python设置环境变量

We can set an environment variable in Python using os module. Python os module environ works as a dictionary that holds the environment variables available to the program at that moment.

我们可以使用os模块在Python中设置环境变量。 Python os模块environ充当字典 ，该字典保存当时可用于程序的环境变量。

打印当前环境变量 (Print Current Environment Variables)

We can print os.environ variable to learn about the existing environment variables that are available to the program.

我们可以打印os.environ变量以了解程序可用的现有环境变量。

import os# current environment variables
print(os.environ)

Output:

输出：

environ({'PATH': '/Library/PostgreSQL/10/bin:/Users/pankaj/Downloads/mongodb/bin:/Library/Frameworks/Python.framework/Versions/3.7/bin:/Users/pankaj/Downloads/apache-maven-3.5.3/bin:/Library/Frameworks/Python.framework/Versions/3.7/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin', 'COMMAND_MODE': 'unix2003', 'MAVEN_OPTS': '-Xmx2048m -XX:MaxPermSize=128m', 'VERSIONER_PYTHON_VERSION': '2.7', 'LOGNAME': 'pankaj', 'XPC_SERVICE_NAME': 'com.apple.xpc.launchd.oneshot.0x10000003.pycharm', 'PWD': '/Users/pankaj/Documents/github/journaldev/Python-3/basic_examples', 'PYCHARM_HOSTED': '1', 'PYTHONPATH': '/Users/pankaj/Documents/github/journaldev/Python-3', 'SHELL': '/bin/zsh', 'PAGER': 'less', 'LSCOLORS': 'Gxfxcxdxbxegedabagacad', 'PYTHONIOENCODING': 'UTF-8', 'SECURITYSESSIONID': '186a8', 'OLDPWD': '/Applications/PyCharm CE.app/Contents/bin', 'VERSIONER_PYTHON_PREFER_32_BIT': 'no', 'USER': 'pankaj', 'ZSH': '/Users/pankaj/.oh-my-zsh', 'TMPDIR': '/var/folders/1t/sx2jbcl534z88byy78_36ykr0000gn/T/', 'SSH_AUTH_SOCK': '/private/tmp/com.apple.launchd.1o59WVsq9I/Listeners', 'XPC_FLAGS': '0x0', 'PYTHONUNBUFFERED': '1', 'M2_HOME': '/Users/pankaj/Downloads/apache-maven-3.5.3', '__CF_USER_TEXT_ENCODING': '0x1F5:0x0:0x0', 'Apple_PubSub_Socket_Render': '/private/tmp/com.apple.launchd.U1NEZUKVjH/Render', 'LESS': '-R', 'LC_CTYPE': 'UTF-8', 'HOME': '/Users/pankaj', '__PYVENV_LAUNCHER__': '/Library/Frameworks/Python.framework/Versions/3.7/bin/python3.7'})

Python Current Environment Variable

Python当前环境变量

检查环境变量是否存在？ (Check if environment variable exists or not?)

We can check if environment variable exists or not using in statement.

我们可以使用in语句检查环境变量是否存在。

if 'HOME' in os.environ:print('HOME environment variable is already defined. Value =', os.environ['HOME'])
else:print('HOME environment variable is not defined.')

Output:

输出：

HOME environment variable is already defined. Value = /Users/pankaj

Changing the environment variable value can have serious implications for the execution of the program. Hence, it’s advisable to first check if the environment variable exists or not. Then it’s up to you whether you want to modify the value or not. You can always define a new environment variable and use it in your program.

更改环境变量值可能会对程序的执行产生严重影响。 因此，建议首先检查环境变量是否存在。 然后由您决定是否要修改该值。 您始终可以定义一个新的环境变量，并在程序中使用它。

Python设置环境变量 (Python set environment variable)

We can set an environment variable like we set the values in the dictionary.

我们可以像在字典中设置值那样设置环境变量。

os.environ['MYSQL_VERSION'] = '5.7.18'

Note that the environment variable key-value pair must be a string, otherwise an error will be raised.

请注意，环境变量键值对必须是字符串，否则将引发错误。

>>> os.environ['Data'] = 123
Traceback (most recent call last):File "<stdin>", line 1, in <module>File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/os.py", line 683, in __setitem__value = self.encodevalue(value)File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/os.py", line 753, in encoderaise TypeError("str expected, not %s" % type(value).__name__)
TypeError: str expected, not int
>>>

Python读取环境变量 (Python Read Environment Variable)

Let’s see how to read the environment variable we have set in the above code snippet.

让我们看看如何读取在以上代码片段中设置的环境变量。

print('MySQL Version =', os.environ['MYSQL_VERSION'])

Output: MySQL Version = 5.7.18

输出： MySQL Version = 5.7.18

But is this the correct way to retrieve environment variable value? Let’s see what happens when the environment variable is not present.

但这是检索环境变量值的正确方法吗？ 让我们看看当环境变量不存在时会发生什么。

>>> print(os.environ['DATA'])
Traceback (most recent call last):File "<stdin>", line 1, in <module>File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/os.py", line 678, in __getitem__raise KeyError(key) from None
KeyError: 'DATA'
>>>

The better way is to use get() function of environ variable. If the environment variable is not present, then it will return None.

更好的方法是使用环境变量的get()函数。 如果环境变量不存在，则它将返回None 。

>>> print(os.environ.get('DATA'))
None

We can also specify a default value to return if the environment variable is not present.

如果环境变量不存在，我们还可以指定默认值以返回。

>>> print(os.environ.get('DATA', 'TXT'))
TXT

Reference: os.environ

参考： os.environ

翻译自: https://www.journaldev.com/24935/python-set-environment-variable

python设置环境变量