Best way to write a file n-bytes long

[Dialtone]

cappy2112 at yahoo.com (Tony C) writes:

> Sure, I could create a string that is n Megabytes long, and pass it to
> write, but it seems as though there should be a better way ???
> String manipulation is typically considered slow.
>
> st="X" * 1048576
> fh=open("junk","wb")
> fh.write(st)
> fh.close()

For little files (less than 2 or 3 Mb I think) your code is the
fastest I can think of. But growing There is a new version which is a
lot faster:

In [14]: def p3 (a):
   ....:     tmp = time.time()
   ....:     str = 'X'*1024
   ....:     fh = file('junk','wb')
   ....:     for i in xrange(a):
   ....:         fh.write(str)
   ....:     print time.time()-tmp
   ....: 

In [14]: def p4 (a):
   ....:     tmp = time.time()
   ....:     str = 'X'*(1024*a) #Parenthesis are fundamental here
   ....:     fh = file('junk','wb')
   ....:     fh.write(str)
   ....:     fh.close()
   ....:     print time.time()-tmp
   ....: 

In [15]: p3(10240)  # 10Mb file
0.478311061859

In [16]: p4(10240)
0.625810027122


And this is only slightly faster, If we go up a lil bit more (100Mb)
we get to:

In [17]: p3(100000)  # This is a 100 Mb file, so not really little 
                     # and performance is not that bad, 25Mb/s
4.00167894363

In [18]: p4(100000)
8.85632002354

Otherwise if we stay down (1K):
In [22]: p3(1)     
0.00810098648071

In [23]: p4(1)
0.000607967376709


HTH

-- 
Valentino Volonghi, Regia SpA, Milan

Linux User #310274, Debian Sid Proud User