how to reading binary data...

Bengt Richter bokr at oz.net
Thu Oct 21 11:03:34 CEST 2004


On Thu, 21 Oct 2004 10:07:32 +0200, Miki Tebeka <miki.tebeka at zoran.com> wrote:

>Hello Sandeep,
>
>>    I opened any particular file with the 'rb' mode that is read binary
>>    mode. so python will treat the data as a raw data. now i want to read
>>    first 4 bytes only then i will convert the first 4 bytes into long
>>    datatype and then again read 4 bytes and will do the same.
>>    But how to set & move the pointer using loop?
>Each time you read from a file the "file pointer" is automatically
>incrmented.
>
>>>> f = open(".bashrc")
>>>> f.tell()
>0L
>>>> f.read(1)
>'#'
>>>> f.tell()
>1L
>See http://docs.python.org/lib/bltin-file-objects.html
>
>>    also how to convert into long?
>http://docs.python.org/lib/module-struct.html
>
>HTH.

Beware of cooked input if you want to convert binary data, if you're not on unix.
E.g., windows: (tell seems to tell the binary truth, but read collapses \r\n to \n
unless you use 'rb' mode)

 >>> open('linesofthree.txt','rb').read()
 '012\r\n567\r\nABC\r\n'
 >>> f = open('linesofthree.txt')
 >>> while True:
 ...     pos = f.tell()
 ...     byt = f.read(1)
 ...     print '[%s:%r]' % (pos, byt),
 ...     if not byt: break
 ...
 [0:'0'] [1:'1'] [2:'2'] [3:'\n'] [5:'5'] [6:'6'] [7:'7'] [8:'\n'] [10:'A'] [11:'B'] [12:'C'] [13
 :'\n'] [15:'']
 >>> f = open('linesofthree.txt','rb')
 >>> while True:
 ...     pos = f.tell()
 ...     byt = f.read(1)
 ...     print '[%s:%r]' % (pos, byt),
 ...     if not byt: break
 ...
 [0:'0'] [1:'1'] [2:'2'] [3:'\r'] [4:'\n'] [5:'5'] [6:'6'] [7:'7'] [8:'\r'] [9:'\n'] [10:'A'] [11
 :'B'] [12:'C'] [13:'\r'] [14:'\n'] [15:'']
 >>> print open('linesofthree.txt','rb').read()
 012
 567
 ABC


Regards,
Bengt Richter



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