Python在字符后讀取一定數量的字節

Question

我正在處理一個字符分隔的十六進制文件，其中每個字段都有一個特定的起始代碼。 我已將文件打開為“ rb”，但我想知道，在使用.find獲取啟動碼的索引之后，如何從該位置讀取一定數量的字節？ 這就是我加載文件的方式以及我要嘗試執行的操作

with open(someFile, 'rb') as fileData:
    startIndex = fileData.find('(G')
    data = fileData[startIndex:7]

其中7是我想從find函數返回的索引中讀取的字節數。 我正在使用python 2.7.3

Answer 1

您可以像這樣在python2.7下獲取字節串中子字符串的位置：

>>> with open('student.txt', 'rb') as f:
...     data = f.read()
... 
>>> data  # holds the French word for student: élève
'\xc3\xa9l\xc3\xa8ve\n'
>>> len(data)  # this shows we are dealing with bytes here, because "élève\n" would be 6 characters long, had it been properly decoded!
8
>>> len(data.decode('utf-8'))
6
>>> data.find('\xa8')  # continue with the bytestring...
4
>>> bytes_to_read = 3
>>> data[4:4+bytes_to_read]  
'\xa8ve'

您可以查找特殊字符，並且為了與Python3k兼容，最好在字符前加上b ，以指示這些是字節（在Python2.x中，它可以正常工作）：

 >>> data.find(b'è')  # in python2.x this works too (unfortunately, because it has lead to a lot of confusion): data.find('è')
3
>>> bytes_to_read = 3
>>> pos = data.find(b'è')
>>> data[pos:pos+bytes_to_read] # when you use the syntax 'n:m', it will read bytes in a bytestring
'\xc3\xa8v'
>>>