Propagation V.S. Levels checking

import logging

root_logger = logging.getLogger()
root_handler = logging.StreamHandler()
root_handler.setFormatter(logging.Formatter('ROOT: %(message)s'))
root_logger.addHandler(root_handler)

# 父Logger配置
parent_logger = logging.getLogger('parent')
parent_handler = logging.StreamHandler()
parent_handler.setFormatter(logging.Formatter('PARENT: %(message)s'))
parent_logger.addHandler(parent_handler)
parent_logger.setLevel(logging.ERROR)

# 子Logger（默认propagate=True）
child_logger = logging.getLogger('parent.child')
child_handler = logging.StreamHandler()
child_handler.setFormatter(logging.Formatter('CHILD: %(message)s'))
child_logger.addHandler(child_handler)
child_logger.setLevel(logging.INFO)
child_logger.info("INFO msg")

Running Result:

/Users/egonever/DjangoByExamples/mysite/logging_demystified/example3.py 
CHILD: INFO msg
PARENT: INFO msg
ROOT: INFO msg

Process finished with exit code 0

Why "INFO msg" was logged three times?

The message "INFO msg" is logged three times due to Python's logging hierarchy and the default propagation behavior (propagate=True). Here's the detailed explanation:

1. ‌Child Logger Handling‌
   The child_logger first processes the message through its own handler (child_handler), which outputs:
   CHILD: INFO msg
   This occurs because the message level (INFO) meets the child logger's level (INFO)‌.

2. ‌Propagation to Parent Logger‌
   Since propagate=True (default), the message is passed to the parent logger (parent_logger). Despite the parent's level being ERROR, the message is still processed by the parent's handler (parent_handler), resulting in:
   PARENT: INFO msg
   Note: The parent logger's level only filters messages before they reach its handlers, but propagation bypasses this check‌.

3. ‌Propagation to Root Logger‌
   The message further propagates to the root logger, which has no level set (defaults to WARNING). However, the root logger's handler (root_handler) still processes the message, producing:
   ROOT: INFO msg
   Similar to the parent logger, the root's level doesn't block propagated messages‌.

Key Points‌

• Propagation allows messages to bubble up the logger hierarchy (child → parent → root)‌.
• Handler levels are independent of logger levels.
• When a handler is created, the level is set to NOTSET with numeric value 0(which causes all messages to be processed).
• To avoid duplicate logs, either:
  • Set child_logger.propagate = False, or
  • Configure logger levels and handlers more carefully‌.
    • Explicitly configure handlers' level by using setLevel method.