Find first sequence item that matches a criterion

python

generator

performance

byAnton Shumikhin·Feb 7, 2025

For pinpointing the first item meeting a condition, utilize this efficient combo: next() and generator:

numbers = [1, 2, 3, 4, 5]
# Hide the kids... the Lambo is coming out!
match = next((num for num in numbers if num > 3), None)

Here, match is the first number over 3 so it becomes 4. If the match is absent, None saves the day by preventing glitches.

How to become an efficiency guru with `next()` and a generator

When dealing with sizeable datasets, next() paired with a generator outshines other approaches. Generators yield items one by one, guzzling less memory and hitting the exit before trawling the entire list.

`next()` vs. list comprehension: Who wins?

Sure, list comprehensions are slick. But they're busybodies - evaluating everything to build a fresh list. In contrast, next() keeps it simple and swift - it finds the first match and skips inspecting beyond that. Making it perfect for times when performance isn't just a buzzword.

Missing items? Here's how to smooth things out using a default value

There might be occasions with no match. But we can be prepared by providing a default_value:

# Some may say this is a default route to happiness
match = next((item for item in sequence if condition), default_value)

Now, instead of a show-stopping StopIteration exception, you'll get your friendly default_value back.

Taking the reins with try-except

If defining a default value upfront isn't your style, embrace try-except strategy. Use it to wrap next() and catch a potential StopIteration:

try:
    # There's gold in them hills
    match = next(item for item in sequence if condition)
except StopIteration:
    # I didn't choose the default value. The default value chose me
    match = 'Default Value'

The alternative world of filter function

You might feel tempted to use the filter() function. But before you jump ship, remember filter() is quirky — it responds with a filter object, not directly your target match.

Why `index()` can be a dark alley

The index() method might seem apt for finding an element's position. But beware, it demands reinforcements — a helper function or a list comprehension, thereby compromising efficiency.