cs-context 20250528
Assorted context managers.
Keywords: python2, python3

Assorted context managers.

*Latest release 20250528*:
* with_self: fix reference to the get_context_from_self parameter.
* with_all: BREAKING: receive the objects as *objs, not objs, easier to use and read.

Short summary:
* `closeall`: Enter all the objects from `objs` using `with` and return a function to close them all.
* `contextif`: A context manager to use `cmgr` conditionally, with a flexible call signature. This yields the context manager if `cmgr` is used or `None` if it is not used, allowing the enclosed code to test whether the context is active.
* `ContextManagerMixin`: A mixin to provide context manager `__enter__` and `__exit__` methods running the first and second steps of a single `__enter_exit__` generator method.
* `pop_cmgr`: 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__`.
* `popattrs`: The "pop" part of `stackattrs`. Restore previous attributes of `o` named by `attr_names` with previous state in `old_values`.
* `popkeys`: The "pop" part of `stackkeys`. Restore previous key values of `d` named by `key_names` with previous state in `old_values`.
* `push_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.
* `pushattrs`: The "push" part of `stackattrs`. Push `attr_values` onto `o` as attributes, return the previous attribute values in a `dict`.
* `pushkeys`: The "push" part of `stackkeys`. Use the mapping provided as `kv` or `kw` to update `d`. Return the previous key values in a `dict`.
* `reconfigure_file`: Context manager flavour of `TextIOBase.reconfigure`.
* `setup_cmgr`: Run the enter phase of the context manager `cmgr`. Return a `(yielded,teardwon)` 2-tuple where `yielded` is the value returned from the context manager's enter step and `callable` is a callable which runs the tear down phase.
* `stack_signals`: Context manager to apply a handler function to `signums` using `signal.signal`. The old handlers are restored on exit from the context manager.
* `stackattrs`: 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 returned mapping.
* `stackkeys`: A context manager to push new values for the key values of `d` and to restore them afterward. The new values are provided as `kv` or `kw` as convenient. Returns a `dict` containing a mapping of the previous key values. Keys not present are not present in the mapping.
* `stackset`: Context manager to add `element` to the set `s` and remove it on return. The element is neither added nor removed if it is already present.
* `twostep`: Return a generator which operates the context manager `cmgr`.
* `with_self`: A decorator to run a method inside `with self:` for classes which need to be "held open"/"marked as in use" while the method runs.
* `withall`: Enter every object `obj` in `objs` except those which are `None` using `with obj:`, then yield.
* `withif`: Return a context manager for `obj`. If `obj` has an `__enter__` attribute, return `obj` otherwise return `nullcontext()`.

Module contents:
- <a name="closeall"></a>`closeall(objs: Iterable) -> Callable`: Enter all the objects from `objs` using `with`
  and return a function to close them all.

  This is for situations where resources must be obtained now
  but released at a later time on completion of some operation,
  for example where we are dispatching a thread to work with
  the resources.
- <a name="contextif"></a>`contextif(cmgr, *cmgr_args, **cmgr_kwargs)`: A context manager to use `cmgr` conditionally,
  with a flexible call signature.
  This yields the context manager if `cmgr` is used or `None`
  if it is not used, allowing the enclosed code to test whether
  the context is active.

  This is to ease uses where the context object is optional,
  for example `None` if not present. Example from `cs.vt.stream`:

      @contextmanager
      def startup_shutdown(self):
        """ Open/close `self.local_store` if not `None`.
        """
        with super().startup_shutdown():
          with contextif(self.local_store):
            with self._packet_connection(self.recv, self.send) as conn:
              with stackattrs(self, _conn=conn):
                yield

  Here `self.local_store` might be `None` if there's no local
  store to present. We still want a nice nested `with` statement
  during the setup. By using `contextif` we run a context manager
  which behaves correctly when `self.local_store=None`.

  The signature is flexible, offering 2 basic modes of use.

  *Flagged use*: `contextif(flag,cmgr,*a,**kw)`: if `flag` is a
  Boolean (`bool`) then it governs whether the context manager `cmgr`
  is used. Historically the driving use case was verbosity
  dependent status lines or progress bars. Example:

      from cs.upd import run_task
      with contextif(verbose, run_task, ....) as proxy:
          ... do stuff, updating proxy if not None ...

  In the `cs.upd` example above, `run_task` is a context manager
  function which pops up an updatable status line, normally
  used as:

      with run_task("doing thing") as proxy:
          ... do the thing, setting proxy.text as needed ...

  *Unflagged use*: `contextif(cmgr,*a,**kw)`: use `cmgr` as the
  flag: if falsey (eg `None`) then `cmgr` is not used.

  Additionally, `cmgr` may be a callable, in which case the
  context manager itself is obtained by calling
  `cmgr(*cmgr_args,**cmgr_kwargs)`. Otherwise `cmgr` is assumed
  to be a context manager already, and it is an error to provide
  `cmgr_args` or `cmgr_kwargs`.

  This last mode can be a bit fiddly. If `cmgr` is a context
  manager _but is also callable for some other purposes_ you will
  need to do a little shuffle to avoid the implied call:

      with contexif(flag, lambda: cmgr):
          ... do stuff ...

  This provides a callable (the lambda) which returns the context
  manager itself.
- <a name="ContextManagerMixin"></a>`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_,
  it is a short generator method.
  Its first `yield` is the normal `yield` for a context manager.
  A second `yield` may be used to indicate that an exception was handled.

  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 `yield`s the value for the `with` statement.
  If `None` is `yield`ed (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_ a `@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

*`ContextManagerMixin.__enter__(self)`*:
Run `super().__enter__` (if any)
then the `__enter__` phase of `self.__enter_exit__()`.

