Context managers. Initially just stackattrs.

Latest release 20200517:

• Add nullcontext like the one from recent contextlib.
• stackattrs: expose the push and pop parts as pushattrs() and popattrs().

Function popattrs(o, attr_names, old_values)

The "pop" part of stackattrs. Restore previous attributes of o named by attr_names with previous state in old_values.

Function pushattrs(o, **attr_values)

The "push" part of stackattrs. Push attr_values onto o as attributes, return the previous attribute values in a dict.

Function stackattrs(o, **attr_values)

Context manager to push new values for the attributes of o and to restore them afterward. Returns a dict containing a mapping of the previous attribute values. Attributes not present are not present in the mapping.

Restoration includes deleting attributes which were not present initially.

Example of fiddling a programme's "verbose" mode:

>>> class RunModes:
...     def __init__(self, verbose=False):
...         self.verbose = verbose
...
>>> runmode = RunModes()
>>> if runmode.verbose:
...     print("suppressed message")
...
>>> with stackattrs(runmode, verbose=True):
...     if runmode.verbose:
...         print("revealed message")
...
revealed message
>>> if runmode.verbose:
...     print("another suppressed message")
...

Example exhibiting restoration of absent attributes:

>>> class O:
...     def __init__(self):
...         self.a = 1
...
>>> o = O()
>>> print(o.a)
1
>>> print(o.b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'O' object has no attribute 'b'
>>> with stackattrs(o, a=3, b=4):
...     print(o.a)
...     print(o.b)
...     o.b = 5
...     print(o.b)
...     delattr(o, 'a')
...
3
4
5
>>> print(o.a)
1
>>> print(o.b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'O' object has no attribute 'b'

Release Log

Release 20200517:

• Add nullcontext like the one from recent contextlib.
• stackattrs: expose the push and pop parts as pushattrs() and popattrs().

Release 20200228.1: Initial release with stackattrs context manager.