python生产环境禁用assert断言的方法

目录

• 1. 背景
• 2.解决方案
• 2.1 禁用assert的策略
• 2.2 禁用的原理
• 3. 实施禁用策略
• 3.1 启动命令行的参数中,添加-O
• 3.2 设置PYTHONOPTIMIZE环境变量
• 4 使用断言的坑

1. 背景

在潜意识中, assert 是应用在unittest或pytest环境中, 不能应用到业务代码中, 因为断言会导致运行中断,对业务有损,并且消耗内存, 影响性能.

但不可否认, 使用断言非常方便调试代码

通过研读assert的文档, 发现断言是可以被关闭的,特此记录下

详细的介绍参考这个文档: 参考这篇文档

2.解决方案

2.1 禁用assert的策略

有两种方案

1. 启动命令行的参数中,添加-O, 也就是大写的o
2. 设置PYTHONOPTIMIZE环境变量为合适的值

2.2 禁用的原理

在python 内部机制中, 有一常量__debug__,值为BOOL类型的, 与断言是强关联的, 因为__debug__是常量, 所以当python 解释器运行时, 不能修改它的值.

Python 3.8.13 (default, Mar 28 2022, 06:16:26)
[Clang 12.0.0 ] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> __debug__
True
>>> __debug__ = False
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>>

既然运行时不能修改, 那没有运行时可以修改吗?当然可以的, 这取决于python 运行模式,python运行模式有两种,和__debug__的关系如下表所示:

那__debug__的值和断言有什么关系呢?

看源码:

if __debug__:
    if not expression:
        raise AssertionError(assertion_message)

所以__debug__==True时会抛出断言异常, __debug__==False时就啥也不干,跳过了

这下清晰了

3. 实施禁用策略

3.1 启动命令行的参数中,添加-O

验证方案:

1、先写这样一个测试脚本,命名为demo.py

# demo.py
print(f"__debug__=={__debug__}"

2、在命令行使用 禁用断言的启动方式

python -O demo.py

输出结果

__debug__==False

辅助理解: 在命令行中运行python -h,其结果也体现了这样的使用方式

mac$ python -h
usage: python [option] ... [-c cmd | -m mod | file | -] [arg] ...
Options and arguments (and corresponding environment variables):
-b     : issue warnings about str(bytes_instance), str(bytearray_instance)
         and comparing bytes/bytearray with str. (-bb: issue errors)
-B     : don't write .pyc files on import; also PYTHONDONTWRITEBYTECODE=x
-c cmd : program passed in as string (terminates option list)
-d     : debug output from parser; also PYTHONDEBUG=x
-E     : ignore PYTHON* environment variables (such as PYTHONPATH)
-h     : print this help message and exit (also --help)
-i     : inspect interactively after running script; forces a prompt even
         if stdin does not appear to be a terminal; also PYTHONINSPECT=x
-I     : isolate Python from the user's environment (implies -E and -s)
-m mod : run library module as a script (terminates option list)
-O     : remove assert and __debug__-dependent statements; add .opt-1 before
         .pyc extension; also PYTHONOPTIMIZE=x
-OO    : do -O changes and also discard docstrings; add .opt-2 before
         .pyc extension
-q     : don't print version and copyright messages on interactive startup
-s     : don't add user site directory to sys.path; also PYTHONNOUSERSITE
-S     : don't imply 'import site' on initialization
-u     : force the stdout and stderr streams to be unbuffered;
         this option has no effect on stdin; also PYTHONUNBUFFERED=x
-v     : verbose (trace import statements); also PYTHONVERBOSE=x
         can be supplied multiple times to increase verbosity
-V     : print the Python version number and exit (also --version)
         when given twice, print more information about the build
-W arg : warning control; arg is action:message:category:module:lineno
         also PYTHONWARNINGS=arg
-x     : skip first line of source, allowing use of non-Unix forms of #!cmd
-X opt : set implementation-specific option. The following options are available:

