core:adds tests for partial_variables (#15427)

**Description:** Added small tests to test partial_variables in
PromptTemplate. It was missing.

libs/core/tests/unit_tests/examples/simple_prompt.yaml

@@ -1,5 +1,7 @@
 _type: prompt
 input_variables:
-    ["adjective", "content"]
+    ["adjective"]
+partial_variables:
+    content: dogs
 template: 
     Tell me a {adjective} joke about {content}.

libs/core/tests/unit_tests/prompts/test_loading.py

@@ -28,7 +28,8 @@ def test_loading_from_YAML() -> None:
     """Test loading from yaml file."""
     prompt = load_prompt(EXAMPLE_DIR / "simple_prompt.yaml")
     expected_prompt = PromptTemplate(
-        input_variables=["adjective", "content"],
+        input_variables=["adjective"],
+        partial_variables={"content": "dogs"},
         template="Tell me a {adjective} joke about {content}.",
     )
     assert prompt == expected_prompt

libs/core/tests/unit_tests/prompts/test_prompt.py

@@ -1,4 +1,6 @@
 """Test functionality related to prompts."""
+from unittest import mock
+
 import pytest
 
 from langchain_core.prompts.prompt import PromptTemplate
@@ -34,6 +36,22 @@ def test_prompt_from_template() -> None:
     assert prompt == expected_prompt
 
 
+def test_prompt_from_template_with_partial_variables() -> None:
+    """Test prompts can be constructed from a template with partial variables."""
+    # given
+    template = "This is a {foo} test {bar}."
+    partial_variables = {"bar": "baz"}
+    # when
+    prompt = PromptTemplate.from_template(template, partial_variables=partial_variables)
+    # then
+    expected_prompt = PromptTemplate(
+        template=template,
+        input_variables=["foo"],
+        partial_variables=partial_variables,
+    )
+    assert prompt == expected_prompt
+
+
 def test_prompt_missing_input_variables() -> None:
     """Test error is raised when input variables are not provided."""
     template = "This is a {foo} test."
@@ -118,6 +136,26 @@ def test_prompt_from_file() -> None:
     assert prompt.template == "Question: {question}\nAnswer:"
 
 
+def test_prompt_from_file_with_partial_variables() -> None:
+    """Test prompt can be successfully constructed from a file
+    with partial variables."""
+    # given
+    template = "This is a {foo} test {bar}."
+    partial_variables = {"bar": "baz"}
+    # when
+    with mock.patch("builtins.open", mock.mock_open(read_data=template)):
+        prompt = PromptTemplate.from_file(
+            "mock_file_name", partial_variables=partial_variables
+        )
+    # then
+    expected_prompt = PromptTemplate(
+        template=template,
+        input_variables=["foo"],
+        partial_variables=partial_variables,
+    )
+    assert prompt == expected_prompt
+
+
 def test_partial_init_string() -> None:
     """Test prompt can be initialized with partial variables."""
     template = "This is a {foo} test."