How does autologging work?

Release:1.2.1

The autologging.logged decorator is rather boring - it simply creates a logging.Logger object and sets it as an attribute of the decorated class (or function).

However, in order to automate tracing while preserving introspection and subclassing capabilities, the autologging.traced decorator has a tougher job.

To trace the call and return of a particular function, Autologging performs the following steps:

  1. Intercept the function call, capturing all positional and keyword arguments that were passed.
  2. Log the call at the autologging.TRACE log level.
  3. Call the original function, passing the positional and keyword arguments, and capturing the return value.
  4. Log the return at the autologging.TRACE log level.
  5. Return the value.

Autologging installs a replacement function for any traced class method or function. That replacement function is responsible for the steps described above.

Note

If the TRACE level is disabled for the tracing logger when a traced function is called, the replacement function delegates directly to the original function.

Which functions are traced?

A quick way to determine which functions have been traced is to look for the __autologging_traced__ attribute. Autologging sets this attribute to the value True on every replacement function. For example:

>>> from autologging import traced
>>> @traced
... def example():
...     return "OK"
...
>>> hasattr(example, "__autologging_traced__")
True
>>> @traced
... class Example:
...     @classmethod
...     def class_method(cls):
...         return "OK"
...     @staticmethod
...     def static_method():
...         return "OK"
...     def method(self):
...         return "OK"
...
>>> hasattr(Example.class_method, "__autologging_traced__")
True
>>> hasattr(Example.static_method, "__autologging_traced__")
True
>>> hasattr(Example.method, "__autologging_traced__")
True

Introspecting traced functions

When Autologging installs a replacement function for tracing, a reference to the original function is stored as the __wrapped__ attribute of the replacement function.

>>> from autologging import traced
>>> @traced
... def example():
        return "OK"
...
>>> traced(example).__wrapped__ is example
True
>>> from autologging import traced
>>> @traced
... class Example:
...     def method(self):
...         return "OK"
...
>>> original_method = Example.__dict__["method"]
>>> traced(Example).__dict__["method"].__wrapped__ is original_method
True

Traced classmethod and staticmethod functions are also replaced by Autologging, but in addition to creating a replacement function, Autologging also creates a replacement method descriptor. To access the original function of a classmethod or staticmethod, you must use the __wrapped__ attribute of the __func__ attribute of the replacement classmethod or staticmethod. An example makes this clear:

>>> from autologging import traced
>>> @traced
... class Example:
...     @classmethod
...     def class_method(cls):
...         return "OK"
...     @staticmethod
...     def static_method():
...         return "OK"
...
>>> original_classmethod = Example.__dict__["class_method"]
>>> original_staticmethod = Example.__dict__["static_method"]
>>> Example = traced(Example)
>>> Example.__dict__["class_method"].__func__.__wrapped__ is original_classmethod.__func__
True
>>> Example.__dict__["static_method"].__func__.__wrapped__ is original_staticmethod.__func__
True

Inheritance and subclassing with traced methods

Autologging is careful to not “break” assumptions about the types of methods, or how those methods are inherited or overridden.

A replacement tracing method (or method descriptor, in the case of classmethods and staticmethods) has the same type, name and signature as the original method:

>>> import inspect
>>> from types import FunctionType, MethodType
>>> from autologging import traced
>>> @traced
... class Example:
...     @classmethod
...     def class_method(cls, arg, keyword=None):
...         return "OK"
...     @staticmethod
...     def static_method(arg, keyword=None):
...         return "OK"
...     def method(self, arg, keyword=None):
...         return "OK"
...
>>> type(Example.__dict__["class_method"]) is classmethod
True
>>> Example.class_method.__name__
'class_method'
>>> inspect.signature(Example.class_method)
<Signature (arg, keyword=None)>
>>> type(Example.__dict__["static_method"]) is staticmethod
True
>>> Example.static_method.__name__
'static_method'
>>> inspect.signature(Example.static_method)
<Signature (arg, keyword=None)>
>>> type(Example.__dict__["method"]) is FunctionType
True
>>> type(Example().method) is MethodType
True
>>> Example.method.__name__
'method'
>>> inspect.signature(Example().method)
<Signature (arg, keyword=None)>