Find the longest word in a sentence in Python

max() normally compares words by character code, which is close to alphabetical order (capital letters come first). With key=len, it compares their lengths instead and gives back the longest word.

The program

import string

sentence = input("Enter a sentence: ")
words = [word.strip(string.punctuation) for word in sentence.split()]

longest = max(words, key=len)
print(f"Longest word: {longest} ({len(longest)} letters)")

Typed in this example: Hedy Lamarr invented frequency hopping.. Press Run to type your own.

Enter a sentence: Hedy Lamarr invented frequency hopping.
Longest word: frequency (9 letters)

How it works

• sentence.split() gives the list of words. The last one is "hopping." with its full stop.
• word.strip(string.punctuation) removes punctuation from the start and the end of each word, so "hopping." becomes "hopping". Without this step, the full stop would be counted as a letter.
• key=len tells max() to call len() on each word and compare those numbers. On a tie, max() gives the first of the longest words.

Ties

When several words have the same length, find the longest length first, then keep every word with that length:

sentence = "Alan wrote about logic and minds"
words = sentence.split()

longest_length = max(len(word) for word in words)
longest_words = [word for word in words if len(word) == longest_length]

print(longest_length, longest_words)

5 ['wrote', 'about', 'logic', 'minds']

On an empty sentence, max() raises a ValueError, because there is nothing to compare. max(words, key=len, default="") gives back an empty string instead.