*`ContextManagerMixin.__exit__(self, exc_type, exc_value, traceback)`*:
Run the `__exit__` step of `self.__enter_exit__()`,
then `super().__exit__` (if any).

*`ContextManagerMixin.as_contextmanager(self)`*:
Run the generator from the `cls` class specific `__enter_exit__`
method via `self` as a context manager.

Example from `RunState` which subclasses `HasThreadState`,
both of which are `ContextManagerMixin` subclasses:

    class RunState(HasThreadState):
        .....
        def __enter_exit__(self):
            with HasThreadState.as_contextmanager(self):
                ... RunState context manager stuff ...

This runs the `HasThreadState` context manager
around the main `RunState` context manager.
- <a name="pop_cmgr"></a>`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__`.
- <a name="popattrs"></a>`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.
- <a name="popkeys"></a>`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.
- <a name="push_cmgr"></a>`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)
- <a name="pushattrs"></a>`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.
- <a name="pushkeys"></a>`pushkeys(d, kv=None, **kw)`: The "push" part of `stackkeys`.
  Use the mapping provided as `kv` or `kw` to update `d`.
  Return the previous key values in a `dict`.

  This can be useful in hooks/signals/callbacks,
  where you cannot inline a context manager using `stackkeys`.
- <a name="reconfigure_file"></a>`reconfigure_file(f, **kw)`: Context manager flavour of `TextIOBase.reconfigure`.
- <a name="setup_cmgr"></a>`setup_cmgr(cmgr)`: Run the enter phase of the context manager `cmgr`.
  Return a `(yielded,teardwon)` 2-tuple where `yielded` is the
  value returned from the context manager's enter step and
  `callable` is 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:

      enter_value, teardown = setup_cmgr(my_cmgr_func(...))

  and _not_:

      enter_value, teardown = 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):
              the_context = stackattrs(o, setting=foo)
              enter_value, self._teardown = setup_cmgr(the_context)
              return enter_value
          def __exit__(self, *_):
              teardown, self._teardown = self._teardown, None
              teardown()
- <a name="stack_signals"></a>`stack_signals(signums, handler, additional=False)`: Context manager to apply a handler function to `signums`
  using `signal.signal`.
  The old handlers are restored on exit from the context manager.

  If the optional `additional` argument is true,
  apply a handler which calls both the new handler and the old handler.
- <a name="stackattrs"></a>`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 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.threads.ThreadState` 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'
- <a name="stackkeys"></a>`stackkeys(d, kv=None, **kw)`: A context manager to push new values for the key values of `d`
  and to restore them afterward.
  The new values are provided as `kv` or `kw` as convenient.
  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': ...}
- <a name="stackset"></a>`stackset(s, element, lock=None)`: Context manager to add `element` to the set `s` and remove it on return.
  The element is neither added nor removed if it is already present.
- <a name="twostep"></a>`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 `my_cmgr_func(...)` returns a context manager instance
  and so the correct use of `twostep()` is like this:

      # steps broken out for clarity
      cmgr = my_cmgr_func(...)
      cmgr_iter = twostep(cmgr)
      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
- <a name="with_self"></a>`with_self(*da, **dkw)`: A decorator to run a method inside `with self:` for classes
  which need to be "held open"/"marked as in use" while the
  method runs.

  The optional `get_context_from_self` parameter may be a
  callable to obtain the required context manager from `self`;
  the default is to just use `with self`.
- <a name="withall"></a>`withall(*objs)`: Enter every object `obj` in `objs` except those which are `None`
  using `with obj:`, then yield.

  Example:

      with withall(
          db1,
          db2,
      ):
          ... work with db1 and db2 ...
- <a name="withif"></a>`withif(obj)`: Return a context manager for `obj`.
  If `obj` has an `__enter__` attribute, return `obj`
  otherwise return `nullcontext()`.

  Example:

      with withif(inner_mapping):
        ... work with inner_mapping ...

# Release Log



*Release 20250528*:
* with_self: fix reference to the get_context_from_self parameter.
* with_all: BREAKING: receive the objects as *objs, not objs, easier to use and read.

*Release 20250412*:
push_cmgr: also pop the pop function attribute.

*Release 20250323*:
Docstring updates.

*Release 20250306*:
BREAKING: setup_cmgr: return an (enter_value,next2) 2-tuple instead of next2 so that users can return/yield the context manager enter value.

*Release 20240630*:
* New closeall(Iterable) wrapping withall() and returning the close step as a callable.
* contextif: document the lambda shuffle needed if cmgr is callable.
* New @with_self decorator for methods which should `with(self)` while the method runs (unused?)

*Release 20240412*:
* contextif: rework to be much easier to use, add new call modes.
* pushkeys, stackkeys: support update dicts whose keys are not identifier strings i.e. a non **kw call mode.
* New withif() function returning a context manager even for objects which do not provide one.
* New withall(iterable-of-context-managers) context manager.

*Release 20240316*:
Fixed release upload artifacts.

*Release 20240212.1*:
Minor doc updates.

*Release 20240212*:
New reconfigure_file(f,**kw), a context manager flavour of `TextIOBase.reconfigure`.

*Release 20240201*:
contextif: require the flag to be a bool.

*Release 20230331*:
stackset: accept optional lock to guard modification of the set.

*Release 20230212*:
* BREAKING: drop StackableState, superceded by cs.threads.State.
* New stackset(set,element) to push and then pop an element to a set unless it is already there.

*Release 20230125*:
New ContextManagerMixin.as_contextmanager(cls,self) class method to run the __enter_exit__ from a specific class, useful in subclasses.

*Release 20230109*:
New contextif(flag,cmgr_func,...) context manager to use cmgr_func if flag is true otherwise nullcontext.

*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.