Where to place imports

John [H2O] washakie at gmail.com
Fri Jan 23 18:08:22 CET 2009


So it isn't inefficient to import a bunch of modules that may not be used,
say if only one function is used that doesn't rely on a larger module like
numpy or pylab?

Thanks.


Diez B. Roggisch-2 wrote:
> 
> John [H2O] schrieb:
>> Hello, Im writing some modules and I am a little confused about where to
>> place my imports...
>> 
>> I don't really do any class programming yet, just defining a bunch of
>> functions, so presently I have something along the lines of:
>> 
>> import sys
>> import os
>> import traceback
>> 
>> 
>> def Foo1(a,b):
>>    import numpy
>>    import datetime
>>    return c
>> 
>> def Foo2(a,b):
>>    import datetime
>> 
>>    c = Foo1(a,b)
>>    return c
>> 
>> etc...
>> 
>> Above obviously just for the form, but the point is, in some cases I may
>> import modules twice, but other times I may not use them at all, so
>> should I
>> import everything into the main module? Or individually import them into
>> functions. Is there a performance penalty either way? 
> 
> Usually, you should import on the top of the module. That's the cleanest 
> way, and the most performant one.
> 
> There is a small performance penalty when doing it inside functions - 
> but not too much. It boils down to a lookup if the imported module has 
> already been imported before.
> 
> 
> The only valid reason for doing imports inside functions is if you 
> otherwise get into circular dependency hell, or have modules that need 
> some manipulation of the sys.path before they actually can be imported. 
> This is never true for system modules, and certainly to avoid if possible.
> 
> Diez
> --
> http://mail.python.org/mailman/listinfo/python-list
> 
> 

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