Overhaul how date & time parsing works.

This commit breaks FHIRDate into four classes:
- FHIRDate
- FHIRDateTime
- FHIRInstant
- FHIRTime

BREAKING CHANGES:
- If consuming code were inspecting the elementProperties array and
  doing a check like `element_type is FHIRDate`, that will now fail
  for `datetime`, `instant`, and `time` elements. Backwards
  compatibility is however maintained for checks written like
  `issubclass(element_type, FHIRDate)`.
  Instances have a similar resolution: `isinstance(obj, FHIRDate)` will
  still work.
- If consuming code were manually creating FHIRDate objects
  themselves with a time component, that will now fail with a
  ValueError.

Since the first item is unavoidable if we want to fix the bugs listed
below and has a workaround that works before and after this change,
and the second item is not an expected workflow, I hope that such
breaking changes do not cause too much harm for consumers.

BUG FIXES:
- FHIR `time` fields are now correctly parsed. Previously, a time of
  "10:12:14" would result in a **date** of "1001-01-01"
- Passing too much detail to a `date` field or too little detail to an
  `instant` field will now correctly throw a validation error.
  For example, a Patient.birthDate field with a time. Or an
  Observation.issued field with just a year.
- Sub-seconds would be incorrectly chopped off of a `datetime`'s
  `.isostring` (which the FHIR spec allows us to do) and an `instant`'s
  `.isostring` (which the FHIR spec **does not** allow us to do).
  The `.date` Python representation and the `.as_json()` call would
  both work correctly and keep the sub-seconds. Only `.isostring` was
  affected.

IMPROVEMENTS:
- Leap seconds are now half-supported. The FHIR spec says clients
  "SHOULD accept and handle leap seconds gracefully", which we do...
  By dropping the leap second on the floor and rolling back to :59.
  But this is an improvement on previous behavior of a validation
  error. The `.as_json()` value will still preserve the leap second.
- The `.date` field is now always the appropriate type (datetime.date
  for FHIRDate, datetime.datetime for FHIRDateTime and FHIRInstant,
  and datetime.time for FHIRTime). Previously, a `datetime` field might
  result in a datetime.date if only given a date portion. (Which isn't
  entirely wrong, but consistently providing the same data type is
  useful.)
- The new classes have appropriately named fields in addition to the
  backwards-compatible `.date` field -- FHIRDateTime.datetime,
  FHIRInstant.datetime, and FHIRTime.time. These will always be the
  same value as `.date` for now - but in a future major release, the
  `.date` alias may be dropped.
- The dependency on isodate can now be dropped. It is lightly
  maintained and the stdlib can handle most of its job nowadays.
- Much better class documentation for what sort of things are
  supported and which are not.

MAINTAINERS.md

@@ -40,3 +40,7 @@ Using flit (*Note*: Alternatively, you can use [twine](https://twine.readthedocs
 
     pip install -U flit
     flit publish
+
+### Announce the release
+
+Make a post in the [Zulip channel](https://chat.fhir.org/#narrow/stream/179218-python) for python.

fhir-parser-resources/fhirabstractresource.py

@@ -211,6 +211,5 @@ def where(cls, struct):
         return fhirsearch.FHIRSearch(cls, struct)
 
 
-from . import fhirdate
 from . import fhirsearch
 from . import fhirelementfactory

fhir-parser-resources/fhirdate.py

@@ -1,81 +1,157 @@
-#!/usr/bin/env python
-# -*- coding: utf-8 -*-
-#
-#  Facilitate working with dates.
-#  2014, SMART Health IT.
-
-import sys
-import logging
-import isodate
+"""Facilitate working with FHIR dates and times."""
+# 2014-2024, SMART Health IT.
+
 import datetime
+import re
+from typing import Any, Union
+
+
+class FHIRDate:
+    """
+    A convenience class for working with FHIR dates in Python.
+
+    http://hl7.org/fhir/R4/datatypes.html#date
 
-logger = logging.getLogger(__name__)
+    Converting to a Python representation does require some compromises:
+    - This class will convert partial dates ("reduced precision dates") like "2024" into full
+      dates using the earliest possible time (in this example, "2024-01-01") because Python's
+      date class does not support partial dates.
 
+    If such compromise is not useful for you, avoid using the `date` or `isostring`
+    properties and just use the `as_json()` method in order to work with the original,
+    exact string.
 
