simple file flow question with csv.reader

[Terry Reedy]

On 11/2/2011 7:06 PM, Dennis Lee Bieber wrote:
> On Wed, 2 Nov 2011 14:13:34 -0700 (PDT), Matt<macmanes at gmail.com>
> declaimed the following in gmane.comp.python.general:
>
>> I have a few hundred .csv files, and to each file, I want to
>> manipulate the data, then save back to the original file.

That is dangerous. Better to replace the file with a new one of the same 
name.

> Option 1:	Read the file completely into memory (your example is
> reading line by line); close the reader and its file; reopen the
> file for "wb" (delete, create new); open CSV writer on that file;
> write the memory contents.

and lose data if your system crashes or freezes during the write.

> Option 2:	Open a temporary file "wb"; open a CSV writer on the file;
> for each line from the reader, update the data, send to the writer;
> at end of reader, close reader and file; delete original file;
> rename temporary file to the original name.

This works best if new file is given a name related to the original 
name, in case rename fails. Alternative is to rename original x to 
x.bak, write or rename new file, then delete .bak file.

-- 
Terry Jan Reedy