This rule raises an issue when os.environ is shallow-copied with copy.copy().

Why is this an issue?

os.environ is not a plain dict. It is a proxy mapping that reads and writes the process environment.

copy.copy(os.environ) creates a shallow copy of that proxy. Mutations on the copy still update the real process environment, which is almost never what developers expect when they ask for a copy.

import copy
import os

env = copy.copy(os.environ)
env["MY_VAR"] = "value"
# os.environ["MY_VAR"] is also "value"

os.environ.copy() returns an independent dict snapshot. Changes to that dictionary do not affect the process environment.

What is the potential impact?

• unintended environment changes: code that mutates what it believes is a local copy can alter the real process environment. Later code, child processes, or libraries that read environment variables observe unexpected values
• hard-to-diagnose bugs: the mutation site and the failure site are often far apart

How to fix it

Replace copy.copy(os.environ) with os.environ.copy().

Code examples

Noncompliant code example

import copy
import os

env = copy.copy(os.environ)  # Noncompliant

Compliant solution

import os

env = os.environ.copy()

Resources

Documentation

• Python Documentation - os.environ
• Python Documentation - copy — Shallow and deep copy operations

External coding guidelines

• Ruff - PLW1507 - Shallow copy of os.environ