Re: Questions on import and datetime
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.si...@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 -- http://mail.python.org/mailman/listinfo/python-list
Re: Questions on import and datetime
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.si...@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. -- http://mail.python.org/mailman/listinfo/python-list
Re: Questions on import and datetime
On Mon, Jun 10, 2013 at 10:37 AM, Yunfei Dai yunfei.dai.si...@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 -- http://mail.python.org/mailman/listinfo/python-list
Re: Questions on import and datetime
On 06/10/2013 01:01 PM, Zachary Ware wrote: On Mon, Jun 10, 2013 at 10:37 AM, Yunfei Dai yunfei.dai.si...@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 -- http://mail.python.org/mailman/listinfo/python-list
