Python Read Certain Number of Bytes After Character

Question

I'm dealing with a character separated hex file, where each field has a particular start code. I've opened the file as 'rb', but I was wondering, after I get the index of the startcode using .find, how do I read a certain number of bytes from this position? This is how I am loading the file and what I am attempting to do

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

where 7 is the number of bytes I want to read from the index returned by the find function. I am using python 2.7.3

Answer 1

You can get the position of a substring in a bytestring under python2.7 like this:

>>> 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'

You can look for the special characters, and for compatibility with Python3k, it's better if you prepend the character with a b, indicating these are bytes (in Python2.x, it will work without though):

 >>> 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'
>>>