Where to place imports

Steve Holden steve at holdenweb.com
Fri Jan 23 19:01:44 CET 2009


Diez B. Roggisch wrote:
[...]
> 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.
> 
Well there is also the somewhat obscure point that an import of specific
names inside a function makes them local, which *might* make a
performance difference with tight loop nesting inside - though obviously
the need for such will best be determined by benchmarking:

>>> def f1():
...   from os.path import join
...   print join
...
>>> from os.path import join
>>> def f2():
...   print join
...
>>> dis.dis(f1)
  2           0 LOAD_CONST               1 (-1)
              3 LOAD_CONST               2 (('join',))
              6 IMPORT_NAME              0 (os.path)
              9 IMPORT_FROM              1 (join)
             12 STORE_FAST               0 (join)
             15 POP_TOP

  3          16 LOAD_FAST                0 (join)
             19 PRINT_ITEM
             20 PRINT_NEWLINE
             21 LOAD_CONST               0 (None)
             24 RETURN_VALUE
>>> dis.dis(f2)
  2           0 LOAD_GLOBAL              0 (join)
              3 PRINT_ITEM
              4 PRINT_NEWLINE
              5 LOAD_CONST               0 (None)
              8 RETURN_VALUE
>>>

Not a major issue, just another aspect of the question ... and of course
the OP should take the advice already given unless and until performance
becomes a problem (since it usually won't).

regards
 Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC              http://www.holdenweb.com/




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