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全面解析python当前路径和导包路径问题

作者:ponponon

这篇文章主要为大家介绍了python当前路径和导包路径问题的全面解析,有需要的朋友可以借鉴参考下,希望能够有所帮助,祝大家多多进步,早日升职加薪

引言

python中的模块、库、包有什么区别?

module:一个 .py 文件就是个 module

lib:抽象概念,和另外两个不是一类,只要你喜欢,什么都是 lib,就算只有个 hello world

package:就是个带 __init__.py 的文件夹,并不在乎里面有什么,不过一般来讲会包含一些 packages/modules

如何你有以下的疑问的话,那这个文章很适合你!

何为当前路径?

所谓的当前路径到底是输入命令的路径还是 py 脚本文件所在的路径?
插一句: Linux 等系统中查看当前路径的命令是 pwd, python 中查看当前路径是 os.getcwd()

❓ 疑问一 👉🏻:python 程序的当前路径是执行 python 脚本等路径还是 python 脚本说在的路径?

即执行下面的命令的时候,所谓的当前路径是 testing 文件夹所在的路径还是 main.py 文件所在路径。

python testing/main.py

✅ 答案:当前路径是输入运行命令的路径,而不是 py 文件所在的路径。

对于👇下面的命令,是无所谓区分这两个路径的,但是👆上面的路径就不一样了

python main.py

导包路径和当前路径的关系?

知道这个知识对写程序避坑有什么帮助?,接下往下看吧!

❓ 疑问二 👉🏻:如何查看 Python 的导包路径?

我在 /Users/bot/Desktop/code/ideaboom 新建一个名为 testing 的文件夹,并在 testing 文件夹下新建一个 main.py 的文件。

main.py 文件的内容如下所示:

import os
import sys
print('当前工作路径: ', os.getcwd())
print('导包路径为: ')
for p in sys.path:
    print(p)

并在 /Users/bot/Desktop/code/ideaboom 处运行命令:python testing/main.py

程序输出如下:

当前工作路径:  /Users/bot/Desktop/code/ideaboom

导包路径为: 

/Users/bot/Desktop/code/ideaboom/testing
/Library/Frameworks/Python.framework/Versions/3.9/lib/python39.zip
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/lib-dynload
/Users/bot/.local/share/virtualenvs/ideaboom-8ZWsq-JB/lib/python3.9/site-packages

python 中的包就是包含 __init__.py 文件的文件夹

首要导包路径不是当前路径有什么问题?

这是一个很典型的问题,我们往往会在项目的根目录下面建一个 testing 文件夹,把需要单元测试相关的文件放在。

但是当我们输入命令 python testing/main.py 的时候,就会出现 ModuleNotFoundError: No module named xxx ,出现的原因就是上面提到的:首要导包路径不是当前路径

本来 xxx 和 testing 文件夹是在项目的根目录下面,sys.path 中的首要导包路径就是项目的根目录,但是当我们 python testing/main.py 的时候,首要导包路变成了 testing 而不是项目根目录了!这还是 main.py 中的 import xxx 当然找不到了。

知道了问题如何解决呢?

解决什么😨?当然是运行 tetsing 文件夹下面的 main.py 文件报错 ModuleNotFoundError: No module named xxx 的问题。🤯

❓ 疑问三:如何改变 Python 程序的首要导包路径?

python 首要导包路径就是 sys.path 列表中的第一个元素,即被运行的 py 文件所在的文件夹路径

方案一:动态修改 sys.path

最常见的方式就是:

把当前路径添加到 sys.path 中,且为了避免命名冲突,最好添加到列表的头部,而不是用 append 添加到尾部。至于本来的(不期望的)首要导包路径 /Users/bot/Desktop/code/ideaboom/testing 可以删除,也可以保留。

import os
import sys
print('当前工作路径: ', os.getcwd())
print('导包路径为: ')
sys.path.insert(0,os.getcwd()) # 把当前路径添加到 sys.path 中
for p in sys.path:
    print(p)

程序输出:👇

当前工作路径:  /Users/bot/Desktop/code/ideaboom
导包路径为: 
/Users/bot/Desktop/code/ideaboom
/Users/bot/Desktop/code/ideaboom/testing
/Library/Frameworks/Python.framework/Versions/3.9/lib/python39.zip
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/lib-dynload
/Users/bot/.local/share/virtualenvs/ideaboom-8ZWsq-JB/lib/python3.9/site-packages

但是这个方案不太好,有一些缺点,比如下面的代码,看起来就很不优雅,因为按照 python 的代码规范,导包相关的代码应该写在最前面,这种 导包+代码+导包 的方式破坏了 pythonic

import os
import sys
import time
import schedule
from pathlib import Path
import os
import sys
import time
import schedule
from pathlib import Path
BASE_DIR = Path(__file__).resolve().parent.parent.parent
sys.path.insert(0, str(BASE_DIR))
import django
django.setup()

方案二:使用环境变量 PYTHONPATH

🥳 更好的方案 👇

因为首要导包路径的设定是 python 解释器的默认执行,那我们能不能在 python 解释器启动之前就指定好我们需要的首要导包路径呢?

通过查看 python --help 命令,我们可以到以下内容:👇

usage: /opt/homebrew/Cellar/python@3.8/3.8.12/bin/python3 [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.

看了一圈下来,感觉这个 PYTHONHOME 是救星,但实际上不是哦,恰恰是那个不起眼的 PYTHONPATH

testing/main.py

import os
import sys
print('当前工作路径: ', os.getcwd())
print('导包路径为: ')
for p in sys.path:
    print(p)

我们可以使用下面的方式来启动程序:👇

PYTHONPATH=$(pwd)  python testing/main.py

此时程序的输出变成了:👇

(ideaboom) ╭─bot@mbp13m1.local ~/Desktop/code/ideaboom  ‹master*› 
╰─➤  PYTHONPATH=$(pwd)  python testing/main.py
当前工作路径:  /Users/bot/Desktop/code/ideaboom
导包路径为: 
/Users/bot/Desktop/code/ideaboom/testing
/Users/bot/Desktop/code/ideaboom
/Library/Frameworks/Python.framework/Versions/3.9/lib/python39.zip
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/lib-dynload
/Users/bot/.local/share/virtualenvs/ideaboom-8ZWsq-JB/lib/python3.9/site-packages

让我们再来看看 PYTHONPATH=$(pwd) python testing/main.py, 它等效于 PYTHONPATH=/Users/bot/Desktop/code/ideaboom python testing/main.py.

在 python testing/main.py 添加 PYTHONPATH=$(pwd) 的环境变量的作用域仅限于本次命令的运行,不会扩散到当前的 shell 环境中。

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