标签归档：python-os

知识问答

os.getenv和os.environ.get之间的区别

问题：os.getenv和os.environ.get之间的区别

两种方法之间有什么区别吗？

>>> os.getenv('TERM')
'xterm'
>>> os.environ.get('TERM')
'xterm'

>>> os.getenv('FOOBAR', "not found") == "not found"
True
>>> os.environ.get('FOOBAR', "not found") == "not found"
True

它们似乎具有完全相同的功能。

点击查看英文原文

Is there any difference at all between both approaches?

>>> os.getenv('TERM')
'xterm'
>>> os.environ.get('TERM')
'xterm'

>>> os.getenv('FOOBAR', "not found") == "not found"
True
>>> os.environ.get('FOOBAR', "not found") == "not found"
True

They seem to have the exact same functionality.

回答 0

观察到的一个区别（Python27）：

os.environ如果环境变量不存在，则会引发异常。 os.getenv不引发异常，但返回None

点击查看英文原文

One difference observed (Python27):

os.environ raises an exception if the environmental variable does not exist. os.getenv does not raise an exception, but returns None

回答 1

请参阅此相关线程。基本上，os.environ可以在import上找到，并且至少在CPython中os.getenv是的包装os.environ.get。

编辑：在CPython中，响应评论os.getenv基本上是os.environ.get; 的快捷方式。因为os.environ在导入时加载os，只有这样，才适用 os.getenv。

点击查看英文原文

See this related thread. Basically, os.environ is found on import, and os.getenv is a wrapper to os.environ.get, at least in CPython.

EDIT: To respond to a comment, in CPython, os.getenv is basically a shortcut to os.environ.get ; since os.environ is loaded at import of os, and only then, the same holds for os.getenv.

回答 2

在带有iPython的Python 2.7中：

>>> import os
>>> os.getenv??
Signature: os.getenv(key, default=None)
Source:
def getenv(key, default=None):
    """Get an environment variable, return None if it doesn't exist.
    The optional second argument can specify an alternate default."""
    return environ.get(key, default)
File:      ~/venv/lib/python2.7/os.py
Type:      function

因此，我们可以得出结论os.getenv只是一个简单的包装os.environ.get。

点击查看英文原文

In Python 2.7 with iPython:

>>> import os
>>> os.getenv??
Signature: os.getenv(key, default=None)
Source:
def getenv(key, default=None):
    """Get an environment variable, return None if it doesn't exist.
    The optional second argument can specify an alternate default."""
    return environ.get(key, default)
File:      ~/venv/lib/python2.7/os.py
Type:      function

So we can conclude os.getenv is just a simple wrapper around os.environ.get.

回答 3

os.environ.get和之间没有功能上的区别os.getenv，但在和上设置条目之间却存在巨大差异。已损坏，因此您应默认设置为仅避免使用鼓励您用于对称的方法。os.putenvos.environos.putenvos.environ.getos.getenvos.putenv

os.putenv改变实际的OS级的环境变量，但在某种程度上，它不是通过露面os.getenv，os.environ或检查环境变量的任何其他方式STDLIB：

>>> import os
>>> os.environ['asdf'] = 'fdsa'
>>> os.environ['asdf']
'fdsa'
>>> os.putenv('aaaa', 'bbbb')
>>> os.getenv('aaaa')
>>> os.environ.get('aaaa')

您可能必须对C级getenv进行ctypes调用才能在调用后查看实际的环境变量os.putenv。（启动shell子进程并要求其提供环境变量也可能会起作用，如果您对转义和--norc/ --noprofile/ / /其他任何操作都非常谨慎，则需要避免启动配置，但这似乎很难解决。）

点击查看英文原文

While there is no functional difference between os.environ.get and os.getenv, there is a massive difference between os.putenv and setting entries on os.environ. os.putenv is broken, so you should default to os.environ.get simply to avoid the way os.getenv encourages you to use os.putenv for symmetry.

os.putenv changes the actual OS-level environment variables, but in a way that doesn’t show up through os.getenv, os.environ, or any other stdlib way of inspecting environment variables:

>>> import os
>>> os.environ['asdf'] = 'fdsa'
>>> os.environ['asdf']
'fdsa'
>>> os.putenv('aaaa', 'bbbb')
>>> os.getenv('aaaa')
>>> os.environ.get('aaaa')

You’d probably have to make a ctypes call to the C-level getenv to see the real environment variables after calling os.putenv. (Launching a shell subprocess and asking it for its environment variables might work too, if you’re very careful about escaping and --norc/--noprofile/anything else you need to do to avoid startup configuration, but it seems a lot harder to get right.)

回答 4

除了以上答案：

$ python3 -m timeit -s 'import os' 'os.environ.get("TERM_PROGRAM")'
200000 loops, best of 5: 1.65 usec per loop

$ python3 -m timeit -s 'import os' 'os.getenv("TERM_PROGRAM")'
200000 loops, best of 5: 1.83 usec per loop
点击查看英文原文

In addition to the answers above:

$ python3 -m timeit -s 'import os' 'os.environ.get("TERM_PROGRAM")'
200000 loops, best of 5: 1.65 usec per loop

$ python3 -m timeit -s 'import os' 'os.getenv("TERM_PROGRAM")'
200000 loops, best of 5: 1.83 usec per loop