cs.context 20221118

Assorted context managers.

Assorted context managers.

Latest release 20221118: stackattrs: improve docstring.

Class ContextManagerMixin

A mixin to provide context manager __enter__ and __exit__ methods running the first and second steps of a single __enter_exit__ generator method.

Note: the __enter_exit__ method is not a context manager, but a short generator method.

This makes it easy to use context managers inside __enter_exit__ as the setup/teardown process, for example:

def __enter_exit__(self):
    with open(self.datafile, 'r') as f:
        yield f

Like a context manager created via @contextmanager it performs the setup phase and then yields the value for the with statement. If None is yielded (as from a bare yield) then self is returned from __enter__. As with @contextmanager, if there was an exception in the managed suite then that exception is raised on return from the yield.

However, and unlike an @contextmanager method, the __enter_exit__ generator may also yield an additional true/false value to use as the result of the __exit__ method, to indicate whether the exception was handled. This extra yield is optional and if it is omitted the __exit__ result will be False indicating that an exception was not handled.

Here is a sketch of a method which can handle a SomeException specially:

class CMgr(ContextManagerMixin):
    def __enter_exit__(self):
        ... do some setup here ...
        # Returning self is common, but might be any relevant value.
        # Note that if you want `self`, you can just use a bare yield
        # and ContextManagerMixin will provide `self` as the default.
        enter_result = self
        exit_result = False
        try:
            yield enter_result
        except SomeException as e:
            ... handle e ...
            exit_result = True
        finally:
            ... do tear down here ...
        yield exit_result

Function pop_cmgr(o, attr)

Run the __exit__ phase of a context manager commenced with push_cmgr. Restore attr as it was before push_cmgr. Return the result of __exit__.

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 hooks/signals/callbacks, 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 hooks/signals/callbacks, where you cannot inline a context manager.

Function push_cmgr(o, attr, cmgr)

A convenience wrapper for twostep(cmgr) to run the __enter__ phase of cmgr and save its value as o.attr. Return the result of the enter` phase.

The __exit__ phase is run by pop_cmgr(o,attr), returning the return value of the exit phase.

Example use in a unit test:

class TestThing(unittest.TestCase):
    def setUp(self):
        # save the temp dir path as self.dirpath
        push_cmgr(self, 'dirpath', TemporaryDirectory())
    def tearDown(self):
        # clean up the temporary directory, discard self.dirpath
        pop_cmgr(self, 'dirpath')

The cs.testutils SetupTeardownMixin class does this allowing the provision of a single setupTeardown() context manager method for test case setUp/tearDown.

Doc test:

>>> from os.path import isdir as isdirpath
>>> from tempfile import TemporaryDirectory
>>> from types import SimpleNamespace
>>> obj = SimpleNamespace()
>>> dirpath = push_cmgr(obj, 'path', TemporaryDirectory())
>>> assert dirpath == obj.path
>>> assert isdirpath(dirpath)
>>> pop_cmgr(obj, 'path')
>>> assert not hasattr(obj, 'path')
>>> assert not isdirpath(dirpath)

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 hooks/signals/callbacks, 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 hooks/signals/callbacks, where you cannot inline a context manager.

Function setup_cmgr(cmgr)

Run the set up phase of the context manager cmgr and return a callable which runs the tear down phase.

This is a convenience wrapper for the lower level twostep() function which produces a two iteration generator from a context manager.

Please see the push_cmgr function, a superior wrapper for twostep().

Note: this function expects cmgr to be an existing context manager. In particular, if you define some context manager function like this:

@contextmanager
def my_cmgr_func(...):
    ...
    yield
    ...

then the correct use of setup_cmgr() is:

teardown = setup_cmgr(my_cmgr_func(...))

and not:

cmgr_iter = setup_cmgr(my_cmgr_func)
...

The purpose of setup_cmgr() is to split any context manager's operation across two steps when the set up and teardown phases must operate in different parts of your code. A common situation is the __enter__ and __exit__ methods of another context manager class.

The call to setup_cmgr() performs the "enter" phase and returns the tear down callable. Calling that performs the tear down phase.

