dunders.Rmd

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# Two double underscore variables

Python often uses variable and function and method names with double
underscores on each end.

For example, as Python sets up to import a module, it defines a variable for
itself called `__file__`.

Experienced Python people often call these variables "dunder" variables,
because they have Double UNDERscores on each side.

When you see a *dunder* variable or function or method, it is almost invariably
a variable or function or method that Python has defined, or that Python is
using in a special way.


## The `__file__` variable

The `__file__` variable contains the path to the file that Python is currently
importing.  You can use this variable inside a module to find the path of the
module.  For example, let's say you have a module like this:

```{python}
# %%file example_module.py

# An example Python module
print("Type of __file__ variable is:", type(__file__))
print("__file__ is:", __file__)
```

If you run this module as a script, `__file__` is set:

```{python}
# Execute as script
%run example_module.py
```

If you `import` the module, `__file__` is also set:

```{python}
import example_module
```

## The `__name__` variable

When Python `import`s a module, it sets the `__name__` variable to be a string
containing the name of the module it is importing:

```{python}
# %%file another_example.py

# Another example Python module
print("Type of __name__ variable is:", type(__name__))
print("__name__ is:", __name__)
```

```{python}
import another_example
```

If you run the same module as a script, Python is not `import`ing when it runs
the code, and `__name__` contains the string `"__main__"`:

```{python}
%run another_example.py
```