diff --git a/stix2/test/v20/test_attack_pattern.py b/stix2/test/v20/test_attack_pattern.py
index cc24fa9..1f53450 100644
--- a/stix2/test/v20/test_attack_pattern.py
+++ b/stix2/test/v20/test_attack_pattern.py
@@ -37,7 +37,7 @@ def test_attack_pattern_example():
         description="...",
     )
 
-    assert str(ap) == EXPECTED
+    assert ap.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
@@ -111,6 +111,6 @@ def test_less_precise_timestamps():
         description="...",
     )
 
-    assert str(ap) == EXPECTED
+    assert ap.serialize(pretty=True) == EXPECTED
 
 # TODO: Add other examples
diff --git a/stix2/test/v20/test_bundle.py b/stix2/test/v20/test_bundle.py
index f53d0cb..3e09192 100644
--- a/stix2/test/v20/test_bundle.py
+++ b/stix2/test/v20/test_bundle.py
@@ -116,7 +116,7 @@ def test_bundle_id_must_start_with_bundle():
 def test_create_bundle1(indicator, malware, relationship):
     bundle = stix2.v20.Bundle(objects=[indicator, malware, relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
     assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
@@ -129,31 +129,31 @@ def test_create_bundle2(indicator, malware, relationship):
 def test_create_bundle_with_positional_args(indicator, malware, relationship):
     bundle = stix2.v20.Bundle(indicator, malware, relationship)
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_positional_listarg(indicator, malware, relationship):
     bundle = stix2.v20.Bundle([indicator, malware, relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_listarg_and_positional_arg(indicator, malware, relationship):
     bundle = stix2.v20.Bundle([indicator, malware], relationship)
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_listarg_and_kwarg(indicator, malware, relationship):
     bundle = stix2.v20.Bundle([indicator, malware], objects=[relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_arg_listarg_and_kwarg(indicator, malware, relationship):
     bundle = stix2.v20.Bundle([indicator], malware, objects=[relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_invalid(indicator, malware, relationship):
diff --git a/stix2/test/v20/test_campaign.py b/stix2/test/v20/test_campaign.py
index 0d4a202..13b47b0 100644
--- a/stix2/test/v20/test_campaign.py
+++ b/stix2/test/v20/test_campaign.py
@@ -21,7 +21,7 @@ EXPECTED = """{
 def test_campaign_example():
     campaign = stix2.v20.Campaign(**CAMPAIGN_MORE_KWARGS)
 
-    assert str(campaign) == EXPECTED
+    assert campaign.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v20/test_course_of_action.py b/stix2/test/v20/test_course_of_action.py
index f648907..c778316 100644
--- a/stix2/test/v20/test_course_of_action.py
+++ b/stix2/test/v20/test_course_of_action.py
@@ -28,7 +28,7 @@ def test_course_of_action_example():
         description="This is how to add a filter rule to block inbound access to TCP port 80 to the existing UDP 1434 filter ...",
     )
 
-    assert str(coa) == EXPECTED
+    assert coa.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v20/test_custom.py b/stix2/test/v20/test_custom.py
index f5e7024..b5d86b5 100644
--- a/stix2/test/v20/test_custom.py
+++ b/stix2/test/v20/test_custom.py
@@ -778,7 +778,7 @@ def test_custom_extension_with_list_and_dict_properties_observable_type(data):
         pass
 
     example = SomeCustomExtension(keys=[{'test123': 123, 'test345': 'aaaa'}])
-    assert data == str(example)
+    assert data == example.serialize(pretty=True)
 
 
 def test_custom_extension_invalid_type_name():
@@ -992,7 +992,7 @@ def test_custom_object_nested_dictionary(data):
         dictionary={'key': {'key_b': 'value', 'key_a': 'value'}},
     )
 
-    assert data == str(example)
+    assert data == example.serialize(pretty=True)
 
 
 @stix2.v20.CustomObject(
diff --git a/stix2/test/v20/test_external_reference.py b/stix2/test/v20/test_external_reference.py
index 07cf42d..04821cd 100644
--- a/stix2/test/v20/test_external_reference.py
+++ b/stix2/test/v20/test_external_reference.py
@@ -26,7 +26,7 @@ def test_external_reference_veris():
         url="https://github.com/vz-risk/VCDB/blob/master/data/json/0001AA7F-C601-424A-B2B8-BE6C9F5164E7.json",
     )
 
-    assert str(ref) == VERIS
+    assert ref.serialize(pretty=True) == VERIS
 
 
 CAPEC = """{
@@ -41,7 +41,7 @@ def test_external_reference_capec():
         external_id="CAPEC-550",
     )
 
-    assert str(ref) == CAPEC
+    assert ref.serialize(pretty=True) == CAPEC
     assert re.match("ExternalReference\\(source_name=u?'capec', external_id=u?'CAPEC-550'\\)", repr(ref))
 
