When a function is used in python, what objects need to be created?

Question

I read somewhere that it is bad to define functions inside of functions in python, because it makes python create a new function object very time the outer function is called. Someone basically said this:

#bad
def f():
    def h():
        return 4
return h()

#faster
def h():
    return 4

def f(h=h):
    return h()

Is this true? Also, what about a case when I have a ton of constants like this:

x = # long tuple of strings
# and several more similar tuples
# which are used to build up data structures

def f(x):
    #parse x using constants above
    return parse dictionary

Is it faster if I put all the constants inside the definition of f? Or should I leave them outside and bind them to local names in a keyword argument? I don't have any data to do timings with unfortunately, so I guess I'm asking about your experiences with similar things.

Answer 1

Short answers to you questions - it is true. Inner functions are created each time outer function is called, and this takes some time. Also access to objects, which were defined outside of a function, is slower, comparing to access to local variables.

However, you also asked more important question - "should I care?". Answer to this, almost always, NO. Performance difference will be really minor, and readability of your code is much more valuable.

So, if you think that this function belongs to body of other function and makes no sense elsewhere - just put it inside and do not care about performance (at least, until your profiler tells you otherwise).