Using “with” statement for CSV files in Python

问题

Is it possible to use the with statement directly with CSV files? It seems natural to be able to do something like this:

import csv
with csv.reader(open("myfile.csv")) as reader:
    # do things with reader

But csv.reader doesn't provide the __enter__ and __exit__ methods, so this doesn't work. I can however do it in two steps:

import csv
with open("myfile.csv") as f:
    reader = csv.reader(f)
    # do things with reader

Is this second way the ideal way to do it? Why wouldn't they make csv.reader directly compatible with the with statement?

回答1:

The primary use of with statement is an exception-safe cleanup of an object used in the statement. with makes sure that files are closed, locks are released, contexts are restored, etc.

Does csv.reader have things to cleanup in case of exception?

I'd go with:

with open("myfile.csv") as f:
    for row in csv.reader(f):
        # process row

You don't need to submit the patch to use csv.reader and with statement together.

import contextlib

Help on function contextmanager in module contextlib:

contextmanager(func)
    @contextmanager decorator.

Typical usage:

    @contextmanager
    def some_generator(<arguments>):
        <setup>
        try:
            yield <value>
        finally:
            <cleanup>

This makes this:

    with some_generator(<arguments>) as <variable>:
        <body>

equivalent to this:

    <setup>
    try:
        <variable> = <value>
        <body>
    finally:
        <cleanup>

Here's a concrete example how I've used it: curses_screen.

回答2:

Yes. The second way is correct.

As to why? Who ever knows. You're right, it's probably an easy change. It's not as high priority as other things.

You can easily make your own patch kit and submit it.

回答3:

The problem is csv.reader doesn't really manage a context. It can accept any iterable, not just a file. Therefore it doesn't call close on its input (incidentally if it did you could use contextlib.closing). So it's not obvious what context support for csv.reader would actually do.

回答4:

import csv

class CSV(object):
    def __init__(self,path,mode):
        self.path = path
        self.mode = mode
        self.file = None

    def __enter__(self):
        self.file = open(self.path,self.mode)
        if self.mode == 'r':
            return csv.reader(self.file)
        elif self.mode == 'w':
            return csv.writer(self.file)

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.file.close()   

with CSV('data.csv','r') as reader:
    for row in reader:
        print row

回答5:

It's easy to create what you want using a generator function:


import csv
from contextlib import contextmanager

@contextmanager
def opencsv(path):
   yield csv.reader(open(path))

with opencsv("myfile.csv") as reader:
   # do stuff with your csvreader

来源：https://stackoverflow.com/questions/441130/using-with-statement-for-csv-files-in-python

标签

python

csv

with-statement