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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: File Read issue by using module binascii
Date: Sun, 28 Apr 2013 09:42:13 +0200
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Tim Roberts wrote:

> Jimmie He <jimmie.he@gmail.com> wrote:
> 
>>When I run the readbmp on an example.bmp(about 100k),the Shell is become
>>to "No respose",when I change f.read() to f.read(1000),it is ok,could
>>someone tell me the excat reason for this? Thank you in advance!
>>
>>Python Code as below!!
>>
>>import binascii
>>
>>def read_bmp():
>>    f = open('example.bmp','rb')
>>    rawdata = f.read()                       #f.read(1000) is ok
>>    hexstr = binascii.b2a_hex(rawdata)       #Get an HEX number
>>    bsstr = bin (int(hexstr,16))[2:]
> 
> I suspect the root of the problem here is that you don't understand what
> this is actually doing.  You should run this code in the command-line
> interpreter, one line at a time, and print the results.
> 
> The "read" instruction produces a string with 100k bytes.  The b2a_hex
> then
> produces a string with 200k bytes.  Then, int(hexstr,16) takes that
> 200,000 byte hex string and converts it to an integer, roughly equal to 10
> to the
> 240,000 power, a number with some 240,000 decimal digits.  You then
> convert
> that integer to a binary string.  That string will contain 800,000 bytes.
> You then drop the first two characters and print the other 799,998 bytes,
> each of which will be either '0' or '1'.
> 
> I am absolutely, positively convinced that's not what you wanted to do.
> What point is there in printing out the binary equavalent of a bitmap?
> 
> Even if you did, it would be much quicker for you to do the conversion one
> byte at a time, completely skipping the conversion to hex and then the
> creation of a massive multi-precision number.  Example:

Hm, if you fix the long integer arithmetic "problem" you should also attack 
the unbounded memory consumption problem in general ;)

>     f = open('example.bmp','rb')
>     rawdata = f.read()
>     bsstr = []
>     for b in rawdata:
>         bsstr.append( bin(ord(b)) )
>     bsstr = ''.join(bsstr)
> 
> or even:
>     f = open('example.bmp','rb')
>     bsstr = ''.join( bin(ord(b))[2:] for b in f.read() )

Yes, the original is horrible newbie code ;) but that's what you tend to 
write while learning to program -- and python can handle it alright. On the 
other hand, Idle becomes unresponsive when I do

>>> print("a"*10**6)

in its shell. I'm still investigating, but the problem seems to be that it's 
a single line.

>>> print(("a"*100+"\n") * 10**4)

takes under 7 secs. Not as good as konsole (KDE's terminal emulation) which 
finishes in 0.5 secs, but acceptable.