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Q&A How can I access and use command-line arguments in Python?

At startup, Python loads the sys standard library module (without waiting for any code to import it) and sets up several useful values. Among these is sys.argv, which stores command-line arguments ...

posted 6mo ago by Karl Knechtel‭

Answer
#1: Initial revision by user avatar Karl Knechtel‭ · 2024-06-15T09:35:12Z (6 months ago)
At startup, Python loads the `sys` standard library module (without waiting for any code to `import` it) and sets up several useful values. Among these is [sys.argv](https://docs.python.org/3/library/sys.html#sys.argv), which stores command-line arguments for the script. This is a plain list of strings, one per token on the command line (as in other programming languages, it's the shell's responsibility to tokenize the command line).

Notice that these are *not* the command-line arguments of the Python process: it won't include `'python'` (or `'py'`), and generally won't contain command-line arguments that were passed to the Python interpreter. It only includes arguments *for the script*: `argv[0]` is the name of the script's file, etc. Therefore, the Python code doesn't need any special cases.

Typically, Python programs don't care about the script's file name, so they will pass `sys.argv[1:]` along to whatever code is used to *parse* (understand) the command-line arguments.

<section class="notice is-warning">

Because of how this system works, it's **not possible** to use the presence or absence of command-line arguments, a `main` function etc. to distinguish Python scripts from importable modules. Indeed, **a given Python source file is allowed to fill both roles**.
</section>

Depending on the interface you're designing, you may want to use a library for this task. The default standard library for this is [`argparse`](https://docs.python.org/3/library/argparse.html), which was added in Python 3.2, deprecating the older [`optparse`](https://docs.python.org/3/library/optparse.html). Both of these are higher-level wrappers; [`getopt`](https://docs.python.org/3/library/getopt.html) is also available as a low-level option that closely mimics the [C POSIX `getopt` function](https://en.wikipedia.org/wiki/Getopt). Many Python coders still find `argparse` lacking in features or annoying to use; one commonly recommended third-party option is [Click](https://pypi.org/project/click/). Note that some of these tools may automatically look up `sys.argv` (at least by default) without you passing it in.