On 2013-10-30, Victor Hooi <victorh...@gmail.com> wrote:
> Hi,
>
> I have a CSV file that I will repeatedly appending to.
>
> I'm using the following to open the file:
>
> with open(self.full_path, 'r') as input, open(self.output_csv, 'ab') as
> output:
> fieldnames = (...)
> csv_writer = DictWriter(output, filednames)
> # Call csv_writer.writeheader() if file is new.
> csv_writer.writerows(my_dict)
>
> I'm wondering what's the best way of calling writeheader() only
> if the file is new?
>
> My understanding is that I don't want to use os.path.exist(),
> since that opens me up to race conditions.
>
> I'm guessing I can't use try-except with IOError, since the
> open(..., 'ab') will work whether the file exists or not.
>
> Is there another way I can execute code only if the file is new?
A heavy-duty approach involves prepending the old contents to a
temporary file.
fieldnames = (...)
with tempfile.TempDirectory() as temp:
tempname = os.path.join(temp, 'output.csv')
with open(tempname, 'wb') as output:
writer = csv.DictWriter(output, fieldnames=fieldnames)
writer.writeheader()
try:
with open(self.output_csv, 'b') old_data:
reader = csv.DictReader(old_data)
for rec in reader:
writer.writerow(rec)
except IOError:
pass
with open(self.full_path, 'b') as infile:
# etc...
shutil.copy(tempname, self.output_csv)
This avoids clobbering output_csv unless new data is succesfully
written. I believe TempDirectory isn't available in Python 2, so
some other way of creating that path will be needed, and I'm too
lazy to look up how. ;)
--
Neil Cerutti
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