Better argparse.FileType - lazily open files passed as arguments rather than them being opened immediately

lazyfiletype.py

import argparse
import pathlib
from typing import IO, Self


class LazyFileType(argparse.FileType):
    """Subclasses `argparse.FileType` in order to provide a way to lazily open
    files for reading/writing from arguments.  Initializes the same as the
    parent, but provides `open` method which returns the file object.
    Usage:
    ```
    parser = argparse.ArgumentParser()
    parser.add_argument('f', type=LazyFileType('w'))
    args = parser.parse_args()
    with args.f.open() as f:
        for line in foo:
            ...
    ```
    Provides an alternate constructor for use with the `default` kwarg to
    `ArgumentParser.add_argument`.
    Usage:
    ```
    parser.add_argument('-f', type=LazyFileType('w'),
                        default=LazyFileType.default('some_file.txt')
    ```
    """

    def __call__(self, string: str) -> typing.Self:  # type: ignore
        self.filename = string  # pylint: disable=attribute-defined-outside-init

        if 'r' in self._mode or 'x' in self._mode:
            if not pathlib.Path(self.filename).exists():
                m = (f"can't open {self.filename}:  No such file or directory: "
                     f"'{self.filename}'")
                raise argparse.ArgumentTypeError(m)

        return self

    def open(self) -> IO:
        """Opens and returns file for reading
                :rtype: io.TextIOWrapper
        """
        return open(self.filename, self._mode, self._bufsize, self._encoding, self._errors)

    @classmethod
    def default(cls, string: str, **kwargs) -> Self:
        """Alternate constructor for a default argument to argparse argument
        Args:
            string: filename to open
            **kwargs: arguments to `__init__`
        Returns:
            instance of `LazyFileType`
        """
        inst = cls(**kwargs)
        inst.filename = string  # pylint: disable=attribute-defined-outside-init
        return inst

z_LICENSE

Copyright 2024 Paul Durivage <[email protected]>

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.