Questions on "import" and "datetime"

[Yunfei Dai]

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 at gmail.com> wrote:
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> > Hi all,
> 
> 
> 
> Hi Yunfei,
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> 
> 
> >
> 
> > 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
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> xlrd.open_workbook" would be a SyntaxError :)
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> 
> 
> > 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
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> 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:
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> 
> 
> from foo import bar
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> 
> 
> import foo;bar = foo.bar
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> 
> 
> If foo.bar happens to be a module (module 'bar' in package 'foo'), you
> 
> could also do this:
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> 
> 
> import foo.bar as bar
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> 
> 
> ...and that restriction is where your problem lies.
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> 
> 
> > But what is the problem of datetime?
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> 
> 
> I'm not familiar with xlrd, but I believe the difference between
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> xlrd.open_workbook and datetime.datetime would be that
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> xlrd.open_workbook is a module in a package, while datetime.datetime
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> is a class in a module.  'from ... import ...' can import any object
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> from the target module/package, and assign it to a local name.
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> 'import ...' on the other hand can only import a module (you'll notice
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> the ImportError you get when you try 'import datetime.datetime' is 'No
> 
> module named datetime'.  This particular example is a bit confusing
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> due to there being a class in a module of the same name, but try
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> 'import datetime.date' for a clearer message.
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> 
> 
> > 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
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> with 'import datetime;type(datetime.datetime)').  In Python 2, this
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> means it is a new-style class (meaning it is a subclass of 'object').
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> In Python 3, it's just a class (since there are no longer old-style
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> classes).
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> 
> 
> > Thanks very much in advance! Very looking forward to your answers.
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> >
> 
> > Best,
> 
> > Yunfei
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> >
> 
> 
> 
> I hope I have actually answered your question and not just muddied
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> things further for you.  You can of course ask again if I've made
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> things worse :)
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> 
> 
> -- 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