Example use in a class:

class SomeClass:
    def __init__(self, foo)
        self.foo = foo
        self._teardown = None
    def __enter__(self):
        self._teardown = setup_cmgr(stackattrs(o, setting=foo))
    def __exit__(self, *_):
        teardown, self._teardown = self._teardown, None
        teardown()

Class StackableState(_thread._local)

An object which can be called as a context manager to push changes to its attributes.

Example:

>>> state = StackableState(a=1, b=2)
>>> state.a
1
>>> state.b
2
>>> state
StackableState(a=1,b=2)
>>> with state(a=3, x=4):
...     print(state)
...     print("a", state.a)
...     print("b", state.b)
...     print("x", state.x)
...
StackableState(a=3,b=2,x=4)
a 3
b 2
x 4
>>> state.a
1
>>> state
StackableState(a=1,b=2)

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 returned mapping.

Restoration includes deleting attributes which were not present initially.

This makes it easy to adjust temporarily some shared context object without having to pass it through the call stack.

See stackkeys for a flavour of this for mappings.

See cs.context.StackableState for a convenient wrapper class.

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.

This makes it easy to adjust temporarily some shared context object without having to pass it through the call stack.

See stackattrs for a flavour of this for object attributes.

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': ...}

Function twostep(cmgr)

Return a generator which operates the context manager cmgr.

The first iteration performs the "enter" phase and yields the result. The second iteration performs the "exit" phase and yields None.

See also the push_cmgr(obj,attr,cmgr) function and its partner pop_cmgr(obj,attr) which form a convenient wrapper for this low level generator.

The purpose of twostep() is to split any context manager's operation across two steps when the set up and tear down phases must operate in different parts of your code. A common situation is the __enter__ and __exit__ methods of another context manager class or the setUp and tearDown methods of a unit test case.

Note: this function expects cmgr to be an existing context manager and not the function which returns the context manager.

In particular, if you define some function like this:

@contextmanager
def my_cmgr_func(...):
    ...
    yield
    ...

then the correct use of twostep() is:

cmgr_iter = twostep(my_cmgr_func(...))
next(cmgr_iter)   # set up
next(cmgr_iter)   # tear down

and not:

cmgr_iter = twostep(my_cmgr_func)
next(cmgr_iter)   # set up
next(cmgr_iter)   # tear down

Example use in a class (but really you should use push_cmgr/pop_cmgr instead):

class SomeClass:
    def __init__(self, foo)
        self.foo = foo
        self._cmgr_ = None
    def __enter__(self):
        self._cmgr_stepped = twostep(stackattrs(o, setting=foo))
        self._cmgr = next(self._cmgr_stepped)
        return self._cmgr
    def __exit__(self, *_):
        next(self._cmgr_stepped)
        self._cmgr = None

Release Log

Release 20221118: stackattrs: improve docstring.

Release 20220619: twostep: the returned "tear down" phase function needs to ignore StopIteration from the context manager, see PEP 479.

Release 20220227: New stack_signals context manager to push signal handlers.

Release 20211115.1: Docstring grammar/phrasing updates.

Release 20211115: Rename enter_exit to __enter_exit__ - the user doesn't call this overtly and it aligns better with __enter__ and __exit__.

Release 20211114.1: ContextManagerMixin: the default enter return is self, supporting a trivial bare yield in the generator.

Release 20211114: New ContextManagerMixin mixin class to implement the enter/exit methods using a simple generator function named enter_exit.

Release 20210727:

• twostep: iteration 1 now returns the result of enter, iteration 2 now returns None.
• New functions push_cmgr(obj,attr,cmgr) and partner pop_cmgr(obj,attr) to run a twostep()ed context manager conveniently, more conveniently than setup_cmgr().

Release 20210420.1: Rerelease after completing stalled merge: docstring updates.

Release 20210420: Docstring corrections and improvements.

Release 20210306:

• New twostep() and setup_cmgr() functions to split a context manager into set up and teardown phases for when these must occur in different parts of the code.
• New thread local StackableState class which can be called to push attribute changes with stackattrs - intended for use as shared global state to avoiod passing through deep function call chains.

Release 20200725.1: Docstring improvements.

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.