This class is like argparse.FileType, except that it supports all keyword arguments for open() and returns a function that opens a handle rather than returning the handle itself.

filetype.py

#!/usr/bin/env python3
import argparse, gettext, sys

class FileType(argparse.FileType):
    '''
    This class is just like argparse.FileType, except that it supports all
    keyword arguments for open() and returns a function that opens a handle
    rather than returning the handle itself. This means that the file is not
    actually opened until that function is called. This also means that the
    result can be used with a "with" block.
    
    Example:
        parser = argparse.ArgumentParser()
        parser.add_argument(
            "--out-file",
            type=filetype.FileType("w", newline="", encoding="UTF-8")
        )
        arguments = parser.parse_args(["--out-file=test.txt"])
        # The file test.txt is opened in the next function call below.
        with arguments.out_file() as f:
            f.write("Hello!\n")
        # At the end of the block, the file is closed.
    
    See: https://bugs.python.org/issue24739
    '''
    def __init__(self, mode="r", **kwargs):
        self._mode = mode
        self._open_kwargs = kwargs
    def __call__(self, string):
        if string == "-":
            if "r" in self._mode:
                return lambda: sys.stdin
            if "w" in self._mode:
                return lambda: sys.stdout
            raise ValueError(
                gettext.gettext('argument "-" with mode %r') % self._mode
            )
        return lambda: open(string, self._mode, **self._open_kwargs)
    def __repr__(self):
        return "%s(%s)" % (
            type(self).__name__,
            ", ".join(
                [self._mode] +
                [
                    "%s=%r" % kwarg
                    for kwarg in self._open_kwargs.items()
                ]
            )
        )
    @property
    def _bufsize(self):
        return self._open_kwargs.get("bufsize", -1)
    @_bufsize.setter
    def _bufsize_setter(self, value):
        self._open_kwargs["bufsize"] = value
    @property
    def _encoding(self):
        return self._open_kwargs.get("encoding")
    @_encoding.setter
    def _encoding_setter(self, value):
        self._open_kwargs["encoding"] = value
    @property
    def _errors(self):
        return self._open_kwargs.get("errors")
    @_errors.setter
    def _errors_setter(self, value):
        self._open_kwargs["errors"] = value