besides the while statement just introduced, python knows the usual control flow statements known from other languages, with some twists.除了之前介绍的while语句，python同样支持其他语言通常用的控制流语句，但也有一些区别。
if statements
perhaps the most well-known statement type is the if statement. for example:
>>> x = int(input(please enter an integer: ))
please enter an integer: 42
>>> if x < 0:
... x = 0
... print('negative changed to zero')
... elif x == 0:
... print('zero')
... elif x == 1:
... print('single')
... else:
... print('more')
...
more
there can be zero or more elif parts, and the else part is optional. the keyword ‘elif’ is short for ‘else if’, and is useful to avoid excessive indentation. an if … elif … elif … sequence is a substitute for the switch or case statements found in other languages.可以有多个elif语句，关键词elif是else if的缩简写，用于缩减语句长度。if … elif … elif … 与其他语言的switch或case语句的作用相近。
if condition_1:
statement_block_1
elif condition_2:
statement_block_2
else:
statement_block_3
eg:例如
var1 = 100
if var1:
print (1 - if 表达式条件为 true)
print (var1)
var2 = 0
if var2:
print (2 - if 表达式条件为 true)
print (var2)
print (good bye!)
结果是：
1-if表达式条件为true100good bye!