Python conformance gaps: parser (PEP 3132 unpacking, inline try) + stdlib (ssl, urllib.request.Request)

[ivarvong]

Summary

Running real LLM-generated Python workloads through Pyex surfaced several conformance gaps where valid Python 3 is rejected or expected stdlib is missing. Each silently turns correct model-generated code into an error, costing the agent turns. Grouping them here; related to #80 (raw-f-string lexer).

All repros run via the sandbox; the control (print(sum(range(10))) → 45) works.

Parser gaps (valid Python 3 rejected)

1. PEP 3132 starred unpacking in for targets

for a, b, *c in [(1, 2, 3, 4)]:
    print(c)

• Expected: [3, 4]
• Actual: expected variable name in for loop at line 1

Valid since Python 3.0 (PEP 3132). Models reach for this when iterating over variable-length tuples.

2. try: with an inline statement

try: x = 1
except Exception: pass
print("ok")

• Expected: ok
• Actual: expected ':' after try at line 1

try: followed by a simple statement on the same line is valid CPython grammar (compound stmt with inline suite), same as if x: y = 1.

3. Raw-f-string prefix rf"..." / fr"..." — see #80

Tracked separately in #80; listing here for completeness as the same theme.

Stdlib gaps

4. ssl module absent

import ssl

• Actual: ImportError: no module named 'ssl'. Available modules: os, abc, base64, bis...

Code falling back from a higher-level client to stdlib HTTPS hits a dead end.

5. urllib.request.Request missing

import urllib.request as u
print(type(u.Request))

• Actual: AttributeError: module 'urllib.request' has no attribute 'Request'

The urllib.request shim lacks Request, the standard way to build a request with headers — a very common idiom.

Why it matters

These are model-agnostic: any model emitting standard Python (starred unpacking, inline try, urllib.request.Request, raw-f-strings) hits a silent fault and burns turns recovering. A conformant parser + more complete stdlib coverage (or, where a construct is intentionally unsupported, a precise error naming it) would remove this class of friction.