检查环境变量是否存在的良好实践是什么?

问题:检查环境变量是否存在的良好实践是什么?

我想检查我的环境中是否存在"FOO"Python 中的变量。为此,我正在使用os标准库。阅读图书馆的文档后,我想出了两种实现目标的方法:

方法1:

if "FOO" in os.environ:
    pass

方法2:

if os.getenv("FOO") is not None:
    pass

我想知道哪种方法是好的/首选条件,以及为什么。

I want to check my environment for the existence of a variable, say "FOO", in Python. For this purpose, I am using the os standard library. After reading the library’s documentation, I have figured out 2 ways to achieve my goal:

Method 1:

if "FOO" in os.environ:
    pass

Method 2:

if os.getenv("FOO") is not None:
    pass

I would like to know which method, if either, is a good/preferred conditional and why.


回答 0

使用第一个;它直接尝试检查是否在中定义了某些内容environ。尽管第二种形式同样可以很好地工作,但是它在语义上是不足的,因为如果存在,您会得到一个返回的值,并且将其用于比较。

你想看看是否有存在 environ,为什么你会得到只是为了进行比较,然后折腾它扔掉

那正是这样getenv做的:

获取一个环境变量None如果不存在则返回。可选的第二个参数可以指定备用默认值。

(这也意味着您的支票可能只是if getenv("FOO")

你不想得到它,你想检查它的存在。

无论哪种方式,getenv都只是一个包装,environ.get但是您看不到有人通过以下方式检查映射中的成员身份:

from os import environ
if environ.get('Foo') is not None:

总结一下,使用:

if "FOO" in os.environ:
    pass

如果您只想检查是否存在,请使用,getenv("FOO")如果您确实想用可能获得的价值做某事。

Use the first; it directly tries to check if something is defined in environ. Though the second form works equally well, it’s lacking semantically since you get a value back if it exists and only use it for a comparison.

You’re trying to see if something is present in environ, why would you get just to compare it and then toss it away?

That’s exactly what getenv does:

Get an environment variable, return None if it doesn’t exist. The optional second argument can specify an alternate default.

(this also means your check could just be if getenv("FOO"))

you don’t want to get it, you want to check for it’s existence.

Either way, getenv is just a wrapper around environ.get but you don’t see people checking for membership in mappings with:

from os import environ
if environ.get('Foo') is not None:

To summarize, use:

if "FOO" in os.environ:
    pass

if you just want to check for existence, while, use getenv("FOO") if you actually want to do something with the value you might get.


回答 1

两种解决方案都有一种情况,这取决于您要根据环境变量的存在来执行什么操作。

情况1

如果您想纯粹基于环境变量的存在而采取不同的措施而又不关心其价值,那么第一个解决方案就是最佳实践。它简要描述了您要测试的内容:环境变量列表中的’FOO’。

if 'KITTEN_ALLERGY' in os.environ:
    buy_puppy()
else:
    buy_kitten()

情况二

如果您想在环境变量中未定义该值的情况下设置默认值,则第二个解决方案实际上很有用,尽管它不是您编写的形式:

server = os.getenv('MY_CAT_STREAMS', 'youtube.com')

也许

server = os.environ.get('MY_CAT_STREAMS', 'youtube.com')

请注意,如果您的应用程序有多个选项,则可能需要查看ChainMap,它允许根据键合并多个字典。ChainMap文档中有一个示例:

[...]
combined = ChainMap(command_line_args, os.environ, defaults)

There is a case for either solution, depending on what you want to do conditional on the existence of the environment variable.

Case 1

When you want to take different actions purely based on the existence of the environment variable, without caring for its value, the first solution is the best practice. It succinctly describes what you test for: is ‘FOO’ in the list of environment variables.

if 'KITTEN_ALLERGY' in os.environ:
    buy_puppy()
else:
    buy_kitten()

Case 2

When you want to set a default value if the value is not defined in the environment variables the second solution is actually useful, though not in the form you wrote it:

server = os.getenv('MY_CAT_STREAMS', 'youtube.com')

or perhaps

server = os.environ.get('MY_CAT_STREAMS', 'youtube.com')

Note that if you have several options for your application you might want to look into ChainMap, which allows to merge multiple dicts based on keys. There is an example of this in the ChainMap documentation:

[...]
combined = ChainMap(command_line_args, os.environ, defaults)

回答 2

为了安全起见

os.getenv('FOO') or 'bar'

上述答案的一个极端情况是设置了环境变量但为空

对于这种特殊情况,您会得到

print(os.getenv('FOO', 'bar'))
# prints new line - though you expected `bar`

要么

if "FOO" in os.environ:
    print("FOO is here")
# prints FOO is here - however its not

为了避免这种情况,只需使用 or

os.getenv('FOO') or 'bar'

然后你得到

print(os.getenv('FOO') or 'bar')
# bar

什么时候有空的环境变量?

您忘记在.env文件中设置值

# .env
FOO=

或导出为

$ export FOO=

或忘记设置它 settings.py

# settings.py
os.environ['FOO'] = ''

更新:如果有疑问,请查看这些单线

>>> import os; os.environ['FOO'] = ''; print(os.getenv('FOO', 'bar'))

$ FOO= python -c "import os; print(os.getenv('FOO', 'bar'))"

To be on the safe side use

os.getenv('FOO') or 'bar'

A corner case with the above answers is when the environment variable is set but is empty

For this special case you get

print(os.getenv('FOO', 'bar'))
# prints new line - though you expected `bar`

or

if "FOO" in os.environ:
    print("FOO is here")
# prints FOO is here - however its not

To avoid this just use or

os.getenv('FOO') or 'bar'

Then you get

print(os.getenv('FOO') or 'bar')
# bar

When do we have empty environment variables?

You forgot to set the value in the .env file

# .env
FOO=

or exported as

$ export FOO=

or forgot to set it in settings.py

# settings.py
os.environ['FOO'] = ''

Update: if in doubt, check out these one-liners

>>> import os; os.environ['FOO'] = ''; print(os.getenv('FOO', 'bar'))

$ FOO= python -c "import os; print(os.getenv('FOO', 'bar'))"

回答 3

如果您要检查是否未设置多个环境变量,可以执行以下操作:

import os

MANDATORY_ENV_VARS = ["FOO", "BAR"]

for var in MANDATORY_ENV_VARS:
    if var not in os.environ:
        raise EnvironmentError("Failed because {} is not set.".format(var))

In case you want to check if multiple env variables are not set, you can do the following:

import os

MANDATORY_ENV_VARS = ["FOO", "BAR"]

for var in MANDATORY_ENV_VARS:
    if var not in os.environ:
        raise EnvironmentError("Failed because {} is not set.".format(var))

回答 4

我的评论可能与给定的标签无关。但是,我是从搜索中转到此页面的。我一直在寻找R中的类似支票,并在@hugovdbeg帖子的帮助下提出了以下内容。我希望这对在R中寻求类似解决方案的人有所帮助

'USERNAME' %in% names(Sys.getenv())

My comment might not be relevant to the tags given. However, I was lead to this page from my search. I was looking for similar check in R and I came up the following with the help of @hugovdbeg post. I hope it would be helpful for someone who is looking for similar solution in R

'USERNAME' %in% names(Sys.getenv())