句
score = float(input("score:"))
if 90 <= score <= 100:
    print ("A")
elif 80 <= score <= 90:
    print ("B")
elif 60 <= score <= 80:
    print ("C")
else:
    print ("D")

x = -1
y = 99
if (x >= 0):
    if (x > 0):
        y = 1
    else:
        y = 0
else:
    y = -1
print ("y =", y)

        3.2.4 switch 語句的替代方案
使用字典實現switch語句
from __future__ import division
x = 1
y = 2
operator = "*"
result = {"+": x + y, "-": x - y, "*": x * y, "/": x / y}
print (result.get(operator))
輸出：2  x*y

class switch(object):
    def __init__(self, value):
        self.value = value
        self.fall = False

    def __iter__(self):
        yield self.match
        raise StopIteration

    def match(self, *args):
        if self.fall or not args:
            return True
        elif self.value in args:
            self.fall = True
            return True
        else:
            return False

operator = "+"
x = 1
y = 2
for case in switch(operator):
    if case("+"):
        print (x + y)
        break
    if case("-"):
        print (x - y)
        break
    if case("*"):
        print (x * y)
        break
    if case("/"):
        print (x / y)
        break
    if case():
        print ""