os_path.Rmd

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# Using the `os.path` module

The `os.path` module is one of two ways of manipulating and using file
paths in Python.  See the [path manipulation](path_manipulation.Rmd) page
for more discussion, and [pathlib](pathlib.Rmd) page for a tutorial on the
alternative `pathlib` approach.

The primary documentation for `os.path` is
<https://docs.python.org/3/library/os.path.html>

This page covers:

* `os.path`
* `dirname, basename, join, splitext, abspath`

```{python}
import os.path
```

In IPython, you can tab complete on `os.path` to list the functions and
attributes there.

The first function we will use from `os.path` is `dirname`.  To avoid
typing `os.path` all the time, we import `os.path` with the shortened name
`op`:

```{python}
import os.path as op
op.dirname
```

The `dirname` function gives the directory name from a full file path. It
works correctly for Unix paths on Unix machines, and Windows paths on Windows
machines:

```{python}
# On Unix
op.dirname('/a/full/path/then_filename.txt')
```

You'll see something like this as output if you run something similar on
Windows.  So `op.dirname('c:\\a\\full\\path\\then_filename.txt') ` will give:

```
'c:\\a\\full\\path'
```

Notice that, on Windows, you need to use double backslashes, because the
backslash in a Python string is a way of *escaping* a character — meaning, to
specify you mean exactly that character.  Double backslash has the meaning
"Yes, I do mean exactly backslash for the next character".

`dirname` also works for relative paths,  A relative path where the starting
directory is relative to the current directory, rather than absolute, in terms
of the root of the file system:

```{python}
# On Unix
op.dirname('relative/path/then_filename.txt')
```

Use `basename` to get the filename rather than the directory name:

```{python}
# On Unix
op.basename('/a/full/path/then_filename.txt')
```

Sometimes you want to join one or more directory names with a filename to get
a path.   Windows and Unix have different characters to separate directories
in a path.  Windows uses the backslash: `\`, Unix uses a forward slash:
`/`. If your code will run on Windows and Unix, you need to take care that
you get the right character joining your paths.  This is what `os.path.join`
does:

```{python}
# On Unix
op.join('relative', 'path', 'then_filename.txt')
```

This also works on Windows.  `op.join('relative', 'path', 'then_filename.txt')` gives output `'relative\\path\\then_filename.txt'`.

To convert a relative to an absolute path, use `abspath`:

```{python}
# Show the current working directory
os.getcwd()
op.abspath('relative/path/then_filename.txt')
```

Use `splitext` to split a path into: the path + filename; and the file
extension:

```{python}
op.splitext('relative/path/then_filename.txt')
```