The sys.argv Array
There are various ways of handling Python command line arguments, but what you need to know is that the underlying support for all Python command line arguments is provided by sys.argv.
The examples will show you how to handle the python command line arguments stored in sys.argv and to solve a few problems that come when trying to access them.
We will learn how to
- Access the contents of sys.argv
- Process whitespaces in python command line arguments
- Handle errors while processing python command line arguments
Displaying Arguments
The sys module exposes an array named argv that includes the following:
- argv[0] contains the name of the current Python program.
- argv[1:], the rest of the list, contains any and all Python command line arguments passed to the program.
for example
Line 1 imports the python module sys
Line 3 gives the name of the program by getting the first item in the list sys.argv
Line 5 displays the python command line arguments by getting all the remaining items of the list sys.argv
Running the script above with a list of arguments as shown
python argv.py cat dog sheep friends eleven
Name of script: argv.py
Arguments passed to script: ['cat', 'dog', 'sheep', 'friends', 'eleven']
The output confirms that the content of sys.argv[0]
is the Python script argv.py
, and that the remaining elements of the sys.argv list contains the arguments of the script, ['cat', 'dog', 'sheep', 'friends', 'eleven']
.
Escaping Whitespace Characters
On Linux, whitespaces can be escaped by doing one of the following:
- Surrounding the arguments with single quotes (')
- Surrounding the arguments with double quotes (")
- Prefixing each space with a backslash () Without one of these solutions, the script will store two arguments instead of one as in the example below
C:\Users\UserName\Desktop\DevPosts>python argv.py "cat dog"
Name of script: argv.py
Arguments passed to script: ['cat dog']
C:\Users\UserName\Desktop\DevPosts>python argv.py cat dog
Name of script: argv.py
Arguments passed to script: ['cat', 'dog']
Handling Errors
Many things can go wrong, so it’s a good idea to provide the users of your program with some guidance in the event they pass incorrect arguments at the command line. For example, kill.py expects one argument, and if you omit it, then you get an error:
Traceback (most recent call last):
File "C:\Users\UserName\Desktop\DevPosts\searcher.py", line 4, in <module>
print("File To Search: {}".format(sys.argv[1]))
IndexError: list index out of range
The Python exception IndexError
is raised, and the corresponding traceback shows that the error is caused by the expression arg = sys.argv[1]
. The message of the exception is list index out of range. So you didn’t pass an argument at the command line, so there’s nothing in the list sys.argv
at index 1.
This is a common problem type that can be addressed in a few different ways. For this initial example, you’ll keep it brief by including the expression arg = sys.argv[1]
in a try block. Modify the code as follows:
import sys
try:
file_to_search = sys.argv[1]
except IndexError:
raise SystemExit("Usage: {} <file_to_search>".format(sys.argv[0]))
The expression is included in a try
block. Line 6 raises the built-in exception SystemExit
. If no argument is passed to searcher.py
, then the process exits with a status code of 1 after printing the usage. Note the addition of sys.argv[0]
in the error message. It shows the name of the program in the usage message. Now, when you run the same program without any Python command line arguments, you can see the following output:
C:\Users\UserName\Desktop\DevPosts>searcher.py
Usage: C:\Users\UserName\Desktop\DevPosts\searcher.py <file_to_search>
Now that we have made ourselves familiar with how sys.argv
works, we can dive into our little project which is to code a simple file searcher
first we import the sys
module
import sys
Next we write checks to make sure we get the right number of arguments from the user
if len(sys.argv) < 3:
sys.stderr.write("Error: Usage " + sys.argv[0] + " <filename> <words>")
sys.stderr.flush()
exit(2)
else:
filename = sys.argv[1]
needle = sys.argv[2]
initalize counter for the number of times we find our needle
counter = 0
Now we open our file in read mode
file_handler = open(filename)
Next step is to iterate through the file while separating words in each line by a delimeter (space) and making it into a sentence
for line in file_handler.readlines():
sentences = line.split(" ")
Then iterate through the sentences word by word and compare against our needle, and if it is a match, we increment counter
for word in sentences:
# this checks if the last word is a new line character and eliminates it
if word == sentences[-1]:
word = word[:-1]
if word == needle:
counter += 1
Finally we print
the number of times the word was found
print (str(counter) + " times " + needle + " appears")
The full code
import sys
if len(sys.argv) < 3:
sys.stderr.write("Error: Usage " + sys.argv[0] + " <filename> <words>")
sys.stderr.flush()
exit(2)
else:
filename = sys.argv[1]
needle = sys.argv[2]
counter = 0
file_handler = open(filename)
for line in file_handler.readlines():
sentences = line.split(" ")
for word in sentences:
if word == sentences[-1]:
word = word[:-1]
if word == needle:
counter += 1
print (str(counter) + " times " + needle + " appears")
In this tutorial, you've learned about Python command line arguments. You should feel a little prepared to apply this new skills to your code.
Happy Coding :)
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