python之-if语句
if语句用来检验一个条件， 如果 条件为真，我们运行一块语句（称为 if-块 ）， 否则 我们处理另外一块语句（称为 else-块 ）。 else 从句是可选的。
#!/usr/bin/python
# Filename: if.py
number = 23
guess = int(raw_input('Enter an integer : '))
if guess == number:
print 'Congratulations, you guessed it.'
# New block starts here
print "(but you do not win any prizes!)"
# New block ends here
elif guess < number:
print 'No, it is a little higher than that'
# Another block
# You can do whatever you want in a block ...
else:
print 'No, it is a little lower than that'
# you must have guess > number to reach here
print 'Done'
# This last statement is always executed, after the if statement is executed
elif和else部分是可选的。一个最简单的有效if语句是：
if True: print 'Yes, it is true'