Python | Exceptional Conditions Testing in Unit Tests

Python | Accepting Script Input

Last Updated : 12 Jun, 2019

A lot of people use Python as a replacement for shell scripts, using it to automate common system tasks, such as manipulating files, configuring systems, and so forth. This article aims to describe accepting Script Input via Redirection, Pipes, or Input Files.

Problem – To have a script to be able to accept input using whatever mechanism is easiest for the user. This should include piping output from a command to the script, redirecting a file into the script, or just passing a filename, or list of filenames, to the script on the command line.

Python’s built-in fileinput module makes this very simple and concise if the script looks like this.

Code #1 :

import fileinput 
  
with fileinput.input() as f_input: 
    for line in f_input: 
        print(line, end ='') 

Then input to the script can already be accepted in all of the previously mentioned ways. If the script is saved and make it executable then the one can get the expected output using all of the following:

Code #2 :

# Prints a directory listing to stdout. 
$ ls | ./filein.py  
  
# Reads/etc/passwd to stdout. 
$ ./filein.py/etc/passwd  
  
# Reads/etc/passwd to stdout. 
$ ./filein.py < /etc/passwd  

The fileinput.input() function creates and returns an instance of the FileInput class. In addition to containing a few handy helper methods, the instance can also be used as a context manager. So, to put all of this together, if one wrote a script that expected to be printing output from several files at once, one might have it include the filename and line number in the output, as shown in the code given below –

Code #3 :

import fileinput 
with fileinput.input('/etc/passwd') as f: 
    for line in f: 
        print(f.filename(), f.lineno(), line, end ='') 

/etc/passwd1
/etc/passwd2
/etc/passwd3 

<other output omitted>

Using it as a context manager ensures that the file is closed when it’s no longer being used, and one leveraged a few handy FileInput helper methods here to get some extra information in the output.

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