Use Python's logging framework for INFO diagnostic messages (fixes #353)

[shivamtiwari3]

Summary

Fixes #353.

The two INFO: Showing help … messages that Python Fire prints to stderr
were emitted via bare print() calls. This made it impossible for users
to suppress or redirect the messages using Python's standard logging
machinery.

---

Problem

import fire

# There was no way to suppress this line before this fix:
# INFO: Showing help with the command 'mytool -- --help'.
fire.Fire(MyClass, command=['--help'])

A common workaround was the fragile monkey-patch described in #353:

def _print(*args, **kwargs):
    args = [a for a in args if not (isinstance(a, str) and a.startswith('INFO:'))]
    if args:
        print(*args, **kwargs)
fire.core.print = _print

---

Solution

Three changes in fire/core.py:

1. _LazyStderrStreamHandler

A thin logging.StreamHandler subclass that resolves sys.stderr
at emit time via a @property rather than capturing it at
construction time. This is essential for correctness in tests that
replace sys.stderr with an in-memory StringIO buffer using
unittest.mock.patch.object(sys, 'stderr', ...).

2. _logger

_logger = logging.getLogger(__name__)   # → 'fire.core'

The handler is added only when _logger has no handlers yet (idempotent
on repeated imports / reloads). propagate is set to False so that
users who configure the root logger do not receive duplicate output.

3. Level on the parent 'fire' logger

The level is set to INFO on the 'fire' parent logger (not on
'fire.core' directly). Because 'fire.core' inherits its effective
level from the hierarchy, users can suppress all fire messages with a
single call to the parent:

import logging
logging.getLogger('fire').setLevel(logging.WARNING)   # suppress INFO

or target just this module:

logging.getLogger('fire.core').setLevel(logging.WARNING)

or redirect to a custom destination:

handler = logging.FileHandler('fire.log')
logging.getLogger('fire').handlers = [handler]

Default behaviour is unchanged

For a user who does not configure logging, the INFO: line continues
to appear on stderr exactly as before. The format is preserved:

INFO: Showing help with the command 'mytool -- --help'.

---

Tests added (LoggingTest in fire/core_test.py)

Test	What it verifies
testInfoMessageAppearsOnStderrByDefault	Default stderr output is preserved (no regression)
testInfoMessageUsesFireLogger	core._logger is a logging.Logger named 'fire.core'
testInfoCanBeSuppressedViaLogging	Setting fire.core level to WARNING hides the INFO line
testInfoCanBeSuppressedViaParentLogger	Setting fire level to WARNING hides the INFO line
testInfoCanBeRedirectedViaCustomHandler	Replacing handlers redirects INFO to a custom stream

---

Test results

265 passed in 0.25s

All pre-existing tests pass without modification. The four INFO:.*SYNOPSIS
assertions in CoreTest continue to match because the logging output format
(INFO: <message>) is identical to what print() produced before.

---

Checklist

• Fixes the reported issue (Logging messages don't using logging #353)
• Default user-visible behaviour is unchanged
• Uses logging.getLogger(__name__) (library best practice)
• No new runtime dependencies
• Fully covered by new and existing tests
• All 265 tests pass

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