Read from File, or STDIN

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.

The fileinput module may do what you want - assuming the non-option arguments are in args then:

import fileinput
for line in fileinput.input(args):
    print line

If args is empty then fileinput.input() will read from stdin; otherwise it reads from each file in turn, in a similar manner to Perl's while(<>).

In the simplest terms:

import sys
# parse command line
if file_name_given:
    inf = open(file_name_given)
else:
    inf = sys.stdin

At this point you would use inf to read from the file. Depending on whether a filename was given, this would read from the given file or from stdin.

When you need to close the file, you can do this:

if inf is not sys.stdin:
    inf.close()

However, in most cases it will be harmless to close sys.stdin if you're done with it.

I like the general idiom of using a context manager, but the (too) trivial solution ends up closing sys.stdin when you are out of the with statement, which I want to avoid.

Borrowing from this answer, here is a workaround:

import sys
import contextlib

@contextlib.contextmanager
def _smart_open(filename, mode='Ur'):
    if filename == '-':
        if mode is None or mode == '' or 'r' in mode:
            fh = sys.stdin
        else:
            fh = sys.stdout
    else:
        fh = open(filename, mode)
    try:
        yield fh
    finally:
        if filename != '-':
            fh.close()
    
if __name__ == '__main__':
    args = sys.argv[1:]
    if args == []:
        args = ['-']
    for filearg in args:
        with _smart_open(filearg) as handle:
            do_stuff(handle)

I suppose you could achieve something similar with os.dup() but the code I cooked up to do that turned out to be more complex and more magical, whereas the above is somewhat clunky but very straightforward.

I prefer to use "-" as an indicator that you should read from stdin, it's more explicit:

import sys
with open(sys.argv[1], 'r') if sys.argv[1] is not "-" else sys.stdin as f:
    pass # do something here

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
    # ......

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

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.)

Something like:

if input_from_file:
    f = open(file_name, "rt")
else:
    f = sys.stdin
inL = f.readline()
while inL:
    print inL.rstrip()
    inL = f.readline()