 
@@ -59,7 +59,7 @@ def test_external_reference_capec_url():
         url="http://capec.mitre.org/data/definitions/550.html",
     )
 
-    assert str(ref) == CAPEC_URL
+    assert ref.serialize(pretty=True) == CAPEC_URL
 
 
 THREAT_REPORT = """{
@@ -76,7 +76,7 @@ def test_external_reference_threat_report():
         url="http://www.example.com/threat-report.pdf",
     )
 
-    assert str(ref) == THREAT_REPORT
+    assert ref.serialize(pretty=True) == THREAT_REPORT
 
 
 BUGZILLA = """{
@@ -93,7 +93,7 @@ def test_external_reference_bugzilla():
         url="https://www.example.com/bugs/1370",
     )
 
-    assert str(ref) == BUGZILLA
+    assert ref.serialize(pretty=True) == BUGZILLA
 
 
 OFFLINE = """{
@@ -108,7 +108,7 @@ def test_external_reference_offline():
         description="Threat report",
     )
 
-    assert str(ref) == OFFLINE
+    assert ref.serialize(pretty=True) == OFFLINE
     assert re.match("ExternalReference\\(source_name=u?'ACME Threat Intel', description=u?'Threat report'\\)", repr(ref))
     # Yikes! This works
     assert eval("stix2." + repr(ref)) == ref
diff --git a/stix2/test/v20/test_identity.py b/stix2/test/v20/test_identity.py
index 750c6f2..c62da46 100644
--- a/stix2/test/v20/test_identity.py
+++ b/stix2/test/v20/test_identity.py
@@ -26,7 +26,7 @@ def test_identity_example():
         identity_class="individual",
     )
 
-    assert str(identity) == EXPECTED
+    assert identity.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v20/test_indicator.py b/stix2/test/v20/test_indicator.py
index 6f3bfea..47f4812 100644
--- a/stix2/test/v20/test_indicator.py
+++ b/stix2/test/v20/test_indicator.py
@@ -48,7 +48,7 @@ def test_indicator_with_all_required_properties():
     )
 
     assert ind.revoked is False
-    assert str(ind) == EXPECTED_INDICATOR
+    assert ind.serialize(pretty=True) == EXPECTED_INDICATOR
     rep = re.sub(r"(\[|=| )u('|\"|\\\'|\\\")", r"\g<1>\g<2>", repr(ind))
     assert rep == EXPECTED_INDICATOR_REPR
 
diff --git a/stix2/test/v20/test_intrusion_set.py b/stix2/test/v20/test_intrusion_set.py
index 69b336e..59a9905 100644
--- a/stix2/test/v20/test_intrusion_set.py
+++ b/stix2/test/v20/test_intrusion_set.py
@@ -38,7 +38,7 @@ def test_intrusion_set_example():
         goals=["acquisition-theft", "harassment", "damage"],
     )
 
-    assert str(intrusion_set) == EXPECTED
+    assert intrusion_set.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v20/test_kill_chain_phases.py b/stix2/test/v20/test_kill_chain_phases.py
index d150757..7f65f00 100644
--- a/stix2/test/v20/test_kill_chain_phases.py
+++ b/stix2/test/v20/test_kill_chain_phases.py
@@ -16,7 +16,7 @@ def test_lockheed_martin_cyber_kill_chain():
         phase_name="reconnaissance",
     )
 
-    assert str(recon) == LMCO_RECON
+    assert recon.serialize(pretty=True) == LMCO_RECON
 
 
 FOO_PRE_ATTACK = """{
@@ -31,7 +31,7 @@ def test_kill_chain_example():
         phase_name="pre-attack",
     )
 
-    assert str(preattack) == FOO_PRE_ATTACK
+    assert preattack.serialize(pretty=True) == FOO_PRE_ATTACK
 
 
 def test_kill_chain_required_properties():
diff --git a/stix2/test/v20/test_malware.py b/stix2/test/v20/test_malware.py
index bd49007..974caf0 100644
--- a/stix2/test/v20/test_malware.py
+++ b/stix2/test/v20/test_malware.py
@@ -33,7 +33,7 @@ def test_malware_with_all_required_properties():
         name="Cryptolocker",
     )
 
-    assert str(mal) == EXPECTED_MALWARE
+    assert mal.serialize(pretty=True) == EXPECTED_MALWARE
 
 
 def test_malware_with_empty_optional_field():
@@ -49,7 +49,7 @@ def test_malware_with_empty_optional_field():
         external_references=[],
     )
 
