Igor Korot wrote:
> Hi, Peter,
> Thank you for the great suggestion.
>
> I tried to implement you code but failed.
>
> Here's what I have:
>
> class FileReader:
> def __init__(self, filename, isSkip):
> self.path = filename
> self.isSkip = isSkip
>
> @contextmanager
> def open(*args):
Selfless OO? Not in Python.
> from StringIO import StringIO
> lines = range(10)
> if self.isSkip:
> lines[0] = "skipped"
> lines[6] = "field1-from-line6,field2-from-line6"
> else:
> lines[0] = "field1-from-line1,field2-from-line1"
> yield StringIO("\r\n".join(map(str, lines)))
>
> def is_arbitrary_text(self,fieldnames):
> return "skipped" in fieldnames
>
> def readData(self):
> with self.open(self.path, "r") as f:
> reader = csv.DictReader(f)
> if self.is_arbitrary_text(reader.fieldnames):
> for _ in range(5):
> next(reader, None)
> reader._fieldnames = None
Here you introduced another bug, ignoring my helpful comments.
>> reader._fieldnames = None # underscore necessary,
>> # fieldnames setter doesn't work
>> reader.fieldnames # used for its side-effect
> for row in reader:
> print row
>
> Unfortunately this does not work as "def open()" does not belong to my
> class and if I comment the "@contextmanager" line
> I will get an exception: "AttributeError: __exit__"
>
> Any idea what to do?
Keeping comments is not an option? But please read and try to understand the
comments before you excise them ;)
As I mentioned in the comment to the open() function you are not supposed to
use it as you have real data -- use Python's built-in open() function.
Anyway, if you insist on doing everything the OO-way, at least add a self in
all the right places and don't introduce bugs that could be avoided with
copy-and-paste.
A working script with mock data and following the OO fashion would be:
$ cat csv_skip_header_oo.py
import csv
from contextlib import contextmanager
class FileReader:
def __init__(self, filename, isSkip):
self.path = filename
self.isSkip = isSkip
@contextmanager
def open(self, *args):
from StringIO import StringIO
lines = range(10)
if self.isSkip:
lines[0] = "skipped"
lines[6] = "field1-from-line6,field2-from-line6"
else:
lines[0] = "field1-from-line1,field2-from-line1"
yield StringIO("\r\n".join(map(str, lines)))
def is_arbitrary_text(self,fieldnames):
return "skipped" in fieldnames
def readData(self):
with self.open(self.path, "r") as f:
reader = csv.DictReader(f)
if self.is_arbitrary_text(reader.fieldnames):
for _ in range(5):
next(reader, None)
reader._fieldnames = None # underscore necessary,
# fieldnames setter doesn't work
reader.fieldnames # used for its side-effect
for row in reader:
print row
if __name__ == "__main__":
import sys
print "Demo with made-up data"
skip = len(sys.argv) > 1 and sys.argv[1] == "--skip"
if skip:
print "Variant 2, header is skipped"
else:
print "Variant 1, no header"
FileReader("whatever.csv", skip).readData()
$ python csv_skip_header_oo.py
Demo with made-up data
Variant 1, no header
{'field2-from-line1': None, 'field1-from-line1': '1'}
{'field2-from-line1': None, 'field1-from-line1': '2'}
{'field2-from-line1': None, 'field1-from-line1': '3'}
{'field2-from-line1': None, 'field1-from-line1': '4'}
{'field2-from-line1': None, 'field1-from-line1': '5'}
{'field2-from-line1': None, 'field1-from-line1': '6'}
{'field2-from-line1': None, 'field1-from-line1': '7'}
{'field2-from-line1': None, 'field1-from-line1': '8'}
{'field2-from-line1': None, 'field1-from-line1': '9'}
$ python csv_skip_header_oo.py --skip
Demo with made-up data
Variant 2, header is skipped
{'field1-from-line6': '7', 'field2-from-line6': None}
{'field1-from-line6': '8', 'field2-from-line6': None}
{'field1-from-line6': '9', 'field2-from-line6': None}
A script using real data would be:
$ cat csv_skip_header_oo.py
import csv
class FileReader:
def __init__(self, filename):
self.path = filename
def is_arbitrary_text(self, fieldnames):
# XXX replace with a test suitable for your actual data
return "skipped" in fieldnames
def read_data(self):
with open(self.path, "rb") as f:
reader = csv.DictReader(f)
if self.is_arbitrary_text(reader.fieldnames):
for _ in range(5):
next(reader, None)
reader = csv.DictReader(f)
for row in reader:
print row
if __name__ == "__main__":
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("file")
args = parser.parse_args()
FileReader(args.file).read_data()
$ cat skipped_header.csv
skipped
1
2
3
4
5
field1-from-line6,field2-from-line6
7
8
9$python csv_skip_header_oo.py skipped_header.csv
{'field1-from-line6': '7', 'field2-from-line6': None}
{'field1-from-line6': '8', 'field2-from-line6': None}
{'field1-from-line6': '9', 'field2-from-line6': None}
$ cat no_header.csv
field1-from-line1,field2-from-line1
1
2
3
4
5
6
7
8
9$python csv_skip_header_oo.py no_header.csv
{'field2-from-line1': None, 'field1-from-line1': '1'}
{'field2-from-line1': None, 'field1-from-line1': '2'}
{'field2-from-line1': None, 'field1-from-line1': '3'}
{'field2-from-line1': None, 'field1-from-line1': '4'}
{'field2-from-line1': None, 'field1-from-line1': '5'}
{'field2-from-line1': None, 'field1-from-line1': '6'}
{'field2-from-line1': None, 'field1-from-line1': '7'}
{'field2-from-line1': None, 'field1-from-line1': '8'}
{'field2-from-line1': None, 'field1-from-line1': '9'}
Please have a look at the cleaned-up implementation of the read_data()
method of this last example. As a result of the discussion on the bug
tracker <http://bugs.python.org/issue20004> I am now convinced that you
should use two `DictReader`s rather than hack internal attributes or broken
properties.
See also <http://www.python.org/dev/peps/pep-0008/> for naming conventions.
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