Making exec safer using RestrictedPython

Hi Pythonistas!,

In the previous post, we discussed how to generate dynamic Python code using the exec() function.

While powerful, exec() can be dangerous if not handled properly, especially when running untrusted code.

Today, we’ll explore a safer alternative: RestrictedPython.

RestrictedPython is a sandboxing library that lets you safely execute

Python code with tight control over what that code can do.

it protects your system from:

Accessing files
Running system commands
Using dangerous built-ins like open(), exec(), or eval()

Why Use It

Let’s say you want to allow users to run custom Python snippets in your app, like:

for i in range(3):
    print(i*2)

Block code like this

import os
os.remove("important_file.txt")

Normal exec() is unsafe.

RestrictedPython only allows "safe" operations.

How to Use It

pip install RestrictedPython

Basic Example

from RestrictedPython import compile_restricted
from RestrictedPython import safe_globals

source_code = """
def example():
    return 'Hello World!'
"""
loc = {}
byte_code = compile_restricted(source_code, '<inline>', 'exec')
exec(byte_code, safe_globals, loc)
loc['example']()

Output

Hello World!

This is a safe, limited environment:

No import allowed
No file or OS access
No exec() or eval() inside the code

If we run a code like this

from RestrictedPython import compile_restricted
from RestrictedPython import safe_globals

source_code = """
import os

os.listdir('/')

"""
byte_code = compile_restricted(source_code, '<inline>', 'exec')
exec(byte_code, safe_globals, {})

Output

Traceback (most recent call last):
  File "/home/afsal/Desktop/experiments/restricted_python/no_working.py", line 11, in <module>
    exec(byte_code, safe_globals, {})
  File "<inline>", line 2, in <module>
ImportError: __import__ not found

How Does It Work?

Compiles the code to Python bytecode
Strips out or restricts unsafe features
Provides a safe set of built-in functions

Great for:

Online code runners / compilers
Plugin systems
Custom business rule scripting
Safe sandboxing for education

Not great for:

Full app environments
Anything requiring I/O, network, or files

Summary

RestrictedPython A sandbox to run safe Python code

Blocks unsafe use Files, OS commands, imports, eval/exec

Use case Untrusted user code, education, plugins