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Read from stdin or files in Python (combining argparse and fileinput)
import argpase
import fileinput
if __name__ == '__main__':
parser = ArgumentParser()
parser.add_argument('--dummy', help='dummy argument')
parser.add_argument('files', metavar='FILE', nargs='*', help='files to read, if empty, stdin is used')
args = parser.parse_args()
# If you would call fileinput.input() without files it would try to process all arguments.
# We pass '-' as only file when argparse got no files which will cause fileinput to read from stdin
for line in fileinput.input(files=args.files if len(args.files) > 0 else ('-', )):
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drmull commented Nov 18, 2016

I found your snippet helpful. But I found out you don't need the if case though, just fileinput.input(args.files) will do, no arguments will make args.files an empty list and and an empty list will make fileinput read from stdin

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This really helps if any of your python3 scripts are suffering from a BrokenPipeError or are not being properly handled when you're piping input into a python script you wrote!

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Thank you, both this and @drmull's comment were very helpful

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Thank you, both this and @drmull's comment were very helpful.

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pinzyte commented Dec 31, 2022

In the special case where only 1 file (or none for stdin) is allowed, use: nargs='?', default=[].

The default= is generally required or the default will be None, which will cause fileinput to think it needs to read from sys.argv. This is a problem if you have options on the command line as it will then try to open them as files, and you'll get errors like No such file or directory: '-a'.

Passing an empty array (which is what nargs='*' does when no files are specified) avoids this.

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