-    assert str(mal) == EXPECTED_MALWARE
+    assert mal.serialize(pretty=True) == EXPECTED_MALWARE
 
 
 def test_malware_autogenerated_properties(malware):
diff --git a/stix2/test/v20/test_markings.py b/stix2/test/v20/test_markings.py
index 0753e86..f7d15fa 100644
--- a/stix2/test/v20/test_markings.py
+++ b/stix2/test/v20/test_markings.py
@@ -71,7 +71,7 @@ EXPECTED_CAMPAIGN_WITH_GRANULAR_MARKINGS = """{
 
 
 def test_marking_def_example_with_tlp():
-    assert str(TLP_WHITE) == EXPECTED_TLP_MARKING_DEFINITION
+    assert TLP_WHITE.serialize(pretty=True) == EXPECTED_TLP_MARKING_DEFINITION
 
 
 def test_marking_def_example_with_statement_positional_argument():
@@ -82,7 +82,7 @@ def test_marking_def_example_with_statement_positional_argument():
         definition=stix2.v20.StatementMarking(statement="Copyright 2016, Example Corp"),
     )
 
-    assert str(marking_definition) == EXPECTED_STATEMENT_MARKING_DEFINITION
+    assert marking_definition.serialize(pretty=True) == EXPECTED_STATEMENT_MARKING_DEFINITION
 
 
 def test_marking_def_example_with_kwargs_statement():
@@ -94,7 +94,7 @@ def test_marking_def_example_with_kwargs_statement():
         definition=stix2.v20.StatementMarking(**kwargs),
     )
 
-    assert str(marking_definition) == EXPECTED_STATEMENT_MARKING_DEFINITION
+    assert marking_definition.serialize(pretty=True) == EXPECTED_STATEMENT_MARKING_DEFINITION
 
 
 def test_marking_def_invalid_type():
@@ -118,7 +118,7 @@ def test_campaign_with_markings_example():
         description="Campaign by Green Group against a series of targets in the financial services sector.",
         object_marking_refs=TLP_WHITE,
     )
-    assert str(campaign) == EXPECTED_CAMPAIGN_WITH_OBJECT_MARKING
+    assert campaign.serialize(pretty=True) == EXPECTED_CAMPAIGN_WITH_OBJECT_MARKING
 
 
 def test_granular_example():
@@ -127,7 +127,7 @@ def test_granular_example():
         selectors=["abc", "abc.[23]", "abc.def", "abc.[2].efg"],
     )
 
-    assert str(granular_marking) == EXPECTED_GRANULAR_MARKING
+    assert granular_marking.serialize(pretty=True) == EXPECTED_GRANULAR_MARKING
 
 
 def test_granular_example_with_bad_selector():
@@ -159,7 +159,7 @@ def test_campaign_with_granular_markings_example():
             ),
         ],
     )
-    assert str(campaign) == EXPECTED_CAMPAIGN_WITH_GRANULAR_MARKINGS
+    assert campaign.serialize(pretty=True) == EXPECTED_CAMPAIGN_WITH_GRANULAR_MARKINGS
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v20/test_observed_data.py b/stix2/test/v20/test_observed_data.py
index bd60383..79ed6c4 100644
--- a/stix2/test/v20/test_observed_data.py
+++ b/stix2/test/v20/test_observed_data.py
@@ -47,7 +47,7 @@ def test_observed_data_example():
         },
     )
 
-    assert str(observed_data) == EXPECTED
+    assert observed_data.serialize(pretty=True) == EXPECTED
 
 
 EXPECTED_WITH_REF = """{
@@ -97,7 +97,7 @@ def test_observed_data_example_with_refs():
         },
     )
 
-    assert str(observed_data) == EXPECTED_WITH_REF
+    assert observed_data.serialize(pretty=True) == EXPECTED_WITH_REF
 
 
 def test_observed_data_example_with_bad_refs():
diff --git a/stix2/test/v20/test_relationship.py b/stix2/test/v20/test_relationship.py
index a0fccf4..f8ad67e 100644
--- a/stix2/test/v20/test_relationship.py
+++ b/stix2/test/v20/test_relationship.py
@@ -32,7 +32,7 @@ def test_relationship_all_required_properties():
         source_ref=INDICATOR_ID,
         target_ref=MALWARE_ID,
     )
-    assert str(rel) == EXPECTED_RELATIONSHIP
+    assert rel.serialize(pretty=True) == EXPECTED_RELATIONSHIP
 
 
 def test_relationship_autogenerated_properties(relationship):
diff --git a/stix2/test/v20/test_report.py b/stix2/test/v20/test_report.py
index 53707ce..557a374 100644
--- a/stix2/test/v20/test_report.py
+++ b/stix2/test/v20/test_report.py
@@ -47,7 +47,7 @@ def test_report_example():
         ],
     )
 
-    assert str(report) == EXPECTED
+    assert report.serialize(pretty=True) == EXPECTED
 
 
 def test_report_example_objects_in_object_refs():
@@ -67,7 +67,7 @@ def test_report_example_objects_in_object_refs():
         ],
     )
 
