call a new function without closing the file

Question

I have a method that opens a csv file and another method that I want to use to process this file -but I also need it to process different types of objects, so I want to keep it as an independent method.

import csv

class Loader():

    def load_from_csv(self, csv_path: str):

        with open(csv_path, mode='r', encoding='utf-8') as file:
            reader = csv.DictReader(file)
            return self._read_and_yield(reader)
    
    @classmethod
    def _read_and_yield(cls,reader):
        for row in reader:
            yield(row)

loader = Loader()
for row in loader.load_from_csv('sample.csv'):
    print(row)

But I get ValueError: I/O operation on closed file, because the file seems to get closed when I call the _read_and_yield method. Here it just yields the rows, but imagine a case in which I have to do more operations: I should repeat this process for each type of file (namely read_from_excel).

How should I handle this?

Answer 1

You could consider rewriting your Loader class as a context manager.

Something like this:

from csv import DictReader

class Loader:
    def __init__(self, filename):
        self._filename = filename
    def __enter__(self):
        self._fd = open(self._filename, "r", encoding="utf-8", newline="")
        return self
    def __exit__(self, *_):
        self._fd.close() #type: ignore
    def row(self):
        yield from DictReader(self._fd) #type: ignore

with Loader("foo.csv") as loader:
    for r in loader.row():
        print(r)