Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Newsgroups: comp.lang.python
Subject: Re: Python path and append
Date: Mon, 25 Apr 2016 19:57:46 -0400
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On Mon, 25 Apr 2016 16:15:15 -0400, Seymore4Head
<Seymore4Head@Hotmail.invalid> declaimed the following:

>Thanks for the tip.
>
>Still broke.  :(
>
>f = open('wout.txt', 'r+')
>for line in f:
>    if line=="":
>        exit
>    line=line[:-1]
>    line=line+" *"
>    f.write(line)
>    print line
>f.close()
>
>
>
>I did notice that it wrote the 3 lines of test file but it didn't
>append the * after the third entry and it starts printing garbage
>after that.
>

	Unless Python (or the C runtime) have implement Xerox CP/V "UPDATE"
file mode*, after the first line is read, the I/O pointer is located at the
end of that line. You now attempt to write the modified line, which will
overwrite parts of the second line (note that you are not writing
new-lines). After that write, you are positioned at some point unknown
(relatively speaking, and do a read operation starting at where the write
ended.

	Any I/O operation that changes the length of the data is going to fail,
even if you reposition (look up "seek()" and "tell()")

	On Windows, where the physical line-ending is <cr><lf>, but gets
transformed to just <lf> by Python text reads, you are stripping the <lf>,
adding " *". If the next record is the same length, that " *" on write will
replace the <cr><lf> pair.

	For the type of operation you are performing, on text files, you really
need to go through the hassle of renames and copies... That is, rename the
input file to some temporary name, open it for read, open the original name
for write, and then read input, modify, write output...

	OR -- if the files are short enough, just read the entire file, and
process it in memory...

fupdate = open("theFile.txt", "r+")
fdata = fupdate.readlines()
#	doing piecemeal, rather than making a one-liner
for l, ln in enumerate(fdata):
	#	strip new-line IF present
	if ln[-1] == "\n":
		ln = ln[:-1]
	fdata[l] = ln + " *"
#	join new data with newline
fdata = "\n".join(fdata)
#rewind the file
fupdate.seek(0)
fupdate.write(fdata)
fupdate.close()






* UPDATE mode maintained two I/O pointers: a read pointer and write
pointer, and required one to read one or more records before doing a write
operation. As long as the data (record) size did not change it allowed for
Read/Modify/Write of records.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/