NameError: name 'os' is not defined in Python [Solved]

# NameError: name 'os' is not defined in Python

The Python "NameError: name 'os' is not defined" occurs when we use the os module without importing it first.

To solve the error, import the os module before using it - import os.

Here is an example of how the error occurs.

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BASE = '/user'

# ⛔️ NameError: name 'os' is not defined
print(os.path.join(BASE, 'articles'))

print(os.environ['VIRTUAL_ENV_PROMPT'])

# Import the os module before using it

To solve the error, we have to import the os module.

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# 👇️ import os module first
import os

BASE = '/user'

# 👇️ /user/articles
print(os.path.join(BASE, 'articles'))

# 👇️ (venv)
print(os.environ['VIRTUAL_ENV_PROMPT'])

Even though the os module is in the Python standard library, we still have to import it before using it.

# Make sure to not import the os module in a nested scope

Also, make sure you haven't imported os in a nested scope, e.g. a function.

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def join_path():
    import os

    BASE = '/user'

    print(os.path.join(BASE, 'articles'))

    print(os.environ['VIRTUAL_ENV_PROMPT'])


# ⛔️ NameError: name 'os' is not defined
print(os.environ['VIRTUAL_ENV_PROMPT'])

We imported the os module in a function, so we aren't able to use it outside of the function.

Import the module at the top level to be able to use it throughout your code.

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import os


def join_path():

    BASE = '/user'

    print(os.path.join(BASE, 'articles'))

    print(os.environ['VIRTUAL_ENV_PROMPT'])


print(os.environ['VIRTUAL_ENV_PROMPT'])

The import statement for the os module has to come at the top of the file before any code that makes use of it.

# Make sure to not import the os module in a try/except statement

You also should be importing the os module in a try/except statement.

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try:
    # 👉️ Code here could raise an error
    import os

    print(os.environ['VIRTUAL_ENV_PROMPT'])
except ImportError:
    print(os.environ['VIRTUAL_ENV_PROMPT'])

print(os.environ['VIRTUAL_ENV_PROMPT'])

The code sample works, however, if the code in the try statement raises an error, the os module won't get imported successfully.

This would cause the error because we are trying to access properties on the os module in the outer scope and the except block.

Instead, move the import statement to the top level of the file.

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import os

try:

    print(os.environ['VIRTUAL_ENV_PROMPT'])
except ImportError:
    print(os.environ['VIRTUAL_ENV_PROMPT'])

print(os.environ['VIRTUAL_ENV_PROMPT'])

# Importing functions directly from the os module

An alternative to importing the entire os module is to import only the functions and constants that your code uses.

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from os import path, environ

BASE = '/user'

# 👇️ /user/articles
print(path.join(BASE, 'articles'))

# 👇️ (venv)
print(environ['VIRTUAL_ENV_PROMPT'])

The example shows how to import the path module and the environ mapping object from the os module.

Instead of accessing the members on the module, e.g. os.environ, we now access them directly.

This should be your preferred approach because it makes your code easier to read.

Conversely, when we import specific functions, it is much easier to see which functions from the os module are being used.

The os module provides a portable way of using operating system-dependent functionality.

You can view all of the functions and constants the os module provides by visiting the official docs.