Wehn copying os.environ usinf copy.copy(), any manipulation on the copied object will change os.environment, too.

$ python
Python 2.7.3 (default, Apr 22 2012, 07:46:58) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import copy, os
>>> os.environ['TITI']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/UserDict.py", line 23, in __getitem__
    raise KeyError(key)
KeyError: 'TITI'
>>> env = copy.copy(os.environ)
>>> env['TITI'] = 'in den Ferien'
>>> os.environ['TITI']
'in den Ferien'
>>> 

Strictly speaking, this is correct, as the os.environment class is meant to manipulate the environment in the background. But user's expectation is different: he thinks, manipulating the copied object is save and does not effect the environment.

os.environ is not a dictionary, so it isn't all that surprising that a shallow copy doesn't behave like a shallow copy of a dictionary.  deepcopy does what you'd expect, as does os.environ.copy().

Perhaps it is worth improving this by adding a __copy__ method to os.environ.  Someone would need to propose a patch, and it would be an enhancement.

Here's a patch.

A new patch with tests.

I agree with David that copy(os.environ) is rather ambiguous. I think the preferred idiom should be to call dict(os.environ).
As for __copy__, I don't know what it should do: perhaps simply raise an error when copying is attempted?

Well, I think the fact that os.environ.copy is explicitly supported (and does indeed do dict(os.environ)) is an argument in favor of giving that same meaning to copy(os.environ).  I think that follows the principle of least surprise.

If add support for the copy protocol, consider also adding support for deepcopying and pickling.

[Antoine]
> I agree with David that copy(os.environ) is rather ambiguous.
> I think the preferred idiom should be to call dict(os.environ).
> As for __copy__, I don't know what it should do: perhaps
> simply raise an error when copying is attempted?

FWIW, I concur with Antoine on every point.  Also, raising an error when copying seems reasonable ("refuse the temptation to guess" at what the user intended).

patch looks OK.
But I prefer `__deepcopy__ = __copy__ = copy`.

I don't know how to support pickling. (should unpickled object
reference os.environ, or copied dict?)
I feel it is different issue.

That patch would be fine except that I really agree with Antoine that an error should be raised.

I agree.
os.environ is not dict, it's a proxy of "current" environment which
is not copyable.
So I agree to copy.copy() shouldn't copy os.environ implicitly.

Would we remove the functionality of os.environ.copy()? It seems very odd for types to have a .copy() method that works, while not supporting copy.copy, especially when there is zero documentation, on the web or the docstring, to even hint at the difference.

I'm strongly in favor of silently doing the right thing and behaving the same way the .copy() method already behaves; if you want a "copy" of os.environ that still modifies the environment, that's just aliasing it (envalias = os.environ), not copying at all. If you're trying to make a shallow copy, not an alias, you're trying to separate it from the parent, which every other dict-like thing does (assuming immutable values), where os.environ is a very weird exception (for copy.copy, but not the .copy() method).

Can someone give an example where you'd want copy.copy to produce a "shallow copy" that acts like an alias, not an actual independent copy?

I think it may be good to deprecate and discourage use of `os.environ.copy()`, and add a new method `os.environ.asdict()`. And possibly have `__copy__` raise an error pointing users to `os.environ.asdict()`.

I'm not sure what to do about pickling.

I think an asdict() method is not necessary. Instanciating a dict from os.environ is enough: 

>>> import os
>>> os.environ
environ({'SHELL': '/bin/bash', ...})
>>> dict(os.environ)
{'SHELL': '/bin/bash', ...})

With 'dict()', it's obvious there is no side effect.


I think os._Environ should not be pickable too because there is an ambiguity too when it will be unpicked: should we set the unpickled data to a basic dict (or another _Environ instance) or to the environment variables?

It seems there are more maintainers in favour of the removal. Removing the copy is a breaking change so it should be done on several releases.

Are you interested by a PR which adds DeprecationWarning when copy.copy(os.environ) or os.environ.copy() are called? It seems to be the first step to implement.

I think the advantage of asdict() method is it's more discoverable and it doesn't leave any uncertainty of whether returned value will update environment or not. If a user sees `dict(os.environ)` in code, they may wonder if it does or does not; and there's no obvious way to confirm.

If they see os.environ.asdict(), it will be safe to assume that the method is provided exactly for this purpose, and the way to confirm is obvious - checking the docstring.

I guess the easy way to test it would be to modify the copy and check os.environ, but still there would be some uncertainty if it's consistent across platforms and python versions, if it's expected to still work in future python versions, etc. asdict() docstring is much more explicit.