-    assert str(report) == EXPECTED
+    assert report.serialize(pretty=True) == EXPECTED
 
 
 def test_report_example_objects_in_object_refs_with_bad_id():
diff --git a/stix2/test/v20/test_sighting.py b/stix2/test/v20/test_sighting.py
index 6bad63c..35cd1f1 100644
--- a/stix2/test/v20/test_sighting.py
+++ b/stix2/test/v20/test_sighting.py
@@ -33,7 +33,7 @@ BAD_SIGHTING = """{
 def test_sighting_all_required_properties():
     now = dt.datetime(2016, 4, 6, 20, 6, 37, tzinfo=pytz.utc)
 
-    s = stix2.v20.Sighting(
+    sighting = stix2.v20.Sighting(
         type='sighting',
         id=SIGHTING_ID,
         created=now,
@@ -41,7 +41,7 @@ def test_sighting_all_required_properties():
         sighting_of_ref=INDICATOR_ID,
         where_sighted_refs=[IDENTITY_ID],
     )
-    assert str(s) == EXPECTED_SIGHTING
+    assert sighting.serialize(pretty=True) == EXPECTED_SIGHTING
 
 
 def test_sighting_bad_where_sighted_refs():
diff --git a/stix2/test/v20/test_threat_actor.py b/stix2/test/v20/test_threat_actor.py
index 854e77a..0214f9b 100644
--- a/stix2/test/v20/test_threat_actor.py
+++ b/stix2/test/v20/test_threat_actor.py
@@ -32,7 +32,7 @@ def test_threat_actor_example():
         name="Evil Org",
     )
 
-    assert str(threat_actor) == EXPECTED
+    assert threat_actor.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v20/test_tool.py b/stix2/test/v20/test_tool.py
index 435b85e..dcb94be 100644
--- a/stix2/test/v20/test_tool.py
+++ b/stix2/test/v20/test_tool.py
@@ -43,7 +43,7 @@ def test_tool_example():
         name="VNC",
     )
 
-    assert str(tool) == EXPECTED
+    assert tool.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v20/test_vulnerability.py b/stix2/test/v20/test_vulnerability.py
index 5a69d82..b87152f 100644
--- a/stix2/test/v20/test_vulnerability.py
+++ b/stix2/test/v20/test_vulnerability.py
@@ -36,7 +36,7 @@ def test_vulnerability_example():
         ],
     )
 
-    assert str(vulnerability) == EXPECTED
+    assert vulnerability.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_attack_pattern.py b/stix2/test/v21/test_attack_pattern.py
index 0beda64..96a2869 100644
--- a/stix2/test/v21/test_attack_pattern.py
+++ b/stix2/test/v21/test_attack_pattern.py
@@ -38,7 +38,7 @@ def test_attack_pattern_example():
         description="...",
     )
 
-    assert str(ap) == EXPECTED
+    assert ap.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
@@ -114,7 +114,7 @@ def test_less_precise_timestamps():
         description="...",
     )
 
-    assert str(ap) == EXPECTED
+    assert ap.serialize(pretty=True) == EXPECTED
 
 
 # TODO: Add other examples
diff --git a/stix2/test/v21/test_bundle.py b/stix2/test/v21/test_bundle.py
index 4e30c84..2eeaff4 100644
--- a/stix2/test/v21/test_bundle.py
+++ b/stix2/test/v21/test_bundle.py
@@ -126,7 +126,7 @@ def test_bundle_id_must_start_with_bundle():
 def test_create_bundle1(indicator, malware, relationship):
     bundle = stix2.v21.Bundle(objects=[indicator, malware, relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
     assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
@@ -139,31 +139,31 @@ def test_create_bundle2(indicator, malware, relationship):
 def test_create_bundle_with_positional_args(indicator, malware, relationship):
     bundle = stix2.v21.Bundle(indicator, malware, relationship)
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_positional_listarg(indicator, malware, relationship):
     bundle = stix2.v21.Bundle([indicator, malware, relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_listarg_and_positional_arg(indicator, malware, relationship):
     bundle = stix2.v21.Bundle([indicator, malware], relationship)
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_listarg_and_kwarg(indicator, malware, relationship):
     bundle = stix2.v21.Bundle([indicator, malware], objects=[relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_with_arg_listarg_and_kwarg(indicator, malware, relationship):
     bundle = stix2.v21.Bundle([indicator], malware, objects=[relationship])
 
-    assert str(bundle) == EXPECTED_BUNDLE
+    assert bundle.serialize(pretty=True) == EXPECTED_BUNDLE
 
 
 def test_create_bundle_invalid(indicator, malware, relationship):
diff --git a/stix2/test/v21/test_campaign.py b/stix2/test/v21/test_campaign.py
index 10f5d7b..edc7d77 100644
--- a/stix2/test/v21/test_campaign.py
+++ b/stix2/test/v21/test_campaign.py
@@ -24,7 +24,7 @@ def test_campaign_example():
         **CAMPAIGN_MORE_KWARGS
     )
 
