diff --git a/stix2/patterns.py b/stix2/patterns.py
index 23ce71b..3f9cbd9 100644
--- a/stix2/patterns.py
+++ b/stix2/patterns.py
@@ -147,6 +147,9 @@ class ListConstant(_Constant):
 
 
 def make_constant(value):
+    if isinstance(value, _Constant):
+        return value
+
     try:
         return parse_into_datetime(value)
     except ValueError:

make_constant() should check if value is already a constant
``` python
>>> from stix2.patterns import StringConstant
>>> from stix2.patterns import make_constant
>>> a = StringConstant('foo')
>>> make_constant(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/lu/Projects/cti-python-stix2/stix2/patterns.py", line 166, in make_constant
    raise ValueError("Unable to create a constant from %s" % value)
ValueError: Unable to create a constant from 'foo'
```
diff --git a/stix2/test/test_pattern_expressions.py b/stix2/test/test_pattern_expressions.py
index 74a7d0f..14e3774 100644
--- a/stix2/test/test_pattern_expressions.py
+++ b/stix2/test/test_pattern_expressions.py
@@ -372,3 +372,9 @@ def test_invalid_startstop_qualifier():
         stix2.StartStopQualifier(datetime.date(2016, 6, 1),
                                  'foo')
     assert 'is not a valid argument for a Start/Stop Qualifier' in str(excinfo)
+
+
+def test_make_constant_already_a_constant():
+    str_const = stix2.StringConstant('Foo')
+    result = stix2.patterns.make_constant(str_const)
+    assert result is str_const

pip install -e .[dev]
parse_log_pytest
requirements.txt
pytest pytest-cov taxii2-client
apt-get update apt-get install -y gcc
3.6
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.13
antlr4-python3-runtime==4.9.3
async-generator==1.10
attrs==22.2.0
Babel==2.11.0
backcall==0.2.0
bleach==4.1.0
bump2version==1.0.1
bumpversion==0.6.0
certifi==2021.5.30
cfgv==3.3.1
charset-normalizer==2.0.12
coverage==6.2
decorator==5.1.1
defusedxml==0.7.1
distlib==0.3.9
docutils==0.18.1
entrypoints==0.4
filelock==3.4.1
identify==2.4.4
idna==3.10
imagesize==1.4.1
importlib-metadata==4.8.3
importlib-resources==5.2.3
iniconfig==1.1.1
ipython==7.16.3
ipython-genutils==0.2.0
jedi==0.17.2
Jinja2==3.0.3
jsonschema==3.2.0
jupyter-client==7.1.2
jupyter-core==4.9.2
jupyterlab-pygments==0.1.2
MarkupSafe==2.0.1
mistune==0.8.4
nbclient==0.5.9
nbconvert==6.0.7
nbformat==5.1.3
nbsphinx==0.3.2
nest-asyncio==1.6.0
nodeenv==1.6.0
packaging==21.3
pandocfilters==1.5.1
parso==0.7.1
pexpect==4.9.0
pickleshare==0.7.5
platformdirs==2.4.0
pluggy==1.0.0
pre-commit==2.17.0
prompt-toolkit==3.0.36
ptyprocess==0.7.0
py==1.11.0
Pygments==2.14.0
pyparsing==3.1.4
pyrsistent==0.18.0
pytest==7.0.1
pytest-cov==4.0.0
python-dateutil==2.9.0.post0
pytz==2025.2
PyYAML==6.0.1
pyzmq==25.1.2
requests==2.27.1
simplejson==3.20.1
six==1.17.0
snowballstemmer==2.2.0
Sphinx==1.5.6
sphinx-prompt==1.5.0
-e git+https://github.com/oasis-open/cti-python-stix2.git@f778a45b33f9b74c5a3b62c38db477bb504c2202#egg=stix2
stix2-patterns==2.0.0
taxii2-client==2.3.0
testpath==0.6.0
toml==0.10.2
tomli==1.2.3
tornado==6.1
tox==3.28.0
traitlets==4.3.3
typing_extensions==4.1.1
urllib3==1.26.20
virtualenv==20.16.2
wcwidth==0.2.13
webencodings==0.5.1
zipp==3.6.0

name: cti-python-stix2
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=21.2.2=py36h06a4308_0
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.13
      - antlr4-python3-runtime==4.9.3
      - async-generator==1.10
      - attrs==22.2.0
      - babel==2.11.0
      - backcall==0.2.0
      - bleach==4.1.0
      - bump2version==1.0.1
      - bumpversion==0.6.0
      - cfgv==3.3.1
      - charset-normalizer==2.0.12
      - coverage==6.2
      - decorator==5.1.1
      - defusedxml==0.7.1
      - distlib==0.3.9
      - docutils==0.18.1
      - entrypoints==0.4
      - filelock==3.4.1
      - identify==2.4.4
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==4.8.3
      - importlib-resources==5.2.3
      - iniconfig==1.1.1
      - ipython==7.16.3
      - ipython-genutils==0.2.0
      - jedi==0.17.2
      - jinja2==3.0.3
      - jsonschema==3.2.0
      - jupyter-client==7.1.2
      - jupyter-core==4.9.2
      - jupyterlab-pygments==0.1.2
      - markupsafe==2.0.1
      - mistune==0.8.4
      - nbclient==0.5.9
      - nbconvert==6.0.7
      - nbformat==5.1.3
      - nbsphinx==0.3.2
      - nest-asyncio==1.6.0
      - nodeenv==1.6.0
      - packaging==21.3
      - pandocfilters==1.5.1
      - parso==0.7.1
      - pexpect==4.9.0
      - pickleshare==0.7.5
      - platformdirs==2.4.0
      - pluggy==1.0.0
      - pre-commit==2.17.0
      - prompt-toolkit==3.0.36
      - ptyprocess==0.7.0
      - py==1.11.0
      - pygments==2.14.0
      - pyparsing==3.1.4
      - pyrsistent==0.18.0
      - pytest==7.0.1
      - pytest-cov==4.0.0
      - python-dateutil==2.9.0.post0
      - pytz==2025.2
      - pyyaml==6.0.1
      - pyzmq==25.1.2
      - requests==2.27.1
      - simplejson==3.20.1
      - six==1.17.0
      - snowballstemmer==2.2.0
      - sphinx==1.5.6
      - sphinx-prompt==1.5.0
      - stix2-patterns==2.0.0
      - taxii2-client==2.3.0
      - testpath==0.6.0
      - toml==0.10.2
      - tomli==1.2.3
      - tornado==6.1
      - tox==3.28.0
      - traitlets==4.3.3
      - typing-extensions==4.1.1
      - urllib3==1.26.20
      - virtualenv==20.16.2
      - wcwidth==0.2.13
      - webencodings==0.5.1
      - zipp==3.6.0
prefix: /opt/conda/envs/cti-python-stix2

