如何在 Python 3.6 中解决这个“NameError”问题？

Question

I'm learning to deal with function in Python 3.6 now. I try to set a function to count the number of times a specific word appears in a text file, but I face the NameError problem.

I tried the tool on pythontutor.com to visualise the steps of my code to find out why. It seems like the value of count_num in function count_word doesn't get pass to the for loop. But I really don't understand why.

filenames = [
    "The Wallypug in London.txt", 
    "Chats on Old Silver.txt", 
    "Jack Jingle, and Sucky Shingle.txt", 
    "Superstition and Force.txt"
]


def count_word(filename, word):
    with open(filename) as open_file:
        file_content = open_file.read()

    count_num = file_content.lower().count(word)
    return count_num


for filename in filenames:
    count_word(filename, "great")
    print('There are ' + str(count_num) + " 'great' in " + filename + ".")

I expect the output would be "There are 89 'great' in The Wallypug in London.txt. But it just returns,

NameError: name 'count_num' is not defined

Answer 1

you're missing the assignment in this line

    count_word(filename, "great")

Change to

    count_num = count_word(filename, "great")

Answer 2

This might help :

filenames = [
    "The Wallypug in London.txt", 
    "Chats on Old Silver.txt", 
    "Jack Jingle, and Sucky Shingle.txt", 
    "Superstition and Force.txt"
]

count_num ="" #define count_num here
def count_word(filename, word):
    count_num=0
    return count_num


for filename in filenames:
    count_word(filename, "great")
    print('There are ' + str(count_num) + " 'great' in " + filename + ".")

Answer 3

It's because count_num in function count_word is a local variable, and cannot be referenced in global area. Replace count_word(filename, "great") with count_num = count_word(filename, "great")