-    assert str(campaign) == EXPECTED
+    assert campaign.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_course_of_action.py b/stix2/test/v21/test_course_of_action.py
index 5eea908..964a556 100644
--- a/stix2/test/v21/test_course_of_action.py
+++ b/stix2/test/v21/test_course_of_action.py
@@ -42,7 +42,7 @@ COA_WITH_REF_DICT = json.loads(COA_WITH_REF_JSON)
 )
 def test_course_of_action_example(sdo_json, sdo_dict):
     coa = stix2.v21.CourseOfAction(**sdo_dict)
-    assert str(coa) == sdo_json
+    assert coa.serialize(pretty=True) == sdo_json
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_custom.py b/stix2/test/v21/test_custom.py
index 79a4d03..ac46d84 100644
--- a/stix2/test/v21/test_custom.py
+++ b/stix2/test/v21/test_custom.py
@@ -965,7 +965,7 @@ def test_custom_extension_with_list_and_dict_properties_observable_type(data):
         pass
 
     example = SomeCustomExtension(keys=[{'test123': 123, 'test345': 'aaaa'}])
-    assert data == str(example)
+    assert data == example.serialize(pretty=True)
 
 
 def test_custom_extension_invalid_type_name():
@@ -1202,7 +1202,7 @@ def test_custom_object_nested_dictionary(data):
         dictionary={'key': {'key_b': 'value', 'key_a': 'value'}},
     )
 
-    assert data == str(example)
+    assert data == example.serialize(pretty=True)
 
 
 @stix2.v21.CustomObject(
diff --git a/stix2/test/v21/test_external_reference.py b/stix2/test/v21/test_external_reference.py
index f347191..9316857 100644
--- a/stix2/test/v21/test_external_reference.py
+++ b/stix2/test/v21/test_external_reference.py
@@ -26,7 +26,7 @@ def test_external_reference_veris():
         url="https://github.com/vz-risk/VCDB/blob/master/data/json/0001AA7F-C601-424A-B2B8-BE6C9F5164E7.json",
     )
 
-    assert str(ref) == VERIS
+    assert ref.serialize(pretty=True) == VERIS
 
 
 CAPEC = """{
@@ -41,7 +41,7 @@ def test_external_reference_capec():
         external_id="CAPEC-550",
     )
 
-    assert str(ref) == CAPEC
+    assert ref.serialize(pretty=True) == CAPEC
     assert re.match("ExternalReference\\(source_name=u?'capec', external_id=u?'CAPEC-550'\\)", repr(ref))
 
 
@@ -59,7 +59,7 @@ def test_external_reference_capec_url():
         url="http://capec.mitre.org/data/definitions/550.html",
     )
 
-    assert str(ref) == CAPEC_URL
+    assert ref.serialize(pretty=True) == CAPEC_URL
 
 
 THREAT_REPORT = """{
@@ -76,7 +76,7 @@ def test_external_reference_threat_report():
         url="http://www.example.com/threat-report.pdf",
     )
 
-    assert str(ref) == THREAT_REPORT
+    assert ref.serialize(pretty=True) == THREAT_REPORT
 
 
 BUGZILLA = """{
@@ -93,7 +93,7 @@ def test_external_reference_bugzilla():
         url="https://www.example.com/bugs/1370",
     )
 
-    assert str(ref) == BUGZILLA
+    assert ref.serialize(pretty=True) == BUGZILLA
 
 
 OFFLINE = """{
@@ -108,7 +108,7 @@ def test_external_reference_offline():
         description="Threat report",
     )
 
-    assert str(ref) == OFFLINE
+    assert ref.serialize(pretty=True) == OFFLINE
     assert re.match("ExternalReference\\(source_name=u?'ACME Threat Intel', description=u?'Threat report'\\)", repr(ref))
     # Yikes! This works
     assert eval("stix2." + repr(ref)) == ref
diff --git a/stix2/test/v21/test_grouping.py b/stix2/test/v21/test_grouping.py
index a92a180..e495888 100644
--- a/stix2/test/v21/test_grouping.py
+++ b/stix2/test/v21/test_grouping.py
@@ -38,7 +38,7 @@ def test_grouping_with_all_required_properties():
         ],
     )
 
-    assert str(grp) == EXPECTED_GROUPING
+    assert grp.serialize(pretty=True) == EXPECTED_GROUPING
 
 
 def test_grouping_autogenerated_properties(grouping):
diff --git a/stix2/test/v21/test_identity.py b/stix2/test/v21/test_identity.py
index 9d17723..c235b4d 100644
--- a/stix2/test/v21/test_identity.py
+++ b/stix2/test/v21/test_identity.py
@@ -27,7 +27,7 @@ def test_identity_example():
         identity_class="individual",
     )
 
-    assert str(identity) == EXPECTED
+    assert identity.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_incident.py b/stix2/test/v21/test_incident.py
index 7ee07b8..27bc254 100644
--- a/stix2/test/v21/test_incident.py
+++ b/stix2/test/v21/test_incident.py
@@ -27,7 +27,7 @@ def test_incident_example():
         description="Intrusion into enterprise network",
     )
 
