Gizmo wrote:
> Hello
> I am learning Python via the excellent Dive Into Python book. I have
> little question
>
> 1)
> >>> import os
> >>> type(os.environ)
> <type 'instance'>
>
> Why is this considered "instance" ? Should'nt it be "dict" type?
> I have a feeling I am missing something deeper here.
'instance' is the type of instances of old-style classes. For example:
In [4]: class Foo: pass
...:
In [6]: f=Foo()
In [7]: type(f)
Out[7]: <type 'instance'>
On the other hand, the type of an instance of a new-style class reflects
its actual class:
In [9]: class Bar(object): pass
...:
In [10]: b=Bar()
In [11]: type(b)
Out[11]: <class '__main__.Bar'>
You can find out the class of an instance by looking at its __class__
attribute:
In [3]: os.environ.__class__
Out[3]: <class os._Environ at 0x00B42F30>
You can also see the base classes of the class:
In [8]: os.environ.__class__.__bases__
Out[8]: (<class UserDict.IterableUserDict at 0x00B4E1E0>,)
So os.environ is not a dict or a subclass of dict. It is a subclass of
UserDict.IterableUserDict. This is a class that is used to make custom
mapping objects.
If you look at the docs for os.environ you see that it does not say it
is a dict, it says it is a mapping object; that is, it is something that
acts like a dict.
See also http://docs.python.org/ref/node33.html
> 2) What would happen if I did this
> >>> os.environ = {}
> Hopefully, I wont be allowed to do that...but I am too scared to try.
You can do it. That would replace the os.environ object with an empty
dict, which would have a bad effect on any code that reads values from
os.environ. On the other hand you can restore the old value just by
restarting Python so it is not such a disaster.
Kent
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