Avoid a shared mutable default argument.
Python: Create a fresh list inside the function when the optional argument is omitted.
def collect(value, items=None):
if items is None:
items = []
items.append(value)
return items
print(collect(1))
print(collect(2))Expected output EXAMPLE
[1] [2]
Details
Save as snippet.py and run python3 snippet.py. Uses the Python standard library. Create a fresh list inside the function when the optional argument is omitted.
Tested on Ubuntu 24.04.5 LTS · · Python 3.12.3 · python3 3.12.3-0ubuntu2.1
Test inputs, results and environment
The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.
- Architecture / host kernel
- x86_64 / 6.18.54.1-microsoft-standard-WSL2
- Snippet identity / locale
- uid=65534, gid=65534 (unprivileged) / C.UTF-8
- Interpreter invocation
python3 -c <published code>- Container image ID
sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd- Code SHA-256
70ead31b315d9eca36e807ae20d5b01abac7dc7ff36b9a1ac89fe4356f29afc7- Dependencies
- python3 3.12.3-0ubuntu2.1
Test input setup
These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.
# No input files required.
Observed standard output
[1] [2]
Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.