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

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

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

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

Here is an example of how the error occurs.

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print('before')

# ⛔️ NameError: name 'sys' is not defined
print(sys.version)

print(sys.exit())

print('after')

# Import the sys module before using it

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

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# 👇️ import sys first
import sys

print('before')

print(sys.version)

print(sys.exit())

print('after')

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

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

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

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def get_version():
    import sys

    print(sys.version)


# ⛔️ NameError: name 'sys' is not defined
print(sys.exit())

We imported the sys 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 sys


def get_version():
    print(sys.version)


get_version()

print(sys.exit())

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

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

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

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

    print(sys.version)
except ImportError:
    print(sys.platform)

print(sys.platform)

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

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

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

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import sys

try:
    print(sys.version)
except ImportError:
    print(sys.platform)

print(sys.platform)

# Importing functions and constants from the sys module

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

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from sys import version, exit

print('before')

print(version)

print(exit())

print('after')

The example shows how to import the exit() function and the version constant from the sys module.

Instead of accessing the members on the module, e.g. sys.exit(), 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 sys module are being used.

The sys module provides access to variables used by the Python interpreter and to functions that interact with the interpreter.

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