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.

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(<>).

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] != "-" 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.)

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.

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()