seek in a file

[Jim Dennis]

In article <a7s4q4$no0i9$1 at ID-69142.news.dfncis.de>, Andreas Penzel wrote:

>"Steven Majewski" wrote:

>> Are the files too big to fit in memory ?

>the file is about 1 MB, i have to test out working

	By today's standards 1Mb is tiny.  It should easily fit
	in memory on any reasonably recent (last decade or so)
	computer.

>> What do you want to do with the lines ?
>> ( I hope you weren't planning on trying to modify a line in a file:
>>   because of the variable length on lines, if you don't write
>>   exactly the same number of chars back, you will either overwite
>>   the next line, or leave an extra partial line after your edited
>>   line. Most text editors read the whole file into memory and then
>>   write out a new file after it's modified in memory. )

>In the file are barcodes of products, one per line.
>The program has to look if the scanned code from the barcode-scanner is
>identically with one in the file included. This is important because the
>barcode-scanner sometimes works not correct. All i need is an search-routine
>that compares the scanned code with the codes in the file. If there is an
>code, the programs waits for the next code.

 	So I'd load the file into a dictionary:

	  f=open('.../barcodes.list','r')
	  barcodes = {}
	  for x in f.readlines():
	  	barcodes[x]=None

	And then my validation would look like:

		while 1:
			code=BCodeReader.readline()
			while code not in barcodes:
				tryAgain()
			# use valid code in transactions
			# until operator signals to quit
			# or forever as you like.

	Of course I'm assuming you have some sort of 
	object "BCodeReader" that acts enough like a 
	file that you could support a readline() method on 
	it.  In some cases it could actually be a regular
	file object, created with the Python open() built-in,
	but I don't know about your hardware.  I'm also assuming
	you'd write a "tryAgain" function.

	Of course I recognize that this might be a multi-user
	system (possibly with lots of POS terminals attached,
	and the bar code scanners attached to those).  In that
	case consuming a meg or two of memory for the barcodes
	dictionary *for each process* might get expensive.

	In that case you could create a client/server pair.  
	The server could simply return "true" or "1" any time
	it receives a valid code and "false" or "0" every time
	it gets an invalid one.  It could be written in about
	a dozen lines of Python and it could probably handle
	a few hundred concurrent connections (over UNIX or Internet
	domain sockets).  The client size could be written as
	a small class to include in your application probably 
	also less than a dozen lines of code.