UnboundLocalError: local variable referenced before assignment doesn’t work in command line call

I am aware that there are many solutions to this kind of question. However, none of them seems to have helped with my case. This is the code I’m referring to:

from nltk.book import text4

def length_frequency(length):
    '''
    Parameter: length as an integer
    '''
    # finds words in given length
    counter = 0
    word_in_length = {}
    for word in text4:
        if len(word) == length and word not in word_in_length:
            word_in_length[word] = text4.count(word)
    for key in word_in_length:
        if word_in_length[key] > counter:
            max = word_in_length[key]
            counter = max
            max_word = key
    print(f'The most frequent word with {length} characters is "{max_word}".nIt occurs {counter} times.')

length_frequency(7)

Output:
The most frequent word with 7 characters is "country".
It occurs 312 times.

When I try this code in PyCharm, it works without problems. However, if I use it via command line call it gives this error:

Traceback (most recent call last):
  File "program5.py", line 67, in <module>
    main()
  File "program5.py", line 60, in main
    length_frequency(input_length)
  File "program5.py", line 35, in length_frequency
    print(f'The most frequent word with {length} characters is "{max_word[0]}".nIt occurs {counter} times.')
UnboundLocalError: local variable 'max_word' referenced before assignment

Of course, for the command line call I import sys and use sys.argv as an argument for length. I have tried adding global max_word at the beginning of the function, but it does not work. I have not assigned any variable like max_word before this function.

Answer

Add some error checking to the function to help you debug:

def length_frequency(length: int) -> None:
    '''
    Parameter: length as an integer
    '''
    assert isinstance(length, int), f"{repr(length)} is not an int!"
    word_counts = {word: text4.count(word) for word in set(text4) if len(word) == length}
    assert word_counts, f"No words in corpus with length {length}!"
    max_word = max(word_counts.keys(), key=word_counts.get)
    print(f"The most frequent word with {length} characters is {max_word}")

(I simplified the implementation a bit just for my own benefit in making it easier to understand — I’m pretty sure it does the same thing with less confusion.)

Note that adding type annotations also means that if you had a line of code like, say:

length_frequency(sys.argv[1])

if you were to run mypy it would tell you about the error, no assert required:

test.py:19: error: Argument 1 to "length_frequency" has incompatible type "str"; expected "int"