stix2/test/test_pattern_expressions.py::test_make_constant_already_a_constant
stix2/test/test_pattern_expressions.py::test_create_comparison_expression stix2/test/test_pattern_expressions.py::test_boolean_expression stix2/test/test_pattern_expressions.py::test_boolean_expression_with_parentheses stix2/test/test_pattern_expressions.py::test_hash_followed_by_registryKey_expression_python_constant stix2/test/test_pattern_expressions.py::test_hash_followed_by_registryKey_expression stix2/test/test_pattern_expressions.py::test_file_observable_expression stix2/test/test_pattern_expressions.py::test_multiple_file_observable_expression[AndObservationExpression-AND] stix2/test/test_pattern_expressions.py::test_multiple_file_observable_expression[OrObservationExpression-OR] stix2/test/test_pattern_expressions.py::test_root_types stix2/test/test_pattern_expressions.py::test_artifact_payload stix2/test/test_pattern_expressions.py::test_greater_than_python_constant stix2/test/test_pattern_expressions.py::test_greater_than stix2/test/test_pattern_expressions.py::test_less_than stix2/test/test_pattern_expressions.py::test_greater_than_or_equal stix2/test/test_pattern_expressions.py::test_less_than_or_equal stix2/test/test_pattern_expressions.py::test_not stix2/test/test_pattern_expressions.py::test_and_observable_expression stix2/test/test_pattern_expressions.py::test_invalid_and_observable_expression stix2/test/test_pattern_expressions.py::test_hex stix2/test/test_pattern_expressions.py::test_multiple_qualifiers stix2/test/test_pattern_expressions.py::test_set_op stix2/test/test_pattern_expressions.py::test_timestamp stix2/test/test_pattern_expressions.py::test_boolean stix2/test/test_pattern_expressions.py::test_binary stix2/test/test_pattern_expressions.py::test_list stix2/test/test_pattern_expressions.py::test_list2 stix2/test/test_pattern_expressions.py::test_invalid_constant_type stix2/test/test_pattern_expressions.py::test_invalid_integer_constant stix2/test/test_pattern_expressions.py::test_invalid_timestamp_constant stix2/test/test_pattern_expressions.py::test_invalid_float_constant stix2/test/test_pattern_expressions.py::test_boolean_constant[True-True0] stix2/test/test_pattern_expressions.py::test_boolean_constant[False-False0] stix2/test/test_pattern_expressions.py::test_boolean_constant[True-True1] stix2/test/test_pattern_expressions.py::test_boolean_constant[False-False1] stix2/test/test_pattern_expressions.py::test_boolean_constant[true-True] stix2/test/test_pattern_expressions.py::test_boolean_constant[false-False] stix2/test/test_pattern_expressions.py::test_boolean_constant[t-True] stix2/test/test_pattern_expressions.py::test_boolean_constant[f-False] stix2/test/test_pattern_expressions.py::test_boolean_constant[T-True] stix2/test/test_pattern_expressions.py::test_boolean_constant[F-False] stix2/test/test_pattern_expressions.py::test_boolean_constant[1-True] stix2/test/test_pattern_expressions.py::test_boolean_constant[0-False] stix2/test/test_pattern_expressions.py::test_invalid_boolean_constant stix2/test/test_pattern_expressions.py::test_invalid_hash_constant[MD5-zzz] stix2/test/test_pattern_expressions.py::test_invalid_hash_constant[ssdeep-zzz==] stix2/test/test_pattern_expressions.py::test_invalid_hex_constant stix2/test/test_pattern_expressions.py::test_invalid_binary_constant stix2/test/test_pattern_expressions.py::test_escape_quotes_and_backslashes stix2/test/test_pattern_expressions.py::test_like stix2/test/test_pattern_expressions.py::test_issuperset stix2/test/test_pattern_expressions.py::test_repeat_qualifier stix2/test/test_pattern_expressions.py::test_invalid_repeat_qualifier stix2/test/test_pattern_expressions.py::test_invalid_within_qualifier stix2/test/test_pattern_expressions.py::test_startstop_qualifier stix2/test/test_pattern_expressions.py::test_invalid_startstop_qualifier
stix2/patterns.py
stix2/patterns.py