general: add a test for __init__.py fallback for modules which are switching to namespace packages

for now just a manual ad-hoc test, will try to set it up on CI later

relevant to the discussion here: https://memex.zulipchat.com/#narrow/stream/279601-hpi/topic/extending.20HPI/near/270465792

also potentially relevant to

- https://github.com/karlicoss/HPI/issues/89 (will try to apply to this to reddit/__init__.py later)
- https://github.com/karlicoss/HPI/issues/102

misc/check_legacy_init_py.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+# NOTE: prerequisites for this test:
+# fbmessengerexport installed
+# config configured (can set it to '' though)
+
+from pathlib import Path
+from subprocess import Popen, run, PIPE
+from tempfile import TemporaryDirectory
+
+
+import logzero # type: ignore[import]
+logger = logzero.logger
+
+
+MSG = 'DEPRECATED! Instead of my.fbmessengerexport'
+
+def expect(*cmd: str, should_warn: bool=True) -> None:
+    res = run(cmd, stderr=PIPE)
+    errb = res.stderr; assert errb is not None
+    err = errb.decode('utf8')
+    if should_warn:
+        assert MSG     in err, res
+    else:
+        assert MSG not in err, res
+    assert res.returncode == 0, res
+
+
+def _check(*cmd: str, should_warn: bool, run_as_cmd: bool=True) -> None:
+    expecter = lambda *cmd: expect(*cmd, should_warn=should_warn)
+    if cmd[0] == '-c':
+        [_, code] = cmd
+        if run_as_cmd:
+            expecter('python3', '-c', code)
+        # check as a script
+        with TemporaryDirectory() as tdir:
+            script = Path(tdir) / 'script.py'
+            script.write_text(code)
+            expecter('python3', str(script))
+    else:
+        expecter('python3', *cmd)
+    what = 'warns' if should_warn else '     ' # meh
+    logger.info(f"PASSED: {what}: {repr(cmd)}")
+
+
+def check_warn(*cmd: str, **kwargs) -> None:
+    _check(*cmd, should_warn=True, **kwargs)
+
+def check_ok(*cmd: str, **kwargs) -> None:
+    _check(*cmd, should_warn=False, **kwargs)
+
+
+# NOTE these three are actually sort of OK, they are allowed when it's a proper namespace package with all.py etc.
+# but more likely it means legacy behaviour or just misusing the package?
+# worst case it's just a warning I guess
+check_warn('-c', 'from   my import fbmessenger')
+check_warn('-c', 'import my.fbmessenger')
+check_warn('-c', 'from   my.fbmessenger import *')
+
+# note: dump_chat_history should really be deprecated, but it's a quick way to check we actually fell back to fbmessenger/export.py
+# NOTE: this is the most common legacy usecase
+check_warn('-c', 'from   my.fbmessenger import messages, dump_chat_history')
+check_warn('-m', 'my.core', 'query' , 'my.fbmessenger.messages')
+check_warn('-m', 'my.core', 'doctor', 'my.fbmessenger')
+
+# todo kinda annoying it doesn't work when executed as -c (but does as script!)
+# presumably because doesn't have proper line number information?
+# either way, it'a a bit of a corner case, the script behaviour is more important
+check_ok  ('-c', 'from   my.fbmessenger import export', run_as_cmd=False)
+check_ok  ('-c', 'import my.fbmessenger.export')
+check_ok  ('-c', 'from   my.fbmessenger.export import *')
+check_ok  ('-c', 'from my.fbmessenger.export import messages, dump_chat_history')
+check_ok  ('-m', 'my.core', 'query' , 'my.fbmessenger.export.messages')
+check_ok  ('-m', 'my.core', 'doctor', 'my.fbmessenger.export')
+
+# TODO with reddit, currently these don't work properly at all
+# only when imported from scripts etc?