How to reference a global variable in more than one function?

Question

I've got headache with the global variable reference problem in a simple word frequency count program. I check the the answers from here and here, and the python docs; however, I still haven't got the idea of global variable reference.

from collections import Counter

with open('c:/Users/Nick/Downloads/sample_file.txt') as f:
    words = f.read().lower().split()

c = Counter(words)

total_words = sum(c.values())


def top_n_words(n):
    global c
    # c = Counter(words)
    top_n = c.most_common(n)
    print("Top %d words are:" % n)
    print("-" * 20)
    for w, c in top_n:
        # print("%10s: %-10s" % (w, c))
        print("{word:>10s}: {counts:<10d}".format(word=w, counts=c))


def word_appears(w):
    # global c
    c = Counter(words)
    print("The word '{word:s}' appears {time:d} times.".format(word = w, time = c[w]))


top_n_words(12)
print("-" * 20)
print("Total words: %d" % total_words)
print("Total words: {t:d}".format(t=sum(c.values())))
word_appears("history")

1. In the top_n_words function, I've declare that c is global. Should I declare it global in word_appears function? It doesn't work.
2. Why I can't reference to the c in the print function? Does the order of the top_n_words, word_appears affect the final print function?
3. What is a good practice to handle this kind of situation?

Answer 1

Put all code in functions and use only arguments to make the counter available. For example:

from collections import Counter


def read_words(file_name):
    with open(file_name) as f:
        words = f.read().lower().split()
    return words


def top_n_words(counter, n):
    top_n = counter.most_common(n)
    print("Top %d words are:" % n)
    print("-" * 20)
    for w, c in top_n:
        # print("%10s: %-10s" % (w, c))
        print("{word:>10s}: {counts:<10d}".format(word=w, counts=c))


def word_appears(counter, w):
    print("The word '{word:s}' appears {time:d} times.".format(word=w,
        time=counter[w]))


if __name__ == '__main__':

    def main(file_name):
        words = read_words(file_name)
        counter = Counter(words)
        top_n_words(counter, 12)
        print("-" * 20)
        print("Total words: %d" % len(words))
        word_appears(counter, "history")

    main('words.txt')