try-except with str.index to find endpoints clear solution for Between Markers by rossras - python coding challenges

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try-except with str.index to find endpoints solution in Clear category for Between Markers by rossras

def between_markers(text: str, begin: str, end: str) -> str:
    """
        returns substring between two given markers
    """
    try:
        start_index = text.index(begin) + len(begin)
    except ValueError:
        start_index = 0
    try:
        end_index = text.index(end)
    except ValueError:
        end_index = len(text)
    return text[start_index:end_index]

Feb. 2, 2020

Comments:

alejandropoveda on March 25, 2020, 10:47 a.m.
Watch out with the try exceptions! They are very inefficient and should only use to catch exceptions. If you want to check a condition you should always use conditionals. Greetings.

rossras on March 25, 2020, 5:59 p.m.
Thanks for your remarks. As far as I understand, try-except has no performance penalty for the try: block, but takes about 10x longer than a conditional when actually catching the exception. On the other hand, if I used a conditional I'd have to check it every time. So, the sensible thing to do would be to use str.index() and catch ValueError if the ValueError is expected less than 1/10 of the time, but to use str.find() and check for -1 if the -1 is expected more than 1/10 of the time. In this case, we don't have that information, so I don't see a performance-based reason to prefer one over the other. As a matter of programming convenience, though, I'm not a huge fan of str.find() returning either an index where a substring begins or -1 if the substring isn't found. My problem is that -1 is a perfectly valid index into the string. If you don't explicitly check for the "not found" condition, it passes silently until some other problem happens later on down the line. That may be the right choice sometimes (like if you expect not to find the substring often), but it's generally not my first choice. Possibly my preference for str.index() trades speed for programmer convenience, but isn't that the tradeoff we're all making by using Python instead of C in the first place? See: <a href="https://docs.python.org/3/faq/design.html">https://docs.python.org/3/faq/design.html</a>#how-fast-are-exceptions <a href="https://mail.python.org/pipermail/tutor/2011-January/081143.html">https://mail.python.org/pipermail/tutor/2011-January/081143.html</a> <a href="https://stackoverflow.com/a/16138864">https://stackoverflow.com/a/16138864</a>

Strings Theory

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