Python:为什么 DictWriter 写入 'NULL' 字节？

Question

class WhatsGoingOn:
    def __init__(self, filename, fieldNames, maxLines):
        self.file_to_write = filename
        self.fieldNames = fieldNames'
        self.maxLines = maxLines

        # Open the file for reading and writing. Create it if it doesn't exist, 
        #  and truncate it if it does.
        self.file = open(self.file_to_write, 'w+b')
        self.csvReader = csv.DictReader(self.file, fieldnames=self.fieldNames)
        self.csvWriter = csv.DictWriter(self.file, fieldnames=self.fieldNames, extrasaction='ignore')

    def looper(self):
        # Infinitly (don't worry about that - this is a daemon), 
        #  write to the file. When a certain number of lines have been written, 
        #  read the file and then truncate it.
        try:
            numRowsWritten = 0
            while True:
                # Nevermind what's being written
                self.csvWriter.writerow({'name_of_field_0': str(numRowsWritten ), 'name_of_field_1': str(numRowsWritten )})
                numRowsWritten  += 1

                if numRowsWritten  >= self.maxLines:
                    # Iterate through each row of the file
                    self.file.seek(0)

                    # This only works the first time...
                    for row in self.csvReader:
                        print row['name_of_field']

                    # Truncate the file, and restart the 
                    self.file.truncate(0)
                    numRowsWritten  = 0

        except Exception as e:
            print 'Exception!: {0}'.format(e)
            self.file.close()

输出: 异常!:行包含 NULL 字节

第二次 for row in self.csvReader: 行被命中，抛出异常，当我查看文件时，文件确实有一大堆 NULL 字节开始。显然，在文件被截断后，DictWriter 写入了一大堆 NULL 字节(或者至少这是我的假设)。 如何防止 NULL 字节写入我的文件？

Answer 1

显然，通过截断文件，您会弄乱编写器的某些内部状态。不要截断，而是关闭并重新打开文件(w+b 截断模式)，然后重新初始化 csvReader 和 csvWriter。