-class FHIRDate(object):
-    """ Facilitate working with dates.
-    
-    - `date`: datetime object representing the receiver's date-time
+    For backwards-compatibility reasons, this class is the parent class of FHIRDateTime,
+    FHIRInstant, and FHIRTime. But they are all separate concepts and in a future major release,
+    they should be split into entirely separate classes.
+
+    Public properties:
+    - `date`: datetime.date representing the JSON value
+    - `isostring`: an ISO 8601 string version of the above Python object
+
+    Public methods:
+    - `as_json`: returns the original JSON used to construct the instance
     """
-
-    def __init__(self, jsonval=None):
-        self.date = None
+
+    def __init__(self, jsonval: Union[str, None] = None):
+        self.date: Union[datetime.date, datetime.datetime, datetime.time, None] = None
+
         if jsonval is not None:
-            isstr = isinstance(jsonval, str)
-            if not isstr and sys.version_info[0] < 3:       # Python 2.x has 'str' and 'unicode'
-                isstr = isinstance(jsonval, basestring)
-            if not isstr:
+            if not isinstance(jsonval, str):
                 raise TypeError("Expecting string when initializing {}, but got {}"
                     .format(type(self), type(jsonval)))
-            try:
-                if 'T' in jsonval:
-                    self.date = isodate.parse_datetime(jsonval)
-                else:
-                    self.date = isodate.parse_date(jsonval)
-            except Exception as e:
-                logger.warning("Failed to initialize FHIRDate from \"{}\": {}"
-                    .format(jsonval, e))
-
-        self.origval = jsonval
-
+            if not self._REGEX.fullmatch(jsonval):
+                raise ValueError("does not match expected format")
+            self.date = self._from_string(jsonval)
+
+        self.origval: Union[str, None] = jsonval
+
     def __setattr__(self, prop, value):
-        if 'date' == prop:
+        if prop in {'date', self._FIELD}:
             self.origval = None
-        object.__setattr__(self, prop, value)
-
+            # Keep these two fields in sync
+            object.__setattr__(self, self._FIELD, value)
+            object.__setattr__(self, "date", value)
+        else:
+            object.__setattr__(self, prop, value)
+
     @property
-    def isostring(self):
+    def isostring(self) -> Union[str, None]:
+        """
+        Returns a standardized ISO 8601 version of the Python representation of the FHIR JSON.
+
+        Note that this may not be a fully accurate version of the input JSON.
+        In particular, it will convert partial dates like "2024" to full dates like "2024-01-01".
+        It will also normalize the timezone, if present.
+        """
         if self.date is None:
             return None
-        if isinstance(self.date, datetime.datetime):
-            return isodate.datetime_isoformat(self.date)
-        return isodate.date_isoformat(self.date)
-
+        return self.date.isoformat()
+
     @classmethod
-    def with_json(cls, jsonobj):
+    def with_json(cls, jsonobj: Union[str, list]):
         """ Initialize a date from an ISO date string.
         """
-        isstr = isinstance(jsonobj, str)
-        if not isstr and sys.version_info[0] < 3:       # Python 2.x has 'str' and 'unicode'
-            isstr = isinstance(jsonobj, basestring)
-        if isstr:
+        if isinstance(jsonobj, str):
             return cls(jsonobj)
-        
+
         if isinstance(jsonobj, list):
             return [cls(jsonval) for jsonval in jsonobj]
-        
+
         raise TypeError("`cls.with_json()` only takes string or list of strings, but you provided {}"
             .format(type(jsonobj)))
-    
+
     @classmethod
-    def with_json_and_owner(cls, jsonobj, owner):
+    def with_json_and_owner(cls, jsonobj: Union[str, list], owner):
         """ Added for compatibility reasons to FHIRElement; "owner" is
         discarded.
         """
         return cls.with_json(jsonobj)
-
-    def as_json(self):
+
+    def as_json(self) -> Union[str, None]:
+        """Returns the original JSON string used to create this instance."""
         if self.origval is not None:
             return self.origval
         return self.isostring
-
+
+    ##################################
+    # Private properties and methods #
+    ##################################
+
+    # Pulled from spec for date
+    _REGEX = re.compile(r"([0-9]([0-9]([0-9][1-9]|[1-9]0)|[1-9]00)|[1-9]000)(-(0[1-9]|1[0-2])(-(0[1-9]|[1-2][0-9]|3[0-1]))?)?")
+    _FIELD = "date"
+
+    @staticmethod
+    def _parse_partial(value: str, cls):
+        """
+        Handle partial dates like 1970 or 1980-12.
+
+        FHIR allows them, but Python's datetime classes do not natively parse them.
+        """
+        # Note that `value` has already been regex-certified by this point,
+        # so we don't have to handle really wild strings.
+        if len(value) < 10:
+            pieces = value.split("-")
+            if len(pieces) == 1:
+                return cls(int(pieces[0]), 1, 1)
+            else:
+                return cls(int(pieces[0]), int(pieces[1]), 1)
+        return cls.fromisoformat(value)
+
+    @staticmethod
+    def _parse_date(value: str) -> datetime.date:
+        return FHIRDate._parse_partial(value, datetime.date)
+
+    @staticmethod
+    def _parse_datetime(value: str) -> datetime.datetime:
+        # Until we depend on Python 3.11+, manually handle Z
+        value = value.replace("Z", "+00:00")
+        value = FHIRDate._strip_leap_seconds(value)
+        return FHIRDate._parse_partial(value, datetime.datetime)
+
+    @staticmethod
+    def _parse_time(value: str) -> datetime.time:
+        value = FHIRDate._strip_leap_seconds(value)
+        return datetime.time.fromisoformat(value)
+
+    @staticmethod
+    def _strip_leap_seconds(value: str) -> str:
+        """
+        Manually ignore leap seconds by clamping the seconds value to 59.
+
+        Python native times don't support them (at the time of this writing, but also watch
+        https://bugs.python.org/issue23574). For example, the stdlib's datetime.fromtimestamp()
+        also clamps to 59 if the system gives it leap seconds.
+
+        But FHIR allows leap seconds and says receiving code SHOULD accept them,
+        so we should be graceful enough to at least not throw a ValueError,
+        even though we can't natively represent the most-correct time.
+        """
+        # We can get away with such relaxed replacement because we are already regex-certified
+        # and ":60" can't show up anywhere but seconds.
+        return value.replace(":60", ":59")
+
+    @staticmethod
+    def _from_string(value: str) -> Any:
+        return FHIRDate._parse_date(value)

