python os.environ, os.putenv, /usr/bin/env

python os.environ, os.putenv, /usr/bin/env

提问by CSJ

I want to ensure os.system('env')not contain some specific variable mynamewhich is export in ~/.bashrcas export myname=csj

我想确保os.system('env')不包含某些myname导出~/.bashrc为的特定变量export myname=csj

Therefore, I wrote below python code:

因此，我写了下面的python代码：

import os

def print_all():
    print "os.environ['myname']=%s" % os.environ.get('myname')
    print "os.getenv('myname')=%s" % os.getenv('myname')
    os.system('env | grep myname')
    print

def delete_myname():
    if 'myname' in os.environ: os.environ.pop('myname')
    if os.getenv('myname'): os.unsetenv('myname')

print_all()

os.putenv('myname', 'csj2')
print "---------------------"
delete_myname()
print_all()

os.putenv('myname', 'csj3')
print "---------------------"
delete_myname()
print_all()

I think examine both os.environ['myname']and os.getenv('myname')and then delete them if exist, can ensure os.system('env | grep myname')get nothing.

我认为检查两者os.environ['myname']，os.getenv('myname')然后删除它们（如果存在），可以确保os.system('env | grep myname')一无所获。

However, the result is:

然而，结果是：

os.environ['myname']=csj
os.getenv('myname')=csj
myname=csj

---------------------
os.environ['myname']=None
os.getenv('myname')=None

---------------------
os.environ['myname']=None
os.getenv('myname')=None
myname=csj3

I don't understand why I still got csj3on os.system('env | grep myname')?

我不明白为什么我还在csj3继续os.system('env | grep myname')？

采纳答案by mata

From the docs:

从文档：

Note:Calling putenv() directly does not change os.environ, so it's better to modify os.environ.

注意：直接调用putenv()不会改变os.environ，所以最好修改os.environ。

For unsetenvthere is a similar warining:

因为unsetenv有一个类似的警告：

however, calls to unsetenv() don't update os.environ, so it is actually preferable to delete items of os.environ.

但是，对 unsetenv() 的调用不会更新 os.environ，因此实际上最好删除 os.environ 的项目。

getenvjust returns the value from os.environ, as it's implementation shows, so by using it you get into a state where it seems the value isn't set when you look it up from python, while it acutally is in the real environment. The only way to get it now I can think of would be to call the c getenv function using ctypes...

getenv只是从 返回值os.environ，正如它的实现所显示的那样，因此通过使用它，您会进入一种状态，当您从 python 中查找该值时，它似乎没有设置该值，而它实际上是在真实环境中。我现在能想到的唯一方法是使用 ctypes 调用 c getenv 函数...

If i modify your code to use os.environisnstead of calling putenv/unsetenveverything works as expected:

如果我修改您的代码以使用os.environis而不是调用putenv/unsetenv一切按预期工作：

import os

def print_all():
    print "os.environ['myname']=%s" % (os.environ['myname'] if 'myname' in os.environ else "None")
    os.system('env | grep myname')
    print

def delete_myname():
    if 'myname' in os.environ: os.environ.pop('myname')

print_all()

os.environ['myname'] = 'csj2'
print "---------------------"
print_all()
delete_myname()
print_all()

os.environ['myname'] = 'csj3'
print "---------------------"
print_all()
delete_myname()
print_all()

output:

输出：

$ myname=somevalue python2 test.py 
os.environ['myname']=somevalue
myname=somevalue

---------------------
os.environ['myname']=csj2
myname=csj2

os.environ['myname']=None

---------------------
os.environ['myname']=csj3
myname=csj3

os.environ['myname']=None

回答by Laurent LAPORTE

A good practice could be:

一个好的做法可能是：

• delete the mynameenvironment variable (if it exists),
• run your function
• restore the mynameenvironment variable at function completion.

• 删除myname环境变量（如果存在），
• 运行你的函数
• myname在函数完成时恢复环境变量。

Yu can do that easily with something like the modified_environcontext manager describe in this question.

Yu 可以使用类似modified_environ上下文管理器在这个问题中描述的东西轻松做到这一点。

with modified_environ('myname'):
    call_my_function()