Assert may not work as expected

2022-10-16

#python

前言

assert (断言) 是在编码过程中常用的手段，使用方法如下:

# example.py
# assert_stmt ::=  "assert" expression ["," expression]
def is_admin(user_roles):
    assert isinstance(user_roles,list) and user_roles != [], "No user roles found"

    assert 'admin' in user_roles, "No admin role found."
    print("You have full access to the application.")

上面的表达式其实等价于

def is_admin(user_roles):
    if __debug__:
        assert isinstance(user_roles,list) and user_roles != [], "No user roles found"
    if __debug__:
        assert 'admin' in user_roles, "No admin role found."
    print("You have full access to the application.")

其中 __debug__ 是内置变量，默认情况下为 True,这个时候我们传入一个 user_roles = ['admin','guest']:

>>> is_admin(['admin','guest'])
You have full access to the application.

如果传入 ['guest']:

 line 4, in is_admin
    assert 'admin' in user_roles, "No admin role found."
AssertionError: No admin role found.

如果我们有办法改变 __debug__ 的值为 False,那么上面的函数将永远不会出现 AssertionError,尝试直接去修改这个参数

>>> __debug__ = False
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__

显然直接修改是行不通的，但是可以在命令行下运行的时候设置 PYTHONOPTIMIZE 达到修改 __debug__ 的目的

❯ python -O example.py
You have full access to the application.

所在在使用 assert 时候需要注意启动程序的命令，避免 assert 失效导致程序出现问题。