fhir-parser-resources/fhirdatetime.py

@@ -0,0 +1,57 @@
+"""Facilitate working with FHIR time fields."""
+# 2024, SMART Health IT.
+
+import datetime
+import re
+from typing import Any, Union
+
+from .fhirdate import FHIRDate
+
+
+# This inherits from FHIRDate as a matter of backwards compatibility.
+# (in case anyone was doing isinstance(obj, FHIRDate))
+# Next time we bump the major version, we can stop that and also drop the
+# backwards-compatible 'date' alias. R4-QUIRK
+
+class FHIRDateTime(FHIRDate):
+    """
+    A convenience class for working with FHIR datetimes in Python.
+
+    http://hl7.org/fhir/R4/datatypes.html#datetime
+
+    Converting to a Python representation does require some compromises:
+    - This class will convert partial dates ("reduced precision dates") like "2024" into full
+      naive datetimes using the earliest possible time (in this example, "2024-01-01T00:00:00")
+      because Python's datetime class does not support partial dates.
+    - FHIR allows arbitrary sub-second precision, but Python only holds microseconds.
+    - Leap seconds (:60) will be changed to the 59th second (:59) because Python's time classes
+      do not support leap seconds.
+
+    If such compromise is not useful for you, avoid using the `date`, `datetime`, or `isostring`
+    properties and just use the `as_json()` method in order to work with the original,
+    exact string.
+
+    Public properties:
+    - `datetime`: datetime.datetime representing the JSON value (naive or aware)
+    - `date`: backwards-compatibility alias for `datetime`
+    - `isostring`: an ISO 8601 string version of the above Python object
+
+    Public methods:
+    - `as_json`: returns the original JSON used to construct the instance
+    """
+
+    def __init__(self, jsonval: Union[str, None] = None):
+        self.datetime: Union[datetime.datetime, None] = None
+        super().__init__(jsonval)
+
+    ##################################
+    # Private properties and methods #
+    ##################################
+
+    # Pulled from spec for datetime
+    _REGEX = re.compile(r"([0-9]([0-9]([0-9][1-9]|[1-9]0)|[1-9]00)|[1-9]000)(-(0[1-9]|1[0-2])(-(0[1-9]|[1-2][0-9]|3[0-1])(T([01][0-9]|2[0-3]):[0-5][0-9]:([0-5][0-9]|60)(\.[0-9]+)?(Z|(\+|-)((0[0-9]|1[0-3]):[0-5][0-9]|14:00)))?)?)?")
+    _FIELD = "datetime"
+
+    @staticmethod
+    def _from_string(value: str) -> Any:
+        return FHIRDate._parse_datetime(value)

