Python timeit module execution confusion

Question

I'm trying to use the timeit module in Python (EDIT: We are using Python 3) to decide between a couple of different code flows. In our code, we have a series of if-statements that test for the existence of a character code in a string, and if it's there replace it like this:

if "<substring>" in str_var:
    str_var = str_var.replace("<substring>", "<new_substring>")

We do this a number of times for different substrings. We're debating between that and using just the replace like this:

str_var = str_var.replace("<substring>", "<new_substring>")

We tried to use timeit to determine which one was faster. If the first code-block above is "stmt1" and the second is "stmt2", and our setup string looks like

str_var = '<string><substring><more_string>',

our timeit statements will look like this:

timeit.timeit(stmt=stmt1, setup=setup)

and

timeit.timeit(stmt=stmt2, setup=setup)

Now, running it just like that, on 2 of our laptops (same hardware, similar processing load) stmt1 (the statement with the if-statement) runs faster even after multiple runs (3-4 hundredths of a second vs. about a quarter of a second for stmt2).

However, if we define functions to do both things (including the setup creating the variable) like so:

def foo():
    str_var = '<string><substring><more_string>'
    if "<substring>" in str_var:
        str_var = str_var.replace("<substring>", "<new_substring>")

and

def foo2():
    str_var = '<string><substring><more_string>'
    str_var = str_var.replace("<substring>", "<new_substring>")

and run timeit like:

timeit.timeit("foo()", setup="from __main__ import foo")
timeit.timeit("foo2()", setup="from __main__ import foo2")

the statement without the if-statement (foo2) runs faster, contradicting the non-functioned results.

Are we missing something about how Timeit works? Or how Python handles a case like this?

edit here is our actual code:

>>> def foo():
    s = "hi 1 2 3"
    s = s.replace('1','5')

>>> def foo2():
    s = "hi 1 2 3"
    if '1' in s:
        s = s.replace('1','5')


>>> timeit.timeit(foo, "from __main__ import foo")
0.4094226634183542
>>> timeit.timeit(foo2, "from __main__ import foo2")
0.4815539780738618

vs this code:

>>> timeit.timeit("""s = s.replace("1","5")""", setup="s = 'hi 1 2 3'")
0.18738432400277816
>>> timeit.timeit("""if '1' in s: s = s.replace('1','5')""", setup="s = 'hi 1 2 3'")
0.02985000199987553

Answer 1

I think I've got it.

Look at this code:

timeit.timeit("""if '1' in s: s = s.replace('1','5')""", setup="s = 'hi 1 2 3'")

In this code, setup is run exactly once. That means that s becomes a "global". As a result, it gets modified to hi 5 2 3 in the first iteration and in now returns False for all successive iterations.

See this code:

timeit.timeit("""if '1' in s: s = s.replace('1','5'); print(s)""", setup="s = 'hi 1 2 3'")

This will print out hi 5 2 3 a single time because the print is part of the if statement. Contrast this, which will fill up your screen with a ton of hi 5 2 3s:

timeit.timeit("""s = s.replace("1","5"); print(s)""", setup="s = 'hi 1 2 3'")

So the problem here is that the non-function with if test is flawed and is giving you false timings, unless repeated calls on an already processed string is what you were trying to test. (If it is what you were trying to test, your function versions are flawed.) The reason the function with if doesn't fair better is because it's running the replace on a fresh copy of the string for each iteration.

The following test does what I believe you intended since it doesn't re-assign the result of the replace back to s, leaving it unmodified for each iteration:

>>> timeit.timeit("""if '1' in s: s.replace('1','5')""", setup="s = 'hi 1 2 3'"
0.3221409016812231
>>> timeit.timeit("""s.replace('1','5')""", setup="s = 'hi 1 2 3'")
0.28558505721252914

This change adds a lot of time to the if test and adds a little bit of time to the non-if test for me, but I'm using Python 2.7. If the Python 3 results are consistent, though, these results suggest that in saves a lot of time when the strings rarely need any replacing. If they usually do require replacement, it appears in costs a little bit of time.