-    assert str(incident) == EXPECTED
+    assert incident.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_indicator.py b/stix2/test/v21/test_indicator.py
index 2b22418..c6220d7 100644
--- a/stix2/test/v21/test_indicator.py
+++ b/stix2/test/v21/test_indicator.py
@@ -50,7 +50,7 @@ def test_indicator_with_all_required_properties():
     )
 
     assert ind.revoked is False
-    assert str(ind) == EXPECTED_INDICATOR
+    assert ind.serialize(pretty=True) == EXPECTED_INDICATOR
     rep = re.sub(r"(\[|=| )u('|\"|\\\'|\\\")", r"\g<1>\g<2>", repr(ind))
     assert rep == EXPECTED_INDICATOR_REPR
 
diff --git a/stix2/test/v21/test_infrastructure.py b/stix2/test/v21/test_infrastructure.py
index 3e9feb7..2beb6aa 100644
--- a/stix2/test/v21/test_infrastructure.py
+++ b/stix2/test/v21/test_infrastructure.py
@@ -28,7 +28,7 @@ def test_infrastructure_with_all_required_properties():
         name="Poison Ivy C2",
     )
 
-    assert str(infra) == EXPECTED_INFRASTRUCTURE
+    assert infra.serialize(pretty=True) == EXPECTED_INFRASTRUCTURE
 
 
 def test_infrastructure_autogenerated_properties(infrastructure):
diff --git a/stix2/test/v21/test_intrusion_set.py b/stix2/test/v21/test_intrusion_set.py
index 778eda7..2b2dc0c 100644
--- a/stix2/test/v21/test_intrusion_set.py
+++ b/stix2/test/v21/test_intrusion_set.py
@@ -39,7 +39,7 @@ def test_intrusion_set_example():
         goals=["acquisition-theft", "harassment", "damage"],
     )
 
-    assert str(intrusion_set) == EXPECTED
+    assert intrusion_set.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_kill_chain_phases.py b/stix2/test/v21/test_kill_chain_phases.py
index 0acc538..9e29e88 100644
--- a/stix2/test/v21/test_kill_chain_phases.py
+++ b/stix2/test/v21/test_kill_chain_phases.py
@@ -16,7 +16,7 @@ def test_lockheed_martin_cyber_kill_chain():
         phase_name="reconnaissance",
     )
 
-    assert str(recon) == LMCO_RECON
+    assert recon.serialize(pretty=True) == LMCO_RECON
 
 
 FOO_PRE_ATTACK = """{
@@ -31,7 +31,7 @@ def test_kill_chain_example():
         phase_name="pre-attack",
     )
 
-    assert str(preattack) == FOO_PRE_ATTACK
+    assert preattack.serialize(pretty=True) == FOO_PRE_ATTACK
 
 
 def test_kill_chain_required_properties():
diff --git a/stix2/test/v21/test_location.py b/stix2/test/v21/test_location.py
index cf9c101..0fc5bcb 100644
--- a/stix2/test/v21/test_location.py
+++ b/stix2/test/v21/test_location.py
@@ -54,7 +54,7 @@ EXPECTED_LOCATION_2_REPR = "Location(" + " ".join(
 def test_location_with_some_required_properties():
     now = dt.datetime(2016, 4, 6, 20, 3, 0, tzinfo=pytz.utc)
 
-    loc = stix2.v21.Location(
+    location = stix2.v21.Location(
         id=LOCATION_ID,
         created=now,
         modified=now,
@@ -62,8 +62,8 @@ def test_location_with_some_required_properties():
         longitude=2.3522,
     )
 
-    assert str(loc) == EXPECTED_LOCATION_1
-    rep = re.sub(r"(\[|=| )u('|\"|\\\'|\\\")", r"\g<1>\g<2>", repr(loc))
+    assert location.serialize(pretty=True) == EXPECTED_LOCATION_1
+    rep = re.sub(r"(\[|=| )u('|\"|\\\'|\\\")", r"\g<1>\g<2>", repr(location))
     assert rep == EXPECTED_LOCATION_1_REPR
 
 
diff --git a/stix2/test/v21/test_malware.py b/stix2/test/v21/test_malware.py
index f111826..d71b273 100644
--- a/stix2/test/v21/test_malware.py
+++ b/stix2/test/v21/test_malware.py
@@ -23,7 +23,7 @@ EXPECTED_MALWARE = """{
 def test_malware_with_all_required_properties():
     now = dt.datetime(2016, 5, 12, 8, 17, 27, tzinfo=pytz.utc)
 
-    mal = stix2.v21.Malware(
+    malware = stix2.v21.Malware(
         type="malware",
         id=MALWARE_ID,
         created=now,
@@ -32,7 +32,7 @@ def test_malware_with_all_required_properties():
         is_family=False,
     )
 