fhir-parser-resources/fhirinstant.py

@@ -0,0 +1,54 @@
+"""Facilitate working with FHIR time fields."""
+# 2024, SMART Health IT.
+
+import datetime
+import re
+from typing import Any, Union
+
+from .fhirdate import FHIRDate
+
+
+# This inherits from FHIRDate as a matter of backwards compatibility.
+# (in case anyone was doing isinstance(obj, FHIRDate))
+# Next time we bump the major version, we can stop that and also drop the
+# backwards-compatible 'date' alias. R4-QUIRK
+
+class FHIRInstant(FHIRDate):
+    """
+    A convenience class for working with FHIR instants in Python.
+
+    http://hl7.org/fhir/R4/datatypes.html#instant
+
+    Converting to a Python representation does require some compromises:
+    - FHIR allows arbitrary sub-second precision, but Python only holds microseconds.
+    - Leap seconds (:60) will be changed to the 59th second (:59) because Python's time classes
+      do not support leap seconds.
+
+    If such compromise is not useful for you, avoid using the `date`, `datetime`, or `isostring`
+    properties and just use the `as_json()` method in order to work with the original,
+    exact string.
+
+    Public properties:
+    - `datetime`: datetime.datetime representing the JSON value (aware only)
+    - `date`: backwards-compatibility alias for `datetime`
+    - `isostring`: an ISO 8601 string version of the above Python object
+
+    Public methods:
+    - `as_json`: returns the original JSON used to construct the instance
+    """
+
+    def __init__(self, jsonval: Union[str, None] = None):
+        self.datetime: Union[datetime.datetime, None] = None
+        super().__init__(jsonval)
+
+    ##################################
+    # Private properties and methods #
+    ##################################
+
+    # Pulled from spec for instant
+    _REGEX = re.compile(r"([0-9]([0-9]([0-9][1-9]|[1-9]0)|[1-9]00)|[1-9]000)-(0[1-9]|1[0-2])-(0[1-9]|[1-2][0-9]|3[0-1])T([01][0-9]|2[0-3]):[0-5][0-9]:([0-5][0-9]|60)(\.[0-9]+)?(Z|(\+|-)((0[0-9]|1[0-3]):[0-5][0-9]|14:00))")
+    _FIELD = "datetime"
+
+    @staticmethod
+    def _from_string(value: str) -> Any:
+        return FHIRDate._parse_datetime(value)

fhir-parser-resources/fhirtime.py

@@ -0,0 +1,54 @@
+"""Facilitate working with FHIR time fields."""
+# 2024, SMART Health IT.
+
+import datetime
+import re
+from typing import Any, Union
+
+from .fhirdate import FHIRDate
+
+
+# This inherits from FHIRDate as a matter of backwards compatibility.
+# (in case anyone was doing isinstance(obj, FHIRDate))
+# Next time we bump the major version, we can stop that and also drop the
+# backwards-compatible 'date' alias. R4-QUIRK
+
+class FHIRTime(FHIRDate):
+    """
+    A convenience class for working with FHIR times in Python.
+
+    http://hl7.org/fhir/R4/datatypes.html#time
+
+    Converting to a Python representation does require some compromises:
+    - FHIR allows arbitrary sub-second precision, but Python only holds microseconds.
+    - Leap seconds (:60) will be changed to the 59th second (:59) because Python's time classes
+      do not support leap seconds.
+
+    If such compromise is not useful for you, avoid using the `date`, `time`, or `isostring`
+    properties and just use the `as_json()` method in order to work with the original,
+    exact string.
+
+    Public properties:
+    - `time`: datetime.time representing the JSON value
+    - `date`: backwards-compatibility alias for `time`
+    - `isostring`: an ISO 8601 string version of the above Python object
+
+    Public methods:
+    - `as_json`: returns the original JSON used to construct the instance
+    """
+
+    def __init__(self, jsonval: Union[str, None] = None):
+        self.time: Union[datetime.time, None] = None
+        super().__init__(jsonval)
+
+    ##################################
+    # Private properties and methods #
+    ##################################
+
+    # Pulled from spec for time
+    _REGEX = re.compile(r"([01][0-9]|2[0-3]):[0-5][0-9]:([0-5][0-9]|60)(\.[0-9]+)?")
+    _FIELD = "time"
+
+    @staticmethod
+    def _from_string(value: str) -> Any:
+        return FHIRDate._parse_time(value)