Python's most efficient way to choose longest string in list?

Gracefully handling edge cases

Consider these edge cases: an empty list or a list with equal length longest strings. Here's how to handle them:

Empty list scenario:

longest = max(my_list, key=len, default='')


This returns an empty string for an empty list. Now that's being graceful!

Multiple longest strings scenario:

longest_strings = [s for s in my_list if len(s) == len(max(my_list, key=len))]


Here, with a one-liner list comprehension, you're fighting for equality rights for all the longest strings.

Efficient looping for extra tasks

If you need more than just the length, here's a loop to the rescue:

max_len = 0
for s in my_list:
    if len(s) > max_len:
        longest = s
        max_len = len(s)
        # Add your spice here


Here's the magic trick: looping like this keeps calls to max() to a minimum, making it run like Usain Bolt.

The nuances of optimization

The max function is a sprinter, but you can make it an Olympic champion event by knowing its strengths and weaknesses:

Dodge the lambda bullet

A key=len over a lambda function keeps your code light on its feet and avoids the extra overhead of a lambda's function call:

# Super fast:
longest = max(my_list, key=len)

# Still fast, but a bit overkill:
longest = max(my_list, key=lambda item: len(item))


Minimizing list accesses for speed

Don't call max() again and again – save the result and recall when needed; It's efficiency mantra!

max_length = len(max(my_list, key=len))
longest_strings = [s for s in my_list if len(s) == max_length]


Python 2.x scenario

When working with Python 2.x, watch out for unicode and byte strings:

# In Python 2.x, lists can have both:
my_list = [u'short', 'longer string', u'very long unicode string']

# Allow unicode_literals for consistency:
from __future__ import unicode_literals


The secret sauce: other resources

Check out Effbot's articles for ninja tips on Python list operations.