In Python what happens when you import inside of a function

Question

What are the pros and cons of importing a Python module and or function inside of a function  with respect to efficiency of speed and of memory   Does it re-import every time the function is run  or perhaps just once at the beginning whether or not the function is run

User · Answer

Might I suggest in general that instead of asking, "Will X improve my performance?" you use profiling to determine where your program is actually spending its time and then apply optimizations according to where you'll get the most benefit?

And then you can use profiling to assure that your optimizations have actually benefited you, too.

User · Answer

Does it re-import every time the function is run    No  or rather  Python modules are essentially cached every time they are imported  so importing a second  or third  or fourth     time doesn t actually force them to go through the whole import process again  1     Does it import once at the beginning whether or not the function is run    No  it is only imported if and when the function is executed  2  3  As for the benefits  it depends  I guess  If you may only run a function very rarely and don t need the module imported anywhere else  it may be beneficial to only import it in that function  Or if there is a name clash or other reason you don t want the module or symbols from the module available everywhere  you may only want to import it in a specific function   Of course  there s always from my module import my function as f for those cases    In general practice  it s probably not that beneficial  In fact  most Python style guides encourage programmers to place all imports at the beginning of the module file

User · Answer

The very first time you import goo from anywhere  inside or outside a function   goo py  or other importable form  is loaded and sys modules  goo   is set to the module object thus built   Any future import within the same run of the program  again  whether inside or outside a function  just look up sys modules  goo   and bind it to barename goo in the appropriate scope   The dict lookup and name binding are very fast operations   Assuming the very first import gets totally amortized over the program s run anyway  having the  appropriate scope  be module-level means each use of goo this  goo that  etc  is two dict lookups -- one for goo and one for the attribute name   Having it be  function level  pays one extra local-variable setting per run of the function  even faster than the dictionary lookup part   but saves one dict lookup  exchanging it for a local-variable lookup  blazingly fast  for each goo this  etc  access  basically halving the time such lookups take   We re talking about a few nanoseconds one way or another  so it s hardly a worthwhile optimization   The one potentially substantial advantage of having the import within a function is when that function may well not be needed at all in a given run of the program  e g   that function deals with errors  anomalies  and rare situations in general  if that s the case  any run that does not need the functionality will not even perform the import  and that s a saving of microseconds  not just nanoseconds   only runs that do need the functionality will pay the  modest but measurable  price   It s still an optimization that s only worthwhile in pretty extreme situations  and there are many others I would consider before trying to squeeze out microseconds in this way

User · Answer

It imports once when the function executes first time   Pros    imports related to the function they re used in easy to move functions around the package   Cons    couldn t see what modules this module might depend on

User · Answer

It imports once when the function is called for the first time   I could imagine doing it this way if I had a function in an imported module that is used very seldomly and is the only one requiring the import  Looks rather far-fetched  though

User · Answer

Importing inside a function will effectively import the module once   the first time the function is run   It ought to import just as fast whether you import it at the top  or when the function is run  This isn t generally a good reason to import in a def  Pros  It won t be imported if the function isn t called   This is actually a reasonable reason if your module only requires the user to have a certain module installed if they use specific functions of yours     If that s not he reason you re doing this  it s almost certainly a yucky idea

[python] In Python, what happens when you import inside of a function?

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