Understanding os.path.dirname() in Python (Q&A Learning Post)

[Rajeev Bagra]

While studying Django’s settings.py file, a learner encountered the following code:

BASE_DIR = os.path.dirname(
    os.path.dirname(
        os.path.abspath(__file__)
    )
)

After learning how BASE_DIR works, the learner asked an interesting question:

Does os.path.dirname() simply remove the final .py file from a path?

At first glance, it certainly appears that way because many examples involve Python files. However, the learner soon discovered that os.path.dirname() does something slightly different.

What Does dirname() Actually Do?

The function:

os.path.dirname(path)

returns the parent directory of a path.

In other words, it removes the final component of the path and returns everything before it.

A common misconception is that it specifically removes Python files. In reality, it removes whatever appears last in the path, whether that is a file or a directory.

Example 1: Removing a File

Consider the following path:

import os

path = "/home/user/wiki/wiki/settings.py"

print(os.path.dirname(path))

Output:

/home/user/wiki/wiki

The last component:

settings.py

was removed.

This often leads beginners to think that dirname() removes Python files.

However, that’s not the real rule.

Example 2: Removing a Directory

Now consider:

import os

path = "/home/user/wiki/wiki"

print(os.path.dirname(path))

Output:

/home/user/wiki

Notice that there is no file involved.

The final directory:

wiki

was removed instead.

This proves that dirname() does not care whether the final component is a file or a folder.

A Simple Mental Model

Think of dirname() as asking:

“Who is the parent folder of this path?”

For example:

/a/b/c.txt

Parent directory:

/a/b

Likewise:

/a/b/c

Parent directory:

/a/b

In both cases, the final component is removed.

How This Works in Django

The learner then revisited Django’s code:

BASE_DIR = os.path.dirname(
    os.path.dirname(
        os.path.abspath(__file__)
    )
)

Suppose the project structure is:

wiki/
│
├── manage.py
│
└── wiki/
    └── settings.py

The special variable:

__file__

might contain:

/home/user/wiki/wiki/settings.py

Django then performs:

os.path.abspath(__file__)

Result:

/home/user/wiki/wiki/settings.py

Nothing changes except that the path is guaranteed to be absolute.

Next comes the first dirname():

os.path.dirname(
    "/home/user/wiki/wiki/settings.py"
)

Result:

/home/user/wiki/wiki

The filename has been removed.

Then Django performs the second dirname():

os.path.dirname(
    "/home/user/wiki/wiki"
)

Result:

/home/user/wiki

Now the final directory has been removed.

The resulting value becomes:

BASE_DIR

which points to the root directory of the Django project.

What If settings.py Were in the Root Directory?

The learner then considered another scenario:

wiki/
│
├── manage.py
├── settings.py
└── encyclopedia/

In this case:

__file__

would be:

/home/user/wiki/settings.py

Only one dirname() would be required:

BASE_DIR = os.path.dirname(
    os.path.abspath(__file__)
)

Result:

/home/user/wiki

Because removing the file already places us at the project root.

A Useful Rule

Whenever you use:

os.path.dirname()

count how many directory levels you need to move upward.

For example:

project/
└── config/
    └── settings/
        └── settings.py

To reach:

project/

you would need:

os.path.dirname(
    os.path.dirname(
        os.path.dirname(
            os.path.abspath(__file__)
        )
    )
)

because three levels must be traversed.

Key Takeaway

The learner eventually realized that os.path.dirname() does not specifically remove Python files.

Instead, it removes the last component of a path and returns the parent directory.

If the final component is:

• A file → the file is removed.
• A directory → the directory is removed.

In Django’s BASE_DIR calculation, the first dirname() removes settings.py, while the second dirname() removes the inner wiki directory. The final result is the root project folder, which Django uses when building paths for databases, templates, static files, and other resources.

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