Post History

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.