-    assert str(mal) == EXPECTED_MALWARE
+    assert malware.serialize(pretty=True) == EXPECTED_MALWARE
 
 
 def test_malware_autogenerated_properties(malware):
diff --git a/stix2/test/v21/test_malware_analysis.py b/stix2/test/v21/test_malware_analysis.py
index bf4bbe6..1390203 100644
--- a/stix2/test/v21/test_malware_analysis.py
+++ b/stix2/test/v21/test_malware_analysis.py
@@ -48,9 +48,9 @@ MALWARE_ANALYSIS_DICT = json.loads(MALWARE_ANALYSIS_JSON)
 
 
 def test_malware_analysis_example():
-    ma = stix2.v21.MalwareAnalysis(**MALWARE_ANALYSIS_DICT)
+    malware_analysis = stix2.v21.MalwareAnalysis(**MALWARE_ANALYSIS_DICT)
 
-    assert str(ma) == MALWARE_ANALYSIS_JSON
+    assert malware_analysis.serialize(pretty=True) == MALWARE_ANALYSIS_JSON
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_markings.py b/stix2/test/v21/test_markings.py
index b0a0d09..d3fd11b 100644
--- a/stix2/test/v21/test_markings.py
+++ b/stix2/test/v21/test_markings.py
@@ -99,7 +99,7 @@ EXPECTED_CAMPAIGN_WITH_GRANULAR_LANG_MARKINGS = u"""{
 
 
 def test_marking_def_example_with_tlp():
-    assert str(TLP_WHITE) == EXPECTED_TLP_MARKING_DEFINITION
+    assert TLP_WHITE.serialize(pretty=True) == EXPECTED_TLP_MARKING_DEFINITION
 
 
 def test_marking_def_example_with_statement_positional_argument():
@@ -110,7 +110,7 @@ def test_marking_def_example_with_statement_positional_argument():
         definition=stix2.StatementMarking(statement="Copyright 2016, Example Corp"),
     )
 
-    assert str(marking_definition) == EXPECTED_STATEMENT_MARKING_DEFINITION
+    assert marking_definition.serialize(pretty=True) == EXPECTED_STATEMENT_MARKING_DEFINITION
 
 
 def test_marking_def_example_with_kwargs_statement():
@@ -122,7 +122,7 @@ def test_marking_def_example_with_kwargs_statement():
         definition=stix2.StatementMarking(**kwargs),
     )
 
-    assert str(marking_definition) == EXPECTED_STATEMENT_MARKING_DEFINITION
+    assert marking_definition.serialize(pretty=True) == EXPECTED_STATEMENT_MARKING_DEFINITION
 
 
 def test_marking_def_invalid_type():
@@ -145,7 +145,7 @@ def test_campaign_with_markings_example():
         description="Campaign by Green Group against a series of targets in the financial services sector.",
         object_marking_refs=TLP_WHITE,
     )
-    assert str(campaign) == EXPECTED_CAMPAIGN_WITH_OBJECT_MARKING
+    assert campaign.serialize(pretty=True) == EXPECTED_CAMPAIGN_WITH_OBJECT_MARKING
 
 
 def test_granular_example():
@@ -154,7 +154,7 @@ def test_granular_example():
         selectors=["abc", "abc.[23]", "abc.def", "abc.[2].efg"],
     )
 
-    assert str(granular_marking) == EXPECTED_GRANULAR_MARKING
+    assert granular_marking.serialize(pretty=True) == EXPECTED_GRANULAR_MARKING
 
 
 def test_granular_example_with_bad_selector():
@@ -185,7 +185,7 @@ def test_campaign_with_granular_markings_example():
             ),
         ],
     )
-    assert str(campaign) == EXPECTED_CAMPAIGN_WITH_GRANULAR_REF_MARKINGS
+    assert campaign.serialize(pretty=True) == EXPECTED_CAMPAIGN_WITH_GRANULAR_REF_MARKINGS
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_note.py b/stix2/test/v21/test_note.py
index 47a191e..2f20c8d 100644
--- a/stix2/test/v21/test_note.py
+++ b/stix2/test/v21/test_note.py
@@ -73,7 +73,7 @@ def test_note_with_required_properties():
         ],
     )
 
-    assert str(note) == EXPECTED_NOTE
+    assert note.serialize(pretty=True) == EXPECTED_NOTE
     rep = re.sub(r"(\[|=| )u('|\"|\\\'|\\\")", r"\g<1>\g<2>", repr(note))
     assert rep == EXPECTED_OPINION_REPR
 
diff --git a/stix2/test/v21/test_observed_data.py b/stix2/test/v21/test_observed_data.py
index c1cb38e..ccf9a70 100644
--- a/stix2/test/v21/test_observed_data.py
+++ b/stix2/test/v21/test_observed_data.py
@@ -155,7 +155,7 @@ def test_observed_data_example_with_object_refs():
         ],
     )
 
