cs.context 20200725

Context managers. Initially just `stackattrs`.

Context managers. Initially just stackattrs.

Latest release 20200725: New stackkeys and components pushkeys and popkeys doing "stackattrs for dicts/mappings".

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.

This can be useful in hook/signal/callback, where you cannot inline a context manager.

Function popkeys(d, key_names, old_values)

The "pop" part of stackkeys. Restore previous key values of d named by key_names with previous state in old_values.

This can be useful in hook/signal/callback, where you cannot inline a context manager.

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.

This can be useful in hook/signal/callback, where you cannot inline a context manager.

Function pushkeys(d, **key_values)

The "push" part of stackkeys. Push key_values onto d as key values. return the previous key values in a dict.

This can be useful in hook/signal/callback, where you cannot inline a context manager.

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.

See stackkeys for a flavour of this for mappings.

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'

Function stackkeys(d, **key_values)

Context manager to push new values for the key values of d and to restore them afterward. Returns a dict containing a mapping of the previous key values. Keys not present are not present in the mapping.

Restoration includes deleting key values which were not present initially.

See stackattrs for a flavour of this for objects.

Example of making log entries which may reference some higher level context log entry:

>>> import time
>>> global_context = {
...     'parent': None,
... }
>>> def log_entry(desc, **kw):
...     print("log_entry: global_context =", repr(global_context))
...     entry = dict(global_context)
...     entry.update(desc=desc, when=time.time())
...     entry.update(kw)
...     return entry
...
>>> log_entry("stand alone entry")    #doctest: +ELLIPSIS
log_entry: global_context = {'parent': None}
{'parent': None, 'desc': 'stand alone entry', 'when': ...}
>>> context_entry = log_entry("high level entry")
log_entry: global_context = {'parent': None}
>>> context_entry                     #doctest: +ELLIPSIS
{'parent': None, 'desc': 'high level entry', 'when': ...}
>>> with stackkeys(global_context, parent=context_entry): #doctest: +ELLIPSIS
...     print(repr(log_entry("low level event")))
...
log_entry: global_context = {'parent': {'parent': None, 'desc': 'high level entry', 'when': ...}}
{'parent': {'parent': None, 'desc': 'high level entry', 'when': ...}, 'desc': 'low level event', 'when': ...}
>>> log_entry("another standalone entry")    #doctest: +ELLIPSIS
log_entry: global_context = {'parent': None}
{'parent': None, 'desc': 'another standalone entry', 'when': ...}

Release Log

Release 20200725: New stackkeys and components pushkeys and popkeys doing "stackattrs for dicts/mappings".

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.