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Questions on "import" and "datetime"

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  Questions on "import" and "datetime" Yunfei Dai <yunfei.dai.sigma@gmail.com> - 2013-06-10 08:37 -0700
    Re: Questions on "import" and "datetime" Zachary Ware <zachary.ware+pylist@gmail.com> - 2013-06-10 12:01 -0500
      Re: Questions on "import" and "datetime" Yunfei Dai <yunfei.dai.sigma@gmail.com> - 2013-06-12 03:29 -0700
    Re: Questions on "import" and "datetime" Dave Angel <davea@davea.name> - 2013-06-10 15:10 -0400
      Re: Questions on "import" and "datetime" Yunfei Dai <yunfei.dai.sigma@gmail.com> - 2013-06-12 03:41 -0700

#47574 — Questions on "import" and "datetime"

Hi all,

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. The only difference here is that if "from xlrd import open_workbook" is used we do not have to write "xlrd.open_workbook" in the following code but just "open_workbook". So my understanding of the difference is "from...import..." shortens the code (just like "using namespace std" in C++) but maybe leads to name clash. But what is the problem of datetime? 

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?

Thanks very much in advance! Very looking forward to your answers.

Best,
Yunfei

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#47582

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.

I assume you mean "import xlrd.open_workbook" here, as "from
xlrd.open_workbook" would be a SyntaxError :)

> The only difference here is that if "from xlrd import open_workbook" is used we do not have to write "xlrd.open_workbook" in the following code but just "open_workbook". So my understanding of the difference is "from...import..." shortens the code (just like "using namespace std" in C++) but maybe leads to name clash.

"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.

> But what is the problem of datetime?

I'm not familiar with xlrd, but I believe the difference between
xlrd.open_workbook and datetime.datetime would be that
xlrd.open_workbook is a module in a package, while datetime.datetime
is a class in a module.  'from ... import ...' can import any object
from the target module/package, and assign it to a local name.
'import ...' on the other hand can only import a module (you'll notice
the ImportError you get when you try 'import datetime.datetime' is 'No
module named datetime'.  This particular example is a bit confusing
due to there being a class in a module of the same name, but try
'import datetime.date' for a clearer message.

> 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?

datetime.datetime is actually a type of type 'type' (as can be seen
with 'import datetime;type(datetime.datetime)').  In Python 2, this
means it is a new-style class (meaning it is a subclass of 'object').
In Python 3, it's just a class (since there are no longer old-style
classes).

> Thanks very much in advance! Very looking forward to your answers.
>
> Best,
> Yunfei
>

I hope I have actually answered your question and not just muddied
things further for you.  You can of course ask again if I've made
things worse :)

-- Zach

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#47771

On Monday, June 10, 2013 7:01:30 PM UTC+2, 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.
> 
> 
> 
> I assume you mean "import xlrd.open_workbook" here, as "from
> 
> xlrd.open_workbook" would be a SyntaxError :)
> 
> 
> 
> > The only difference here is that if "from xlrd import open_workbook" is used we do not have to write "xlrd.open_workbook" in the following code but just "open_workbook". So my understanding of the difference is "from...import..." shortens the code (just like "using namespace std" in C++) but maybe leads to name clash.
> 
> 
> 
> "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.
> 
> 
> 
> > But what is the problem of datetime?
> 
> 
> 
> I'm not familiar with xlrd, but I believe the difference between
> 
> xlrd.open_workbook and datetime.datetime would be that
> 
> xlrd.open_workbook is a module in a package, while datetime.datetime
> 
> is a class in a module.  'from ... import ...' can import any object
> 
> from the target module/package, and assign it to a local name.
> 
> 'import ...' on the other hand can only import a module (you'll notice
> 
> the ImportError you get when you try 'import datetime.datetime' is 'No
> 
> module named datetime'.  This particular example is a bit confusing
> 
> due to there being a class in a module of the same name, but try
> 
> 'import datetime.date' for a clearer message.
> 
> 
> 
> > 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?
> 
> 
> 
> datetime.datetime is actually a type of type 'type' (as can be seen
> 
> with 'import datetime;type(datetime.datetime)').  In Python 2, this
> 
> means it is a new-style class (meaning it is a subclass of 'object').
> 
> In Python 3, it's just a class (since there are no longer old-style
> 
> classes).
> 
> 
> 
> > Thanks very much in advance! Very looking forward to your answers.
> 
> >
> 
> > Best,
> 
> > Yunfei
> 
> >
> 
> 
> 
> I hope I have actually answered your question and not just muddied
> 
> things further for you.  You can of course ask again if I've made
> 
> things worse :)
> 
> 
> 
> -- Zach

Hi Zach,

Thanks so much for your quick, long and detailed reply and sorry for replying you late. It is really helpful for me understanding "import". 

Yunfei

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#47588

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

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#47774

On Monday, June 10, 2013 9:10:16 PM UTC+2, Dave Angel wrote:
> 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

Thank you Dave for your reply! It is very helpful.

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