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Date: Mon, 10 Jun 2013 15:10:16 -0400
From: Dave Angel <davea@davea.name>
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Subject: Re: Questions on "import" and "datetime"
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On 06/10/2013 01:01 PM, Zachary Ware wrote:
> On Mon, Jun 10, 2013 at 10:37 AM, Yunfei Dai <yunfei.dai.sigma@gmail.com> wrote:
>> Hi all,
>
> Hi Yunfei,
>
>>
>> I have some questions on "import":
>>
>> 1."from datetime import datetime" works well. But I am confused why "import datetime.datetime" leads to importerror. "from xlrd import open_workbook" could be replaced by "from xlrd.open_workbook" without any problem.
>

It's a historical flaw in datetime that the class has the same name as 
the module it's in.  It should have been called class Datetime 
(according to pep 10, class names should be capitalized).  If that were 
the case, it'd be clear that datetime.Datetime is a class.

It would also be clearer that
     from datetime import Datetime

creates an alias in the present global namespace for the 
datetime.Datetime class, as simply Datetime.

This class has attributes that you can access, like Datetime.hour, and 
it has static methods like Datetime.fromTimeStamp().
  And like all classes, it's "callable", meaning that you can create an 
instance by pretending it's a function:
    obj = Datetime(2013, 12, 1)

But since it's not a module inside a package, you can't use the form:

import datetime.Datetime

Now, just interpret all the above with a lowercase "D" and the confusion 
becomes clearer.  The compiler/interpreter doesn't care either way.

    <SNIP>
> "from ... import ..." imports an object from a module and assigns it
> to a local name that is the same as the name in the other module.  In
> other words, the following two examples do the same thing:
>
> from foo import bar
>
> import foo;bar = foo.bar
>
> If foo.bar happens to be a module (module 'bar' in package 'foo'), you
> could also do this:
>
> import foo.bar as bar
>
> ...and that restriction is where your problem lies.
>

In other words, since datetime.datetime is a class, not a module, you 
can't just import it.

     <SNIP>
>
>> 2.I am also comfused that "datetime.datetime" is a function but whithin "datetime.datetime" there are lots of other functions. So what is the type of "datetime.datetime" on earth? is it a function, or a class or a folder(library) here?
>

As I said before, datetime.datetime is a class, and the functions within 
it are called methods.

You can see it all for yourself very easily.  Use the __file__ attribute 
to locate the source for datetime module on your system.  Here's what it 
looks like on mine:

 >>> import datetime
 >>> datetime.__file__
'/usr/local/lib/python3.3/datetime.py'

Then you can go look at that file.  For my copy, the datetime class 
begins at 1301.  But you can just search for the following line:



class datetime(date):

HTH

-- 
DaveA