[python] Is nested function a good approach when required by only one function?

I found this question because I wanted to pose a question why there is a performance impact if one uses nested functions. I ran tests for the following functions using Python 3.2.5 on a Windows Notebook with a Quad Core 2.5 GHz Intel i5-2530M processor

def square0(x):
    return x*x

def square1(x):
    def dummy(y):
        return y*y
    return x*x

def square2(x):
    def dummy1(y):
        return y*y
    def dummy2(y):
        return y*y
    return x*x

def square5(x):
    def dummy1(y):
        return y*y
    def dummy2(y):
        return y*y
    def dummy3(y):
        return y*y
    def dummy4(y):
        return y*y
    def dummy5(y):
        return y*y
    return x*x

I measured the following 20 times, also for square1, square2, and square5:

s=0
for i in range(10**6):
    s+=square0(i)

and got the following results

>>> 
m = mean, s = standard deviation, m0 = mean of first testcase
[m-3s,m+3s] is a 0.997 confidence interval if normal distributed

square? m     s       m/m0  [m-3s ,m+3s ]
square0 0.387 0.01515 1.000 [0.342,0.433]
square1 0.460 0.01422 1.188 [0.417,0.503]
square2 0.552 0.01803 1.425 [0.498,0.606]
square5 0.766 0.01654 1.979 [0.717,0.816]
>>> 

square0 has no nested function, square1 has one nested function, square2 has two nested functions and square5 has five nested functions. The nested functions are only declared but not called.

So if you have defined 5 nested funtions in a function that you don't call then the execution time of the function is twice of the function without a nested function. I think should be cautious when using nested functions.

The Python file for the whole test that generates this output can be found at ideone.

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