cs.deco 20190601

Assorted decorator functions.

Assorted decorator functions.

Function cached(*da, **dkw)

Decorator to cache the result of a method and keep a revision counter for changes.

The cached values are stored on the instance (self). The revision counter supports the @revised decorator.

This decorator may be used in 2 modes. Directly:

@cached
def method(self, ...)

or indirectly:

@cached(poll_delay=0.25)
def method(self, ...)

Optional keyword arguments:

• attr_name: the basis name for the supporting attributes. Default: the name of the method.
• poll_delay: minimum time between polls; after the first access, subsequent accesses before the poll_delay has elapsed will return the cached value. Default: None, meaning no poll delay.
• sig_func: a signature function, which should be significantly cheaper than the method. If the signature is unchanged, the cached value will be returned. The signature function expects the instance (self) as its first parameter. Default: None, meaning no signature function; the first computed value will be kept and never updated.
• unset_value: the value to return before the method has been called successfully. Default: None.

If the method raises an exception, this will be logged and the method will return the previously cached value, unless there is not yet a cached value in which case the exception will raise.

If the signature function raises an exception then a log message is issued and the signature is considered unchanged.

An example use of this decorator might be to keep a "live" configuration data structure, parsed from a configuration file which might be modified after the program starts. One might provide a signature function which called os.stat() on the file to check for changes before invoking a full read and parse of the file.

Function decorator(deco)

Wrapper for decorator functions to support optional arguments. The actual decorator function ends up being called as:

deco(func, *da, **dkw)

allowing da and dkw to affect the behaviour of the decorator deco.

Examples:

@decorator
def deco(func, *da, kw=None):
  ... decorate func subject to the values of da and kw
@deco
def func1(...):
  ...
@deco('foo', arg2='bah')
def func2(...):
  ...

Function fmtdoc(func)

Decorator to replace a function's docstring with that string formatted against the function's module's __dict__.

This supports simple formatted docstrings:

ENVVAR_NAME = 'FUNC_DEFAULT'

@fmtdoc
def func():
    """Do something with os.environ[{ENVVAR_NAME}]."""
    print(os.environ[ENVVAR_NAME])

This gives func this docstring:

Do something with os.environ[FUNC_DEFAULT].

Warning: this decorator is intended for wiring "constants" into docstrings, not for dynamic values. Use for other types of values should be considered with trepidation.

Function observable_class(property_names, only_unequal=False)

Class decorator to make various instance attributes observable.

Parameters:

• property_names: an interable of instance property names to set up as observable properties. As a special case a single str can be supplied of only one attribute is to be observed.
• only_unequal: only call the observers if the new property value is not equal to the previous proerty value. This requires property values to be comparable for inequality. Default: False, meaning that all updates will be reported.

Function strable(*da, **dkw)

Decorator for functions which may accept a str instead of their core type.

Parameters:

• func: the function to decorate
• open_func: the "open" factory to produce the core type form the string if a string is provided; the default is the builtin "open" function

The usual (and default) example is a function to process an open file, designed to be handed a file object but which may be called with a filename. If the first argument is a str then that file is opened and the function called with the open file.

Examples:

@strable
def count_lines(f):
  return len(line for line in f)

class Recording:
  "Class representing a video recording."
  ...
@strable
def process_video(r, open_func=Recording):
  ... do stuff with `r` as a Recording instance ...