Read from File, or STDIN

user_144278 0
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I've written a command line utility that uses getopt for parsing arguments given on the command line. I would also like to have a filename be an optional argument, such as it is in other utilities like grep, cut etc. So, I would like it to have the following usage

tool -d character -f integer [filename]

How can I implement the following?

  • if a filename is given, read from the file.
  • if a filename is not given, read from STDIN.
stdin arguments command-line file python
9 Answers

To make use of python's with statement, one can use the following code:

import sys
with open(sys.argv[1], 'r') if len(sys.argv) > 1 else sys.stdin as f:
    # read data using f
    # ......
user_6966650

Not a direct answer but related.

Normally when you write a python script you could use the argparse package. If this is the case you can use:

parser = argparse.ArgumentParser()
parser.add_argument('infile', nargs='?', type=argparse.FileType('r'), default=sys.stdin)

'?'. One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from default will be produced.

and here we set default to sys.stdin;

so If there is a file it will read it , and if not it will take the input from stdin "Note: that we are using positional argument in the example above"

for more visit: https://docs.python.org/2/library/argparse.html#nargs

user_17982980

Switch to argparse (it's also part of the standard library) and use an argparse.FileType with a default value of stdin:

import  argparse, sys

p = argparse.ArgumentParser()
p.add_argument('input', nargs='?',
  type=argparse.FileType(), default=sys.stdin)
args = p.parse_args()

print(args.input.readlines())

This will not let you specify encoding and other parameters for stdin, however; if you want to do that you need to make the argument non-optional and let FileType do its thing with stdin when - is given as an argument:

p.add_argument('input', type=FileType(encoding='UTF-8'))

Take heed that this latter case will not honour binary mode ('b') I/O. If you need only that, you can use the default argument technique above, but extract the binary I/O object, e.g., default=sys.stdout.buffer for stdout. However, this will still break if the user specifies - anyway. (With - stdin/stdout is always wrapped in a TextIOWrapper.)

If you want it to work with -, or have any other arguments you need to provide when opening the file, you can fix the argument if it got wrapped wrong:

p.add_argument('output', type=argparse.FileType('wb'))
args = p.parse_args()
if hasattr(args.output, 'buffer'):
    #   If the argument was '-', FileType('wb') ignores the 'b' when
    #   wrapping stdout. Fix that by grabbing the underlying binary writer.
    args.output = args.output.buffer

(Hat tip to medhat for mentioning add_argument()'s type parameter.)

user_1072940

A KISS solution is:

if file == "-":
    content = sys.stdin.read()
else:
    with open(file) as f:
        content = f.read()
print(content)   # Or whatever you want to do with the content of the file.
user_42755940
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