core[patch]: extract input variables for path and detail keys in order to format an ImagePromptTemplate (#22613)

- Description: Add support for `path` and `detail` keys in
`ImagePromptTemplate`. Previously, only variables associated with the
`url` key were considered. This PR allows for the inclusion of a local
image path and a detail parameter as input to the format method.
- Issues:
    - fixes #20820 
    - related to #22024 
- Dependencies: None
- Twitter handle: @DeschampsTho5

---------

Co-authored-by: tdeschamps <tdeschamps@kameleoon.com>
Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>
Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>

libs/core/langchain_core/prompts/chat.py

@@ -473,6 +473,7 @@ class _StringImageMessagePromptTemplate(BaseMessagePromptTemplate):
                     )
                 elif isinstance(tmpl, dict) and "image_url" in tmpl:
                     img_template = cast(_ImageTemplateParam, tmpl)["image_url"]
+                    input_variables = []
                     if isinstance(img_template, str):
                         vars = get_template_variables(img_template, "f-string")
                         if vars:
@@ -483,20 +484,19 @@ class _StringImageMessagePromptTemplate(BaseMessagePromptTemplate):
                                     f"\nFrom: {tmpl}"
                                 )
                             input_variables = [vars[0]]
-                        else:
-                            input_variables = None
                         img_template = {"url": img_template}
                         img_template_obj = ImagePromptTemplate(
                             input_variables=input_variables, template=img_template
                         )
                     elif isinstance(img_template, dict):
                         img_template = dict(img_template)
-                        if "url" in img_template:
-                            input_variables = get_template_variables(
-                                img_template["url"], "f-string"
-                            )
-                        else:
-                            input_variables = None
+                        for key in ["url", "path", "detail"]:
+                            if key in img_template:
+                                input_variables.extend(
+                                    get_template_variables(
+                                        img_template[key], "f-string"
+                                    )
+                                )
                         img_template_obj = ImagePromptTemplate(
                             input_variables=input_variables, template=img_template
                         )

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

@@ -1,3 +1,5 @@
+import base64
+import tempfile
 from pathlib import Path
 from typing import Any, List, Union
 
@@ -559,6 +561,7 @@ async def test_chat_tmpl_from_messages_multipart_text_with_template() -> None:
 
 
 async def test_chat_tmpl_from_messages_multipart_image() -> None:
+    """Test multipart image URL formatting."""
     base64_image = "iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAA"
     other_base64_image = "iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAA"
     template = ChatPromptTemplate.from_messages(
@@ -641,6 +644,65 @@ async def test_chat_tmpl_from_messages_multipart_image() -> None:
     assert messages == expected
 
 
+async def test_chat_tmpl_from_messages_multipart_formatting_with_path() -> None:
+    """Verify that we can pass `path` for an image as a variable."""
+    in_mem = "base64mem"
+    in_file_data = "base64file01"
+
+    with tempfile.NamedTemporaryFile(delete=True, suffix=".jpg") as temp_file:
+        temp_file.write(base64.b64decode(in_file_data))
+        temp_file.flush()
+
+        template = ChatPromptTemplate.from_messages(
+            [
+                ("system", "You are an AI assistant named {name}."),
+                (
+                    "human",
+                    [
+                        {"type": "text", "text": "What's in this image?"},
+                        {
+                            "type": "image_url",
+                            "image_url": "data:image/jpeg;base64,{in_mem}",
+                        },
+                        {
+                            "type": "image_url",
+                            "image_url": {"path": "{file_path}"},
+                        },
+                    ],
+                ),
+            ]
+        )
+        expected = [
+            SystemMessage(content="You are an AI assistant named R2D2."),
+            HumanMessage(
+                content=[
+                    {"type": "text", "text": "What's in this image?"},
+                    {
+                        "type": "image_url",
+                        "image_url": {"url": f"data:image/jpeg;base64,{in_mem}"},
+                    },
+                    {
+                        "type": "image_url",
+                        "image_url": {"url": f"data:image/jpeg;base64,{in_file_data}"},
+                    },
+                ]
+            ),
+        ]
+        messages = template.format_messages(
+            name="R2D2",
+            in_mem=in_mem,
+            file_path=temp_file.name,
+        )
+        assert messages == expected
+
+        messages = await template.aformat_messages(
+            name="R2D2",
+            in_mem=in_mem,
+            file_path=temp_file.name,
+        )
+        assert messages == expected
+
+
 def test_messages_placeholder() -> None:
     prompt = MessagesPlaceholder("history")
     with pytest.raises(KeyError):