28|如何合理利用assert？

1.什么是assert?

在python中assert是debug的一个好工具，主要用于测试一个条件是否满足，如果满足则什么也不做，相当于执行了pass语句，如果测试条件不满足，便会抛出异常AssertionError，并返回具体的错误信息。

具体用法：

assert_stmt ::= "assert" expression ["," expression]


简单形式assert expression，例如下面：

assert_stmt ::= "assert" expression ["," expression]


相当于：

if __debug__:
if not expression: raise AssertionError


再看下，assert expression1,expression2如下面的例子：

assert 1 == 2, 'assertion is wrong'


相当于这两行代码：

if __debug__:
if not expression1: raise AssertionError(expression2)


2.assert的用法？

eg1:

def apply_discount(price, discount):
updated_price = price * (1 - discount)
assert 0 <= updated_price <= price, 'price should be greater or equal to 0 and less or equal to original price'
return updated_price


验证:

apply_discount(100, 0.2)
80.0
apply_discount(100, 2)
AssertionError: price should be greater or equal to 0 and less or equal to original price


assert可以有效的预防bug发生，提高程序的健壮性。

eg2:

def func(input):
assert isinstance(input, list), 'input must be type of list'
# 下面的操作都是基于前提：input 必须是 list
if len(input) == 1:
...
elif len(input) == 2:
...
else:
...


这个函数func()里的所有操作，都是基于输入必须是list这个前提，必要在开头加一句assert检查，防止程序出错。

3.assert错误示例

def delete_course(user, course_id):
assert user_is_admin(user), 'user must be admin'
assert course_exist(course_id), 'course id must exist'
delete(course_id)


eg1：

def delete_course(user, course_id):
if not user_is_admin(user):
raise Exception('user must be admin')
if not course_exist(course_id):
raise Exception('coursde id must exist')
delete(course_id)


def read_and_process(path):
assert file_exist(path), 'file must exist'
with open(path) as f:
...


def read_and_process(path):
assert file_exist(path), 'file must exist'
with open(path) as f:
...