ENH: Add cron scheduler

docs/code/conds/api/cron.py

@@ -0,0 +1,10 @@
+from rocketry.conds import cron
+
+@app.task(cron('* * * * *'))
+def do_minutely():
+    ...
+
+@app.task(cron('*/2 12-18 * Oct Fri'))
+def do_complex():
+    "Run at every 2nd minute past every hour from 12 through 18 on Friday in October."
+    ...

docs/code/conds/api/cron_kwargs.py

@@ -0,0 +1,16 @@
+from rocketry.conds import cron
+
+@app.task(cron(minute="*/5"))
+def do_simple():
+    "Run at every 5th minute"
+    ...
+
+@app.task(cron(minute="*/2", hour="7-18", day_of_month="1,2,3", month="Feb-Aug/2"))
+def do_complex():
+    """Run at: 
+        - Every second minute
+        - Between 07:00 (7 a.m.) - 18:00 (6 p.m.)
+        - On 1st, 2nd and 3rd day of month
+        - From February to August every second month
+    """
+    ...

docs/handbooks/conditions/api/cron.rst

@@ -0,0 +1,28 @@
+
+.. _cron:
+
+Cron Scheduling
+===============
+
+Rocketry also natively supports `cron-like scheduling <https://en.wikipedia.org/wiki/Cron>`_.
+
+You can input a cron statement as a string:
+
+Examples
+--------
+
+.. literalinclude:: /code/conds/api/cron.py
+    :language: py
+
+Or you can use named arguments:
+
+.. literalinclude:: /code/conds/api/cron_kwargs.py
+    :language: py
+
+See more from the official definition of cron.
+
+.. note::
+
+    Unlike most of the condition, the cron condition checks whether the task 
+    has run on the period (as standard with cron schedulers) and not whether 
+    the task has finished on the given period. 

docs/handbooks/conditions/api/index.rst

@@ -21,6 +21,7 @@ Here are some examples:
 
    logical
    periodical
+   cron
    pipeline
    task_status
    scheduler

docs/index.rst

@@ -49,7 +49,7 @@ applications. It is simple, clean and extensive.
 **Core functionalities:**
 
 - Powerful scheduling syntax
-- A lot of built-in scheduling options
+- A lot of built-in scheduling options (including :ref:`cron <cron>`)
 - Task parallelization
 - Task parametrization
 - Task pipelining

docs/versions.rst

@@ -2,6 +2,15 @@
 Version history
 ===============
 
+- ``2.3.0``
+
+    - Add: Cron style scheduling
+    - Add: New condition, ``TaskRunnable``
+    - Add: ``always`` time period
+    - Fix: Various bugs related to ``Any``, ``All`` and ``StaticInterval`` time periods
+    - Fix: Integers as start and end in time periods
+    - Upd: Now time periods are immutable 
+
 - ``2.2.0``
 
     - Add: Async support

rocketry/conditions/api.py

@@ -1,6 +1,6 @@
 from typing import Callable, Union
 from rocketry.conditions.scheduler import SchedulerStarted
-from rocketry.conditions.task.task import DependFailure, DependFinish, DependSuccess, TaskFailed, TaskFinished, TaskStarted, TaskSucceeded
+from rocketry.conditions.task.task import DependFailure, DependFinish, DependSuccess, TaskFailed, TaskFinished, TaskRunnable, TaskStarted, TaskSucceeded
 from rocketry.core import (
     BaseCondition
 )
@@ -9,6 +9,7 @@
 )
 from rocketry.core.condition.base import All, Any, Not
 from rocketry.core.task import Task
+from rocketry.time import Cron
 from .time import IsPeriod
 from .task import TaskExecutable, TaskRunning
 from rocketry.time import (
@@ -137,6 +138,15 @@ def every(past:str, based="run"):
     else:
         raise ValueError(f"Invalid status: {based}")
 
+def cron(__expr=None, **kwargs):
+    if __expr:
+        args = __expr.split(" ")
+    else:
+        args = ()
+
+    period = Cron(*args, **kwargs)
+    return TaskRunnable(period=period)
+
 # Task pipelining
 # ---------------
 
@@ -188,4 +198,4 @@ def running(more_than:str=None, less_than=None, task=None):
 # ---------
 
 def scheduler_running(more_than:str=None, less_than=None):
-    return SchedulerStarted(period=TimeSpanDelta(near=more_than, far=less_than))
+    return SchedulerStarted(period=TimeSpanDelta(near=more_than, far=less_than))

rocketry/conditions/task/task.py

@@ -257,6 +257,7 @@ def get_state(self, task=Task(), session=Session()):
         isin_period = (
             # TimeDelta has no __contains__. One cannot say whether now is "past 2 hours".
             #   And please tell why this does not raise an exception then? - Future me
+            #   Because the period is used in the sub statements and TimeDelta is still accepted - Senior me
             True  
             if isinstance(period, TimeDelta) 
             else IsPeriod(period=period).observe()
@@ -292,6 +293,36 @@ def _from_period(cls, span_type=None, **kwargs):
         return cls(period=period)
 
 
+class TaskRunnable(BaseCondition):
+    """Condition for checking whether a given
+    task has not run (for given period).
+    Useful to set the given task to run once 
+    in given period.
+    """
+
+    def __init__(self, task=None, period=None):
+        self.period = period
+        self.task = task
+        super().__init__()
+
+    def get_state(self, task=Task(), session=Session()):
+        task = self.task if self.task is not None else task
+        period = self.period
+
+        has_not_run = TaskStarted(period=period, task=task) == 0
+
+        isin_period = (
+            True  
+            if isinstance(period, TimeDelta) 
+            else IsPeriod(period=period).observe()
+        )
+
+        return (
+            isin_period
+            and has_not_run.observe(task=task, session=session)
+        )
+
+
 class DependFinish(DependMixin):
     """Condition for checking whether a given
     task has not finished after running a dependent 

rocketry/conds/__init__.py

@@ -13,5 +13,6 @@
     started, succeeded, failed, finished,
 
     scheduler_running,
-    running
+    running,
+    cron
 )

rocketry/core/time/__init__.py

@@ -5,7 +5,10 @@
     StaticInterval,
     All, Any,
 
-    PARSERS
+    PARSERS,
+
+    # Constants
+    always,
 )