reference  : https://docs.python.org/3/reference/expressions.html#conditional-expressions

6.11. Conditional expressions

conditional_expression ::=  or_test ["if" or_test "else" expression]expression             ::=  conditional_expression | lambda_exprexpression_nocond      ::=  or_test | lambda_expr_nocond

Conditional expressions (sometimes called a “ternary operator”) have the lowest priority of all Python operations.

The expression 

x if C else y

 first evaluates the condition, C rather than x. If C is true, x is evaluated and its value is returned; otherwise, y is evaluated and its value is returned.

See PEP 308 for more details about conditional expressions.

举例：

1 先判断字符类型，然后把list转换为小写字符的list

L1=[‘Hello‘, ‘World‘, 18, ‘Apple‘, None]L2=[x.lower() if isinstance(x, str) else x for x in L1]print (L2)

2 取（0-100）的一个偶数序列

L1=[x for x in range(100) if x%2==0]print (L1)

Python在的if使用