Is there an overhead when nesting functions in Python?

Question

In Python, if I have a child function within a parent function, is the child function \"initialised\" (created) every time the parent function is called? Is there any overhe

Answer 1

There is an impact, but in most situations it is so small that you shouldn't worry about it - most non-trivial applications probably already have performance bottlenecks whose impacts are several orders of magnitude larger than this one. Worry instead about the readability and reusability of the code.

Here some code that compares the performance of redefining a function each time through a loop to reusing a predefined function instead.

import gc
from datetime import datetime

class StopWatch:
     def __init__(self, name):
         self.name = name

     def __enter__(self):
         gc.collect()
         self.start = datetime.now()

     def __exit__(self, type, value, traceback):
         elapsed = datetime.now()-self.start
         print '** Test "%s" took %s **' % (self.name, elapsed)

def foo():
     def bar():
          pass
     return bar

def bar2():
    pass

def foo2():
    return bar2

num_iterations = 1000000

with StopWatch('FunctionDefinedEachTime') as sw:
    result_foo = [foo() for i in range(num_iterations)]

with StopWatch('FunctionDefinedOnce') as sw:
    result_foo2 = [foo2() for i in range(num_iterations)]

When I run this in Python 2.7 on my Macbook Air running OS X Lion I get:

** Test "FunctionDefinedEachTime" took 0:00:01.138531 **
** Test "FunctionDefinedOnce" took 0:00:00.270347 **