What is the type of os.environ? and Why does it not support viewkeys method

Question

If I print os.environ, the output looks like a dictionary. Some posts I read online say that it is a memory based dictionary. But it does not support .viewkeys() method and tells me that: _Environ instance does not support this method. So what is the exact type of os.environ. If I try:

print type(os.environ)

I get instance as the answer.

Can please clarify this behavior of os.environ?

Answer 1

>>> os.environ.__class__
<class os._Environ at 0xb7865e6c>

It is a subclass of UserDict.IterableUserDict.

In python 2.7 the source can be found is in os.py on line 413 (Windows) and line 466 (Posix). Here is the python 3.2 source.

Answer 2

It is an os._Environ instance:

>>> os.environ.__class__
<class os._Environ at 0x01DDA928>

It is defined in Python's library, file os.py and cannot be a plain dictionary because updating the dictionary must also update the process's environment. Also the key lookups need to be case insensitive on Windows.

In Python 2.x it subclasses UserDict.IterableUserDict which presumably doesn't have the new viewkeys() method. In Python 3.x it implements the MutableMapping abc but has no other explicit base classes.