         -X faulthandler: enable faulthandler
         -X showrefcount: output the total reference count and number of used
             memory blocks when the program finishes or after each statement in the
             interactive interpreter. This only works on debug builds
         -X tracemalloc: start tracing Python memory allocations using the
             tracemalloc module. By default, only the most recent frame is stored in a
             traceback of a trace. Use -X tracemalloc=NFRAME to start tracing with a
             traceback limit of NFRAME frames
         -X showalloccount: output the total count of allocated objects for each
             type when the program finishes. This only works when Python was built with
             COUNT_ALLOCS defined
         -X importtime: show how long each import takes. It shows module name,
             cumulative time (including nested imports) and self time (excluding
             nested imports). Note that its output may be broken in multi-threaded
             application. Typical usage is python3 -X importtime -c 'import asyncio'
         -X dev: enable CPython's "development mode", introducing additional runtime
             checks which are too expensive to be enabled by default. Effect of the
             developer mode:
                * Add default warning filter, as -W default
                * Install debug hooks on memory allocators: see the PyMem_SetupDebugHooks() C function
                * Enable the faulthandler module to dump the Python traceback on a crash
                * Enable asyncio debug mode
                * Set the dev_mode attribute of sys.flags to True
                * io.IOBase destructor logs close() exceptions
         -X utf8: enable UTF-8 mode for operating system interfaces, overriding the default
             locale-aware mode. -X utf8=0 explicitly disables UTF-8 mode (even when it would
             otherwise activate automatically)
         -X pycache_prefix=PATH: enable writing .pyc files to a parallel tree rooted at the
             given directory instead of to the code tree

--check-hash-based-pycs always|default|never:
    control how Python invalidates hash-based .pyc files
file   : program read from script file
-      : program read from stdin (default; interactive mode if a tty)
arg ...: arguments passed to program in sys.argv[1:]

Other environment variables:
PYTHONSTARTUP: file executed on interactive startup (no default)
PYTHONPATH   : ':'-separated list of directories prefixed to the
               default module search path.  The result is sys.path.
PYTHONHOME   : alternate <prefix> directory (or <prefix>:<exec_prefix>).
               The default module search path uses <prefix>/lib/pythonX.X.
PYTHONCASEOK : ignore case in 'import' statements (Windows).
PYTHONUTF8: if set to 1, enable the UTF-8 mode.
PYTHONIOENCODING: Encoding[:errors] used for stdin/stdout/stderr.
PYTHONFAULTHANDLER: dump the Python traceback on fatal errors.
PYTHONHASHSEED: if this variable is set to 'random', a random value is used
   to seed the hashes of str and bytes objects.  It can also be set to an
   integer in the range [0,4294967295] to get hash values with a
   predictable seed.
PYTHONMALLOC: set the Python memory allocators and/or install debug hooks
   on Python memory allocators. Use PYTHONMALLOC=debug to install debug
   hooks.
PYTHONCOERCECLOCALE: if this variable is set to 0, it disables the locale
   coercion behavior. Use PYTHONCOERCECLOCALE=warn to request display of
   locale coercion and locale compatibility warnings on stderr.
PYTHONBREAKPOINT: if this variable is set to 0, it disables the default
   debugger. It can be set to the callable of your debugger of choice.
PYTHONDEVMODE: enable the development mode.
PYTHONPYCACHEPREFIX: root directory for bytecode cache (pyc) files.

5: 新的需求: 怎么在命令行中禁用断言,看下面的对比代码:

最关键是修改__debug__的值

(sse38) lzh mac$ python
Python 3.8.13 (default, Mar 28 2022, 06:16:26)
[Clang 12.0.0 ] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> __debug__
True
>>> exit()
(sse38) lzh mac$ python -O
Python 3.8.13 (default, Mar 28 2022, 06:16:26)
[Clang 12.0.0 ] :: Anaconda, Inc. on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> __debug__
False
>>>

3.2 设置PYTHONOPTIMIZE环境变量

这部分是在命令行中操作的

mac or linux 系统中,

(sse38)  mac$ export PYTHONOPTIMIZE=0
(sse38)  mac$ python demo.py
True
(sse38)  mac$ export PYTHONOPTIMIZE=1
(sse38)  mac$ python demo.py
False
(sse38)  mac$ export PYTHONOPTIMIZE=2
(sse38)  mac$ python demo.py
False

window 系统中,

(sse38)  mac$ set PYTHONOPTIMIZE=0
(sse38)  mac$ python demo.py
True
(sse38)  mac$ set PYTHONOPTIMIZE=1
(sse38)  mac$ python demo.py
False
(sse38)  mac$ set PYTHONOPTIMIZE=2
(sse38)  mac$ python demo.py
False

4 使用断言的坑

处理或验证数据

处理异常和异常

运行常有副作用的操作

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