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