-    assert str(observed_data) == EXPECTED_OBJECT_REFS
+    assert observed_data.serialize(pretty=True) == EXPECTED_OBJECT_REFS
 
 
 def test_observed_data_object_constraint():
diff --git a/stix2/test/v21/test_opinion.py b/stix2/test/v21/test_opinion.py
index 9634d6e..21c96d7 100644
--- a/stix2/test/v21/test_opinion.py
+++ b/stix2/test/v21/test_opinion.py
@@ -56,7 +56,7 @@ def test_opinion_with_required_properties():
         explanation=EXPLANATION,
     )
 
-    assert str(opi) == EXPECTED_OPINION
+    assert opi.serialize(pretty=True) == EXPECTED_OPINION
     rep = re.sub(r"(\[|=| )u('|\"|\\\'|\\\")", r"\g<1>\g<2>", repr(opi))
     assert rep == EXPECTED_OPINION_REPR
 
diff --git a/stix2/test/v21/test_relationship.py b/stix2/test/v21/test_relationship.py
index 386e24b..9f76d92 100644
--- a/stix2/test/v21/test_relationship.py
+++ b/stix2/test/v21/test_relationship.py
@@ -33,7 +33,7 @@ def test_relationship_all_required_properties():
         source_ref=INDICATOR_ID,
         target_ref=MALWARE_ID,
     )
-    assert str(rel) == EXPECTED_RELATIONSHIP
+    assert rel.serialize(pretty=True) == EXPECTED_RELATIONSHIP
 
 
 def test_relationship_autogenerated_properties(relationship):
diff --git a/stix2/test/v21/test_report.py b/stix2/test/v21/test_report.py
index e54e11d..87ac1c1 100644
--- a/stix2/test/v21/test_report.py
+++ b/stix2/test/v21/test_report.py
@@ -48,7 +48,7 @@ def test_report_example():
         ],
     )
 
-    assert str(report) == EXPECTED
+    assert report.serialize(pretty=True) == EXPECTED
 
 
 def test_report_example_objects_in_object_refs():
@@ -68,7 +68,7 @@ def test_report_example_objects_in_object_refs():
         ],
     )
 
-    assert str(report) == EXPECTED
+    assert report.serialize(pretty=True) == EXPECTED
 
 
 def test_report_example_objects_in_object_refs_with_bad_id():
diff --git a/stix2/test/v21/test_sighting.py b/stix2/test/v21/test_sighting.py
index 0ef5faa..e997d62 100644
--- a/stix2/test/v21/test_sighting.py
+++ b/stix2/test/v21/test_sighting.py
@@ -38,7 +38,7 @@ BAD_SIGHTING = """{
 def test_sighting_all_required_properties():
     now = dt.datetime(2016, 4, 6, 20, 6, 37, tzinfo=pytz.utc)
 
-    s = stix2.v21.Sighting(
+    sighting = stix2.v21.Sighting(
         type='sighting',
         id=SIGHTING_ID,
         created=now,
@@ -46,7 +46,7 @@ def test_sighting_all_required_properties():
         sighting_of_ref=INDICATOR_ID,
         where_sighted_refs=[IDENTITY_ID, LOCATION_ID],
     )
-    assert str(s) == EXPECTED_SIGHTING
+    assert sighting.serialize(pretty=True) == EXPECTED_SIGHTING
 
 
 def test_sighting_bad_where_sighted_refs():
diff --git a/stix2/test/v21/test_threat_actor.py b/stix2/test/v21/test_threat_actor.py
index 6a782ef..94dea8e 100644
--- a/stix2/test/v21/test_threat_actor.py
+++ b/stix2/test/v21/test_threat_actor.py
@@ -34,7 +34,7 @@ def test_threat_actor_example():
         threat_actor_types=["crime-syndicate"],
     )
 
-    assert str(threat_actor) == EXPECTED
+    assert threat_actor.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_tool.py b/stix2/test/v21/test_tool.py
index 6a7d89c..427f8de 100644
--- a/stix2/test/v21/test_tool.py
+++ b/stix2/test/v21/test_tool.py
@@ -45,7 +45,7 @@ def test_tool_example():
         tool_types=["remote-access"],
     )
 
-    assert str(tool) == EXPECTED
+    assert tool.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(
diff --git a/stix2/test/v21/test_vulnerability.py b/stix2/test/v21/test_vulnerability.py
index ee63a0e..bcea913 100644
--- a/stix2/test/v21/test_vulnerability.py
+++ b/stix2/test/v21/test_vulnerability.py
@@ -37,7 +37,7 @@ def test_vulnerability_example():
         ],
     )
 
-    assert str(vulnerability) == EXPECTED
+    assert vulnerability.serialize(pretty=True) == EXPECTED
 
 
 @pytest.mark.parametrize(