Why is calling eval so slow?

[Steven D'Aprano]

I understand that calling eval() on a string is going to be slow, since 
the string has to be parsed, compiled, and then executed.

But why is it so slow when called on function __code__ objects and pre-
compiled byte-code?


Here are the tests I ran:

# calling a regular function
python3.5 -m timeit -s "f = lambda: 99" "f()"
# evaluating a function code object
python3.5 -m timeit -s "f = (lambda: 99).__code__" "eval(f)"
# estimate the overhead of the eval name lookup
python3.5 -m timeit "eval"
# evaluating a pre-compiled byte-code object
python3.5 -m timeit -s "f = compile('99', '', 'eval')" "eval(f)"
# evaluating a string
python3.5 -m timeit "eval('99')"


And the results on my computer:

# call a regular function
1000000 loops, best of 3: 0.245 usec per loop
# evaluate the function __code__ object
1000000 loops, best of 3: 1.16 usec per loop
# overhead of looking up "eval"
10000000 loops, best of 3: 0.11 usec per loop


which means it takes four times longer to execute the code object alone, 
compared to calling the function object which executes the code object.

Why so slow?


# evaluate the pre-compiled byte-code
1000000 loops, best of 3: 1.13 usec per loop
# parse, compile and evaluate a string
10000 loops, best of 3: 23.5 usec per loop

That one, at least, makes perfect sense to me :-)



-- 
Steve