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
