How to read "-" (dash) as standard input with Python without writing extra code?

Question

Using Python 3.5.x, not any greater version than that.

https://stackoverflow.com/a/30254551/257924 is the right answer, but doesn't provide a solution that is built into Python, but requires writing code from scratch:

I need to have a string that has a value of "-" to represent stdin, or its value is a path to a text file I want to read from. I want to use the with operator to open up either type of those files, without using conditional logic to check for "-" in my scripts. I have something that works, but it seems like it should be something that is built into Python core and not requiring me to roll my own context-manager, like this:

from contextlib import contextmanager

@contextmanager
def read_text_file_or_stdin(path):
    """Return a file object from stdin if path is '-', else read from path as a text file."""
    if path == '-':
        with open(0) as f:
            yield f
    else:
        with open(path, 'r') as f:
            yield f


# path = '-'  # Means read from stdin
path = '/tmp/paths'  # Means read from a text file given by this value
with read_text_file_or_stdin(path) as g:
    paths = [path for path in g.read().split('\n') if path]

print("paths", paths)

I plan to pass in the argument to a script via something like -p - to mean "read from standard-input" or -p some_text_file meaning "read from some_text_file".

Does this require me to do the above, or is there something built into Python 3.5.x that provides this already? This seems like such a common need for writing CLI utilities, that it could have already been handled by something in the Python core or standard libraries.

I don't want to install any module/package from repositories outside of the Python standard library in 3.5.x, just for this.

Answer 1

The argparse module provides a FileType factory which knows about the - convention.

import argparse

p = argparse.ArgumentParser()

p.add_argument("-p", type=argparse.FileType("r"))

args = p.parse_args()

Note that args.p is an open file handle, so there's no need to open it "again". While you can still use it with a with statement:

with args.p:
    for line in args.p:
        ...

this only ensures the file is closed in the event of an error in the with statement itself. Also, you may not want to use with, as this will close the file, even if you meant to use it again later.

You should probably use the atexit module to make sure the file gets closed by the end of the program, since it was already opened for you at the beginning.

import atexit

...

args = p.parse_args()
atexit.register(args.p.close)