Python Command Line Arguments and Options

Command Line Arguments And Options

This is a detailed tutorial of the Python Command Line Arguments. Learn to parse the command-line options and arguments to get their values in the program.

Python Command Line Arguments

Let’s say there’s a Python file named hello.py and we want to execute it from the command line. We’ve to write the following command.

python hello.py

You can also pass any number of arguments along with this command as demonstrated in the following command.

python hello.py arg1 arg2 arg3

Here arg1, arg2 and arg3 are the three different arguments. These arguments can be accessed within the Python program code written in the file hello.py

Python sys module

The command-line arguments can be handled with the Python sys module. You can use the argv property of the sys module to get the full list of the arguments passed while executing the current python code file.

The following code illustrates the syntax to fetch the list of arguments, number of arguments and the name of the currently executing Python file.

Note. Always keep in mind that the file name itself is counted as the first argument.

Example

We have saved the code written below in a file named try.py

We executed the above file using the following command.

python try.py hello how are you

Output.

List of Arguments:  ['try.py', 'hello', 'how', 'are', 'you']
Number of Arguments:  5
Currently Executing File:  try.py

As you can observe in the output, the sys.argv contains a Python List with 5 different items. Each of these items is actually the command line arguments including the file name itself. Then we have used the built-in function len() to check the count of this list that gives us the total number of arguments. In the end, we’ve printed the first item of the list or the item at the index 0 i.e. the name of the currently executing python file.

You can use the Python For Loop to iterate the arguments list to process each of the arguments one by one.

Python Command-Line Options

We have learned how you can get all of the different arguments and now let’s figure out the same about the command-line options. One command-line option basically comprises of two command-line arguments. Observe the following command.

python hello.py -opt1 value1 -opt2 value2

Here in the above command, there are five command-line arguments but there are only two command-line options. The five command-line arguments are hello.py, -opt1, value1, -opt2 and value2. On the other hand side, the two options are opt1 and opt2 with values value1 and value2 respectively.

To get the values for the different options, you can first fetch the list of all of the argument using sys.argv[1:] and then can process this list using the getopt module to fetch the option values.

Python getopt Module

This module can be used to get the values of the different options for the currently executing python file. All you need to do is to import the getopt module and then further have to use its method which is also named getopt().

Syntax

Parameters

This method can take upto three different arguments that are described below.

  • args. This is a Python list that contains the list of arguments. We can pass here the list retrieved using sys.argv[1:]. Make sure the list that you are passing here should not contain the file name as the argument. This list of arguments should contain single-character options. For example, a, b, c, etc. For long options, use the third argument.
  • options. This is the string of options for which we want to get the values. Multiple option names have to be separated by a : (colon) symbol. For example, "option1:option2:option3"
  • [long_options]. This is an optional argument and should belong to list data type. This is for the options with more than one character. For example, option1=, opt1=, some-option=, etc. These options are always followed by an equal sign (=).

Return Value

The method getopt() returns a value that further consists of two different values. The first value is basically a list of tuples. Each of these tuples contains the (option, value) pair. The second value returned by this method is also a list that contains the arguments that are still left after parsing the recognizable options and getting their values.

Example 1

Have a look at the following code.

We saved the above code in a file named try.py and executing it using the following command.

python try.py -a VALUE_A -b VALUE_B -c VALUE_C hello how are you

Output.

[('-a', 'VALUE_A'), ('-b', 'VALUE_B'), ('-c', 'VALUE_C')]
['hello', 'how', 'are', 'you']

Each tuple in the list of option-value pairs contains the option name itself prepended with - at the first position and the value of it in the second position. Also, the remaining arguments are also printed in the second line.

Example 2

The following example illustrates the mixed usage of short and long options.

We ran the above code saved in try.py file using the following command.

python try.py -i VALUE_i -o VALUE_o --opt1 VALUE_opt1 --some-option VALUE_some_option hello how are you

Output.

[('-i', 'VALUE_i'), ('-o', 'VALUE_o'), ('--opt1', 'VALUE_opt1'), ('--some-option', 'VALUE_some_option')]
['hello', 'how', 'are', 'you']

getopt.GetoptError Exception

This is the exception raised in case the method finds an unrecognized option and when no argument is given while the option requires one. The argument passed to the exception is basically a string that contains the reason for why the exception raised. Its attributes msg and opt gives further information about the exception.

You can handle such exceptions using Try Except.

I hope you found this guide useful. If so, do share it with others who are willing to learn Python. If you have any questions related to this article, feel free to ask us in the comments section.

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