[Tutor] Importing Modules Within Classes

[Eric Walker]

That was me using the import within the class. I only did it due to my 
ignorance. I am not advanced enough to use it like that and know what I am 
doing, so as suggested I just moved all of my imports to the top.

Python Newbie.....
On Thursday 06 October 2005 08:21 pm, Kent Johnson wrote:
> Daniel Watkins wrote:
> > Recently, there has been an example of someone importing modules within
> > a class definition. eg:
> >
> > class Exemplar:
> >  import re
> >  ...use re...
> >
> > It seems obvious to me that this is incorrect, though more through
> > training than actual observation on my part, and it should be:
> >
> > import re
> >
> > class Exemplar:
> >  ...use re...
>
> It's not really incorrect, it is legal syntax and it will do what you
> expect. It is not idiomatic and in general it is simpler to just put all
> the imports at the top.
>
> > However, someone (I don't recall who) said that there were occasions
> > when it would be appropriate to import modules the former way. I was
> > just wondering under what circumstances importing should be done this
> > way?
>
> That was me. I nest imports quite frequently in Jython code where the first
> import of a module is fairly expensive in time. Putting the import in the
> function that needs it delays the import and perhaps the module won't be
> imported at all.
>
> import is an executable statement. When it runs, the interpreter first
> looks to see if the module has already been imported. If so, the existing
> module is bound to the import name in the current namespace. If the module
> has not yet been imported, it is loaded *and executed*, then bound to the
> name in the current namespace.
>
> So if module A imports B which imports C and D, then importing A will also
> load and execute B, C and D. If any of these are time-consuming you may
> want to defer them.
>
> I found with my Jython programs that I could shorten start-up time quite a
> bit by deferring some imports until they were needed.
>
> Another reason for the nested style of imports is to resolve problems with
> circular imports. There are some subtle problems that can occur when A
> imports B and B imports A. By nesting one of the imports you can defer it
> and sometimes avoid the problem. In this case I think removing the circular
> import is a much better solution - circular dependencies are evil!
>
> Kent
>
> > Cheers,
> > Dan
> >
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