Questions on "import" and "datetime"

[Yunfei Dai]

Hi all,

I have some questions on "import":

1."from datetime import datetime" works well. But I am confused why "importdatetime.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 donot 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 toname 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

 

[Zachary Ware]

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 maybeleads 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

 

[Dave Angel]

Hi Yunfei,

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.

"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.


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:
'/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

 

[Yunfei Dai]

Hi Yunfei,







I assume you mean "import xlrd.open_workbook" here, as "from

xlrd.open_workbook" would be a SyntaxError






"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.






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.






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).






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

 

[Yunfei Dai]

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.











In other words, since datetime.datetime is a class, not a module, you

can't just import it.








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:




'/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

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