feature: Add support for the const keyword.

Fixes #229

python_jsonschema_objects/classbuilder.py

@@ -4,6 +4,7 @@
 import logging
 import sys
 
+import jsonschema.exceptions
 import referencing._core
 import six
 
@@ -184,11 +185,23 @@ def __init__(self, **props):
  # but only for the ones that have defaults set.
  for name in self.__has_default__:
  if name not in props:
- default_value = copy.deepcopy(self.__propinfo__[name]["default"])
+ # "defaults" could come from either the 'default' keyword or the 'const' keyword
+ try:
+ default_value = self.__propinfo__[name]["default"]
+ except KeyError:
+ try:
+ default_value = self.__propinfo__[name]["const"]
+ except KeyError:
+ raise jsonschema.exceptions.SchemaError(
+ "Schema parsing error. Expected {0} to have default or const value".format(
+ name
+ )
+ )
+
  logger.debug(
  util.lazy_format("Initializing '{0}' to '{1}'", name, default_value)
  )
- setattr(self, name, default_value)
+ setattr(self, name, copy.deepcopy(default_value))
 
  for prop in props:
  try:
@@ -626,7 +639,7 @@ def _build_literal(self, nm, clsdata):
  "__propinfo__": {
  "__literal__": clsdata,
  "__title__": clsdata.get("title"),
- "__default__": clsdata.get("default"),
+ "__default__": clsdata.get("default") or clsdata.get("const"),
  }
  },
  )
@@ -670,6 +683,17 @@ def _build_object(self, nm, clsdata, parents, **kw):
  )
  defaults.add(prop)
 
+ if "const" in detail:
+ logger.debug(
+ util.lazy_format(
+ "Setting const for {0}.{1} to: {2}",
+ nm,
+ prop,
+ detail["const"],
+ )
+ )
+ defaults.add(prop)
+
  if detail.get("type", None) == "object":
  uri = "{0}/{1}_{2}".format(nm, prop, "<anonymous>")
  self.resolved[uri] = self.construct(uri, detail, (ProtocolBase,), **kw)

python_jsonschema_objects/literals.py

@@ -44,6 +44,10 @@ def __init__(self, value, typ=None):
 
  self.validate()
 
+ constval = self.const()
+ if constval is not None:
+ self._value = constval
+
  def as_dict(self):
  return self.for_json()
 
@@ -54,6 +58,10 @@ def for_json(self):
  def default(cls):
  return cls.__propinfo__.get("__default__")
 
+ @classmethod
+ def const(cls):
+ return cls.__propinfo__.get("__literal__", {}).get("const", None)
+
  @classmethod
  def propinfo(cls, propname):
  if propname not in cls.__propinfo__:

python_jsonschema_objects/validators.py

@@ -58,6 +58,12 @@ def enum(param, value, _):
  raise ValidationError("{0} is not one of {1}".format(value, param))
 
 
+@registry.register()
+def const(param, value, _):
+ if value != param:
+ raise ValidationError("{0} is not constant {1}".format(value, param))
+
+
 @registry.register()
 def minimum(param, value, type_data):
  exclusive = type_data.get("exclusiveMinimum")

test/test_229.py

@@ -0,0 +1,51 @@
+import pytest
+
+import python_jsonschema_objects as pjo
+
+
+def test_const_properties():
+ schema = {
+ "title": "Example",
+ "type": "object",
+ "properties": {
+ "url": {
+ "type": "string",
+ "default": "https://example.com/your-username/my-project",
+ },
+ "type": {"type": "string", "const": "git"},
+ },
+ }
+
+ ob = pjo.ObjectBuilder(schema)
+ ns1 = ob.build_classes()
+ ex = ns1.Example()
+ ex.url = "can be anything"
+
+ # we expect the value to be set already for const values
+ assert ex.type == "git"
+ with pytest.raises(pjo.ValidationError):
+ # Trying to set the value to something else should throw validation errors
+ ex.type = "mercurial"
+
+ # setting the value to the const value is a no-op, but permitted
+ ex.type = "git"
+
+
+def test_const_bare_type():
+ schema = {
+ "title": "Example",
+ "type": "string",
+ "const": "I stand alone",
+ }
+
+ ob = pjo.ObjectBuilder(schema)
+ ns1 = ob.build_classes()
+ ex = ns1.Example("I stand alone")
+ # we expect the value to be set already for const values
+ assert ex == "I stand alone"
+ with pytest.raises(pjo.ValidationError):
+ # Trying to set the value to something else should throw validation errors
+ ex = ns1.Example("mercurial")
+
+ # setting the value to the const value is a no-op, but permitted
+ ex = ns1.Example("I stand alone")

test/test_default_values.py

@@ -53,15 +53,17 @@ def test_nullable_types_are_still_nullable(ns):
 
  thing1.p1 = None
  thing1.validate()
- assert thing1.as_dict() == {"p1": 0, "p2": None}
+ assert thing1.as_dict() == {"p1": None, "p2": None}
 
 
 def test_null_types_without_defaults_do_not_serialize(ns):
  thing1 = ns.DefaultTest()
 
+ assert thing1.as_dict() == {"p1": 0, "p2": None}
+
  thing1.p3 = 10
  thing1.validate()
  thing1.p1 = None
  thing1.validate()
 
- assert thing1.as_dict() == {"p1": 0, "p2": None, "p3": 10}
+ assert thing1.as_dict() == {"p1": None, "p2": None, "p3": 10}