UnboundLocalError：分配前已引用本地變量“ txt”

Question

到目前為止，這是我的程序。 運行它時，出現錯誤：“ UnboundLocalError：分配前引用了本地變量'txt'”。 我嘗試在txt之前添加global，以將其聲明為全局變量，但是這樣做時又遇到了另一個錯誤。 有什么想法我做錯了嗎？ 提前致謝。

def getwords():
    #function to get words in the input file
    try:
        global txt
        txt=open("sample.txt",'r')
    except IOError:
        print('Unable to open file')

    words=[]

    #read the file line by line
    for line in txt:
        #convert each line into words with space as delimiter
        words=words+line.split()

    return words

def wordcount(wordlist):
    #function to count words in the file
    #worddic is dictionary to store words frequency
    worddic=dict()

    for x in wordlist:
        #convert word to lowercase to ignorecase
        t=x.lower()
        if(t not in worddic):
            worddic[t]=0

        worddic[t]=worddic[t]+1

    max=-1
    t=''

    for x in worddic:
        if(worddic[x]>max):
            max=worddic[x]
            t=x

    return t

def letters(wordlist,lettercount):
    #function to count letters in the file
    for x in wordlist:
        #For each word in the list
        t=x.lower()

        for y in t:
            #for each letter in the word
            if(not (y in lettercount)):
                #if the letter is not in dictionary add it
                #and set frequency to zero
                lettercount[y]=0

            #increment the frequency of letter in dictionary
            lettercount[y] = lettercount[y]+1

def createoutput(lettercount,wordlist,mostword):
    #creates an empty file 'statistics.txt'
    try:
        txt2=open("statistics.txt",'w+')
    except IOError:
        print('Unable to create file')

    txt2.write('Number of words in the file are '+str(len(wordlist))+'\n')
    txt2.write('Most repeated word in the file is '+mostword+'\n')

    for x in lettercount:
        #write to the file 'statistics.txt'
        txt2.write(x+' appeared in the file for '+str(lettercount[x])+' times \n')

def main():
    wordlist=getwords()

    #lettercount is a dictionary with letters as keys
    #and their frequency in the input file as data

    lettercount=dict()
    mostword=wordcount(wordlist)
    letters(wordlist,lettercount)
    createoutput(lettercount,wordlist,mostword)

main()

Answer 1

當open()調用失敗時，您將吞下異常：

try:
    global txt
    txt=open("sample.txt",'r')
except IOError:
    print('Unable to open file')

現在txt 從未分配給 ，因為它是open()調用，此處失敗。 而不是繼續執行該功能，您應該在此時返回：

try:
    txt=open("sample.txt",'r')
except IOError:
    print('Unable to open file')
    return

Answer 2

您想像這樣構造異常：

try:
    # normal code here
except:
    # handle the exception

例如，此代碼是錯誤的，因為它運行的代碼可能會在異常處理程序之外失敗

def createoutput(lettercount,wordlist,mostword):
    #creates an empty file 'statistics.txt'
    try:
        txt2=open("statistics.txt",'w+')
    except IOError:
        print('Unable to create file')

    txt2.write('Number of words in the file are '+str(len(wordlist))+'\n')
    txt2.write('Most repeated word in the file is '+mostword+'\n')

將普通讀取移到異常處理程序中將如下所示

def createoutput(lettercount,wordlist,mostword):
    #creates an empty file 'statistics.txt'
    try:
        txt2=open("statistics.txt",'w+')
        txt2.write('Number of words in the file are '+str(len(wordlist))+'\n')
        txt2.write('Most repeated word in the file is '+mostword+'\n')
    except IOError:
        print('Unable to create file')

但是您從未關閉過文件。 您將close文件命令放在哪里？ 顯然，將其打開后，如果txt2.write引發異常怎么辦？ 如果在那里發生其他問題怎么辦？ 你永遠不會關閉！

對於文件，您將需要執行此操作，因此當文件離開作用域時將其關閉

with open("statistics.txt", "w+") as statsfile:
    statsfile.write("Number of words ...

當然，您將其放入異常處理程序中

def getwords(filename="sample.txt"):
    words = []
    try:
        with open (filename, 'r') as txt:
            for line in txt:
                words = words + line.split()
    except IOError:
        print "unable to open file"
    return words