Python: weird iterator behaviour after filtering out empty strings

#python #iterators #string #filter

I was fiddling around with Python trying to learn some basics and while making a string replacer script I ran across a weird problem.
The following script was supposed to replace all instances of "Input" by each element in subs and print out all the replaced strings.

text = """
MyInput = GetSubsystem<Input>();
extern Input* MyInput;
Input* MyInput = 0;
"""
texts = text.split("\n");
texts = filter(None,texts)#commenting this out fixes the iteration??
original = "Input";
subs = ["Time","WorkQueue","FileSystem","Log"]
sortbysubs = 1
separate = 0
print("number of subs:{} \n".format(len(subs)))
if (sortbysubs):
    for s in subs:
        for t in texts:
            print(t.replace(original,s))
        if separate:print("")
else:
    for t in texts:
        for s in subs:
            print(t.replace(original,s))
        if separate:print("")

When sortbysubs = 0, it seems to work correctly: printing each line of text with each element in subs, and filtering out the empties beforehand.

When sortbysubs = 1, it only prints each text for the first element of subs.

Somehow there is a difference betweem printing "for s for t" rather than "for t for s".

Further more, this behaviour only emerges when filtering the texts for empties.
Commenting out the line "texts = filter(None,texts)" results in the problem disappearing.

Can anyone shed light on what exactly is happening here and how to fix it?